[Paleopsych] Science and Creationism: A View from the National Academy of Science, 2 ed. 1999

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Science and Creationism: A View from the National Academy of Science, 2 ed. 
1999

Preface

In his preface to the original 1984 version of this document, Frank Press, my 
predecessor as president of the National Academy of Sciences, called attention 
to a pair of illustrations similar to the ones on the front and back of this 
booklet. The first is a photograph of Earth from space--the one on this booklet 
was taken by the GOES-7 satellite in 1992 as it passed over Earth and captured 
in graphic detail Hurricane Andrew. The second shows a map of the world 
prepared during the 7th century by the scholar Isidore of Seville. As Press 
pointed out, both illustrations reflect the efforts of humans to understand the 
natural world. "How then," he wrote, "can the two views be so different? The 
answer lies at the very heart of the nature of this system of study we call 
science."  Since those words were written, the mapping of Earth has provided 
further powerful examples of how science and science-based technologies 
progress. Beginning in the early 1990s, a network of satellites has allowed 
anyone with a hand-held receiver to know his or her position on Earth to within 
a few feet. This Global Positioning System* (GPS) now is being used to locate 
vessels lost at sea, study plate tectonics, trace open routes through crowded 
city streets, and survey Earth's surface. Yet the technology originated with a 
purely scientific objective--the desire to build extremely accurate clocks to 
test Einstein's theory of relativity.  The tremendous success of science in 
explaining natural phenomena and fostering technological innovation arises from 
its focus on explanations that can be inferred from confirmable data. 
Scientists seek to relate one natural phenomenon to another and to recognize 
the causes and effects of phenomena. In this way, they have developed 
explanations for the changing of the seasons, the movements of the sun and 
stars, the structure of matter, the shaping of mountains and valleys, the 
changes in the positions of continents over time, the history of life on Earth, 
and many other natural occurrences. By the same means, scientists have also 
deciphered which substances in our environment are harmful to humans and which 
are not, developed cures for diseases, and generated the knowledge needed to 
produce innumerable labor-saving devices.  The concept of biological evolution 
is one of the most important ideas ever generated by the application of 
scientific methods to the natural world. The evolution of all the organisms 
that live on Earth today from ancestors that lived in the past is at the core 
of genetics, biochemistry, neurobiology, physiology, ecology, and other 
biological disciplines. It helps to explain the emergence of new infectious 
diseases, the development of antibiotic resistance in bacteria, the 
agricultural relationships among wild and domestic plants and animals, the 
composition of Earth's atmosphere, the molecular machinery of the cell, the 
similarities between human beings and other primates, and countless other 
features of the biological and physical world. As the great geneticist and 
evolutionist Theodosius Dobzhansky wrote in 1973, "Nothing in biology makes 
sense except in the light of evolution." Nevertheless, the teaching of 
evolution in our schools remains controversial. Some object to it on the 
grounds that evolution contradicts the accounts of origins given in the first 
two chapters of Genesis. Some wish to see "creation science"--which posits that 
scientific evidence exists to prove that the universe and living things were 
specially created in their present form--taught together with evolution as two 
alternative scientific theories.  Scientists have considered the hypotheses 
proposed by creation science and have rejected them because of a lack of 
evidence. Furthermore, the claims of creation science do not refer to natural 
causes and cannot be subject to meaningful tests, so they do not qualify as 
scientific hypotheses. In 1987 the U.S. Supreme Court ruled that creationism is 
religion, not science, and cannot be advocated in public school classrooms. And 
most major religious groups have concluded that the concept of evolution is not 
at odds with their descriptions of creation and human origins.  This new 
edition of Science and Creationism: A View from the National Academy of 
Sciences is a companion volume to a publication released in 1998 by the 
Academy, Teaching About Evolution and the Nature of Science. That longer 
document is addressed to the teachers, educators, and policymakers who design, 
deliver, and oversee classroom instruction in biology. It summarizes the 
overwhelming observational evidence for evolution and explains how science 
differs from other human endeavors. It also suggests effective ways of teaching 
the subject and offers sample teaching exercises, curriculum guides, and 
"dialogues" among fictional teachers discussing the difficulties of presenting 
evolution in the classroom.  This new edition of Science and Creationism has a 
somewhat different purpose. It, too, summarizes key aspects of several of the 
most important lines of the evidence supporting evolution. But it also 
describes some of the positions taken by advocates of creation science and 
presents an analysis of these claims. As such, this document lays out for a 
broader audience the case against presenting religious concepts in science 
classes. Both this document, and the earlier Teaching About Evolution and the 
Nature of Science, are freely available online at the Academy website 
(www.nap.edu).  Scientists, like many others, are touched with awe at the order 
and complexity of nature. Indeed, many scientists are deeply religious. But 
science and religion occupy two separate realms of human experience. Demanding 
that they be combined detracts from the glory of each.

Bruce Alberts President National Academy of Sciences

*"The Global Positioning System: The Role of Atomic Clocks." Part of the series 
Beyond Discovery: The Path from Research to Human Benefit by the National 
Academy of Sciences (Washington, D.C.: National Academy Press, 1997). This 
document is also available at www2.nas.edu/bsi.

Introduction

Science is a particular way of knowing about the world. In science, 
explanations are limited to those based on observations and experiments that 
can be substantiated by other scientists. Explanations that cannot be based on 
empirical evidence are not a part of science.  In the quest for understanding, 
science involves a great deal of careful observation that eventually produces 
an elaborate written description of the natural world. Scientists communicate 
their findings and conclusions to other scientists through publications, talks 
at conferences, hallway conversations, and many other means. Other scientists 
then test those ideas and build on preexisting work. In this way, the accuracy 
and sophistication of descriptions of the natural world tend to increase with 
time, as subsequent generations of scientists correct and extend the work done 
by their predecessors.

Progress in science consists of the development of better explanations for the 
causes of natural phenomena. Scientists never can be sure that a given 
explanation is complete and final. Some of the hypotheses advanced by 
scientists turn out to be incorrect when tested by further observations or 
experiments. Yet many scientific explanations have been so thoroughly tested 
and confirmed that they are held with great confidence.  The theory of 
evolution is one of these well-established explanations. An enormous amount of 
scientific investigation since the mid-19th century has converted early ideas 
about evolution proposed by Darwin and others into a strong and well-supported 
theory. Today, evolution is an extremely active field of research, with an 
abundance of new discoveries that are continually increasing our understanding 
of how evolution occurs.  This booklet considers the science that supports the 
theory of evolution, focusing on three categories of scientific evidence: 
Evidence for the origins of the universe, Earth, and life Evidence for 
biological evolution, including findings from paleontology, comparative 
anatomy, biogeography, embryology, and molecular biology Evidence for human 
evolution At the end of each of these sections, the positions held by advocates 
of "creation science" are briefly presented and analyzed as well. The theory of 
evolution has become the central unifying concept of biology and is a critical 
component of many related scientific disciplines. In contrast, the claims of 
creation science lack empirical support and cannot be meaningfully tested. 
These observations lead to two fundamental conclusions: the teaching of 
evolution should be an integral part of science instruction, and creation 
science is in fact not science and should not be presented as such in science 
classes.

Terms Used in Describing the Nature of Science*

Fact: In science, an observation that has been repeatedly confirmed and for all 
practical purposes is accepted as "true." Truth in science, however, is never 
final, and what is accepted as a fact today may be modified or even discarded 
tomorrow.  Hypothesis: A tentative statement about the natural world leading to 
deductions that can be tested. If the deductions are verified, the hypothesis 
is provisionally corroborated. If the deductions are incorrect, the original 
hypothesis is proved false and must be abandoned or modified. Hypotheses can be 
used to build more complex inferences and explanations.  Law: A descriptive 
generalization about how some aspect of the natural world behaves under stated 
circumstances.  Theory: In science, a well-substantiated explanation of some 
aspect of the natural world that can incorporate facts, laws, inferences, and 
tested hypotheses.  The contention that evolution should be taught as a 
"theory, not as a fact" confuses the common use of these words with the 
scientific use. In science, theories do not turn into facts through the 
accumulation of evidence. Rather, theories are the end points of science. They 
are understandings that develop from extensive observation, experimentation, 
and creative reflection. They incorporate a large body of scientific facts, 
laws, tested hypotheses, and logical inferences. In this sense, evolution is 
one of the strongest and most useful scientific theories we have.  *Adapted 
from Teaching About Evolution and the Nature of Science by the National Academy 
of Sciences (Washington, D.C.: National Academy Press, 1998).

The Origin of the Universe, Earth, and Life

The term "evolution" usually refers to the biological evolution of living 
things. But the processes by which planets, stars, galaxies, and the universe 
form and change over time are also types of "evolution." In all of these cases 
there is change over time, although the processes involved are quite different. 
In the late 1920s the American astronomer Edwin Hubble made a very interesting 
and important discovery. Hubble made observations that he interpreted as 
showing that distant stars and galaxies are receding from Earth in every 
direction. Moreover, the velocities of recession increase in proportion with 
distance, a discovery that has been confirmed by numerous and repeated 
measurements since Hubble's time The implication of these findings is that the 
universe is expanding.

Hubble's hypothesis of an expanding universe leads to certain deductions. One 
is that the universe was more condensed at a previous time. From this deduction 
came the suggestion that all the currently observed matter and energy in the 
universe were initially condensed in a very small and infinitely hot mass. A 
huge explosion, known as the Big Bang, then sent matter and energy expanding in 
all directions.  This Big Bang hypothesis led to more testable deductions. One 
such deduction was that the temperature in deep space today should be several 
degrees above absolute zero. Observations showed this deduction to be correct. 
In fact, the Cosmic Microwave Background Explorer (COBE) satellite launched in 
1991 confirmed that the background radiation field has exactly the spectrum 
predicted by a Big Bang origin for the universe.  As the universe expanded, 
according to current scientific understanding, matter collected into clouds 
that began to condense and rotate, forming the forerunners of galaxies. Within 
galaxies, including our own Milky Way galaxy, changes in pressure caused gas 
and dust to form distinct clouds. In some of these clouds, where there was 
sufficient mass and the right forces, gravitational attraction caused the cloud 
to collapse. If the mass of material in the cloud was sufficiently compressed, 
nuclear reactions began and a star was born.

Some proportion of stars, including our sun, formed in the middle of a 
flattened spinning disk of material. In the case of our sun, the gas and dust 
within this disk collided and aggregated into small grains, and the grains 
formed into larger bodies called planetesimals ("very small planets"), some of 
which reached diameters of several hundred kilometers. In successive stages 
these planetesimals coalesced into the nine planets and their numerous 
satellites. The rocky planets, including Earth, were near the sun, and the 
gaseous planets were in more distant orbits.  The ages of the universe, our 
galaxy, the solar system, and Earth can be estimated using modern scientific 
methods. The age of the universe can be derived from the observed relationship 
between the velocities of and the distances separating the galaxies. The 
velocities of distant galaxies can be measured very accurately, but the 
measurement of distances is more uncertain. Over the past few decades, 
measurements of the Hubble expansion have led to estimated ages for the 
universe of between 7 billion and 20 billion years, with the most recent and 
best measurements within the range of 10 billion to 15 billion years.

The age of the Milky Way galaxy has been calculated in two ways. One involves 
studying the observed stages of evolution of different-sized stars in globular 
clusters. Globular clusters occur in a faint halo surrounding the center of the 
Galaxy, with each cluster containing from a hundred thousand to a million 
stars. The very low amounts of elements heavier than hydrogen and helium in 
these stars indicate that they must have formed early in the history of the 
Galaxy, before large amounts of heavy elements were created inside the initial 
generations of stars and later distributed into the interstellar medium through 
supernova explosions (the Big Bang itself created primarily hydrogen and helium 
atoms). Estimates of the ages of the stars in globular clusters fall within the 
range of 11 billion to 16 billion years.  A second method for estimating the 
age of our galaxy is based on the present abundances of several long-lived 
radioactive elements in the solar system. Their abundances are set by their 
rates of production and distribution through exploding supernovas. According to 
these calculations, the age of our galaxy is between 9 billion and 16 billion 
years. Thus, both ways of estimating the age of the Milky Way galaxy agree with 
each other, and they also are consistent with the independently derived 
estimate for the age of the universe.  Radioactive elements occurring naturally 
in rocks and minerals also provide a means of estimating the age of the solar 
system and Earth. Several of these elements decay with half lives between 700 
million and more than 100 billion years (the half life of an element is the 
time it takes for half of the element to decay radioactively into another 
element). Using these time-keepers, it is calculated that meteorites, which are 
fragments of asteroids, formed between 4.53 billion and 4.58 billion years ago 
(asteroids are small "planetoids" that revolve around the sun and are remnants 
of the solar nebula that gave rise to the sun and planets). The same 
radioactive time-keepers applied to the three oldest lunar samples returned to 
Earth by the Apollo astronauts yield ages between 4.4 billion and 4.5 billion 
years, providing minimum estimates for the time since the formation of the 
moon.  The oldest known rocks on Earth occur in northwestern Canada (3.96 
billion years), but well-studied rocks nearly as old are also found in other 
parts of the world. In Western Australia, zircon crystals encased within 
younger rocks have ages as old as 4.3 billion years, making these tiny crystals 
the oldest materials so far found on Earth.  The best estimates of Earth's age 
are obtained by calculating the time required for development of the observed 
lead isotopes in Earth's oldest lead ores. These estimates yield 4.54 billion 
years as the age of Earth and of meteorites, and hence of the solar system.

The origins of life cannot be dated as precisely, but there is evidence that 
bacteria-like organisms lived on Earth 3.5 billion years ago, and they may have 
existed even earlier, when the first solid crust formed, almost 4 billion years 
ago. These early organisms must have been simpler than the organisms living 
today. Furthermore, before the earliest organisms there must have been 
structures that one would not call "alive" but that are now components of 
living things. Today, all living organisms store and transmit hereditary 
information using two kinds of molecules: DNA and RNA. Each of these molecules 
is in turn composed of four kinds of subunits known as nucleotides. The 
sequences of nucleotides in particular lengths of DNA or RNA, known as genes, 
direct the construction of molecules known as proteins, which in turn catalyze 
biochemical reactions, provide structural components for organisms, and perform 
many of the other functions on which life depends. Proteins consist of chains 
of subunits known as amino acids. The sequence of nucleotides in DNA and RNA 
therefore determines the sequence of amino acids in proteins; this is a central 
mechanism in all of biology.  Experiments conducted under conditions intended 
to resemble those present on primitive Earth have resulted in the production of 
some of the chemical components of proteins, DNA, and RNA. Some of these 
molecules also have been detected in meteorites from outer space and in 
interstellar space by astronomers using radiotelescopes. Scientists have 
concluded that the "building blocks of life" could have been available early in 
Earth's history.

An important new research avenue has opened with the discovery that certain 
molecules made of RNA, called ribozymes, can act as catalysts in modern cells. 
It previously had been thought that only proteins could serve as the catalysts 
required to carry out specific biochemical functions. Thus, in the early 
prebiotic world, RNA molecules could have been "autocatalytic"--that is, they 
could have replicated themselves well before there were any protein catalysts 
(called enzymes). Laboratory experiments demonstrate that replicating 
autocatalytic RNA molecules undergo spontaneous changes and that the variants 
of RNA molecules with the greatest autocatalytic activity come to prevail in 
their environments. Some scientists favor the hypothesis that there was an 
early "RNA world," and they are testing models that lead from RNA to the 
synthesis of simple DNA and protein molecules. These assemblages of molecules 
eventually could have become packaged within membranes, thus making up 
"protocells"--early versions of very simple cells.  For those who are studying 
the origin of life, the question is no longer whether life could have 
originated by chemical processes involving nonbiological components. The 
question instead has become which of many pathways might have been followed to 
produce the first cells.  Will we ever be able to identify the path of chemical 
evolution that succeeded in initiating life on Earth? Scientists are designing 
experiments and speculating about how early Earth could have provided a 
hospitable site for the segregation of molecules in units that might have been 
the first living systems. The recent speculation includes the possibility that 
the first living cells might have arisen on Mars, seeding Earth via the many 
meteorites that are known to travel from Mars to our planet.  Of course, even 
if a living cell were to be made in the laboratory, it would not prove that 
nature followed the same pathway billions of years ago. But it is the job of 
science to provide plausible natural explanations for natural phenomena. The 
study of the origin of life is a very active research area in which important 
progress is being made, although the consensus among scientists is that none of 
the current hypotheses has thus far been confirmed. The history of science 
shows that seemingly intractable problems like this one may become amenable to 
solution later, as a result of advances in theory, instrumentation, or the 
discovery of new facts.  Creationist Views of the Origin of the Universe, 
Earth, and Life Many religious persons, including many scientists, hold that 
God created the universe and the various processes driving physical and 
biological evolution and that these processes then resulted in the creation of 
galaxies, our solar system, and life on Earth. This belief, which sometimes is 
termed "theistic evolution," is not in disagreement with scientific 
explanations of evolution. Indeed, it reflects the remarkable and inspiring 
character of the physical universe revealed by cosmology, paleontology, 
molecular biology, and many other scientific disciplines.  The advocates of 
"creation science" hold a variety of viewpoints. Some claim that Earth and the 
universe are relatively young, perhaps only 6,000 to 10,000 years old. These 
individuals often believe that the present physical form of Earth can be 
explained by "catastrophism," including a worldwide flood, and that all living 
things (including humans) were created miraculously, essentially in the forms 
we now find them.  Other advocates of creation science are willing to accept 
that Earth, the planets, and the stars may have existed for millions of years. 
But they argue that the various types of organisms, and especially humans, 
could only have come about with supernatural intervention, because they show 
"intelligent design."  In this booklet, both these "Young Earth" and "Old 
Earth" views are referred to as "creationism" or "special creation."  There are 
no valid scientific data or calculations to substantiate the belief that Earth 
was created just a few thousand years ago. This document has summarized the 
vast amount of evidence for the great age of the universe, our galaxy, the 
solar system, and Earth from astronomy, astrophysics, nuclear physics, geology, 
geochemistry, and geophysics. Independent scientific methods consistently give 
an age for Earth and the solar system of about 5 billion years, and an age for 
our galaxy and the universe that is two to three times greater. These 
conclusions make the origin of the universe as a whole intelligible, lend 
coherence to many different branches of science, and form the core conclusions 
of a remarkable body of knowledge about the origins and behavior of the 
physical world.  Nor is there any evidence that the entire geological record, 
with its orderly succession of fossils, is the product of a single universal 
flood that occurred a few thousand years ago, lasted a little longer than a 
year, and covered the highest mountains to a depth of several meters. On the 
contrary, intertidal and terrestrial deposits demonstrate that at no recorded 
time in the past has the entire planet been under water. Moreover, a universal 
flood of sufficient magnitude to form the sedimentary rocks seen today, which 
together are many kilometers thick, would require a volume of water far greater 
than has ever existed on and in Earth, at least since the formation of the 
first known solid crust about 4 billion years ago. The belief that Earth's 
sediments, with their fossils, were deposited in an orderly sequence in a 
year's time defies all geological observations and physical principles 
concerning sedimentation rates and possible quantities of suspended solid 
matter. Geologists have constructed a detailed history of sediment deposition 
that links particular bodies of rock in the crust of Earth to particular 
environments and processes. If petroleum geologists could find more oil and gas 
by interpreting the record of sedimentary rocks as having resulted from a 
single flood, they would certainly favor the idea of such a flood, but they do 
not. Instead, these practical workers agree with academic geologists about the 
nature of depositional environments and geological time. Petroleum geologists 
have been pioneers in the recognition of fossil deposits that were formed over 
millions of years in such environments as meandering rivers, deltas, sandy 
barrier beaches, and coral reefs.  The example of petroleum geology 
demonstrates one of the great strengths of science. By using knowledge of the 
natural world to predict the consequences of our actions, science makes it 
possible to solve problems and create opportunities using technology. The 
detailed knowledge required to sustain our civilization could only have been 
derived through scientific investigation.  The arguments of creationists are 
not driven by evidence that can be observed in the natural world. Special 
creation or supernatural intervention is not subjectable to meaningful tests, 
which require predicting plausible results and then checking these results 
through observation and experimentation. Indeed, claims of "special creation" 
reverse the scientific process. The explanation is seen as unalterable, and 
evidence is sought only to support a particular conclusion by whatever means 
possible.

Evidence Supporting Biological Evolution

A long path leads from the origins of primitive "life," which existed at least 
3.5 billion years ago, to the profusion and diversity of life that exists 
today. This path is best understood as a product of evolution. Contrary to 
popular opinion, neither the term nor the idea of biological evolution began 
with Charles Darwin and his foremost work, On the Origin of Species by Means of 
Natural Selection (1859). Many scholars from the ancient Greek philosophers on 
had inferred that similar species were descended from a common ancestor. The 
word "evolution" first appeared in the English language in 1647 in a 
nonbiological connection, and it became widely used in English for all sorts of 
progressions from simpler beginnings. The term Darwin most often used to refer 
to biological evolution was "descent with modification," which remains a good 
brief definition of the process today.

Darwin proposed that evolution could be explained by the differential survival 
of organisms following their naturally occurring variation--a process he termed 
"natural selection." According to this view, the offspring of organisms differ 
from one another and from their parents in ways that are heritable--that is, 
they can pass on the differences genetically to their own offspring. 
Furthermore, organisms in nature typically produce more offspring than can 
survive and reproduce given the constraints of food, space, and other 
environmental resources. If a particular off spring has traits that give it an 
advantage in a particular environment, that organism will be more likely to 
survive and pass on those traits. As differences accumulate over generations, 
populations of organisms diverge from their ancestors.  Darwin's original 
hypothesis has undergone extensive modification and expansion, but the central 
concepts stand firm. Studies in genetics and molecular biology--fields unknown 
in Darwin's time--have explained the occurrence of the hereditary variations 
that are essential to natural selection. Genetic variations result from 
changes, or mutations, in the nucleotide sequence of DNA, the molecule that 
genes are made from. Such changes in DNA now can be detected and described with 
great precision.  Genetic mutations arise by chance. They may or may not equip 
the organism with better means for surviving in its environment. But if a gene 
variant improves adaptation to the environment (for example, by allowing an 
organism to make better use of an available nutrient, or to escape predators 
more effectively--such as through stronger legs or disguising coloration), the 
organisms carrying that gene are more likely to survive and reproduce than 
those without it. Over time, their descendants will tend to increase, changing 
the average characteristics of the population. Although the genetic variation 
on which natural selection works is based on random or chance elements, natural 
selection itself produces "adaptive" change--the very opposite of chance. 
Scientists also have gained an understanding of the processes by which new 
species originate. A new species is one in which the individuals cannot mate 
and produce viable descendants with individuals of a preexisting species. The 
split of one species into two often starts because a group of individuals 
becomes geographically separated from the rest. This is particularly apparent 
in distant remote islands, such as the Galápagos and the Hawaiian archipelago, 
whose great distance from the Americas and Asia means that arriving colonizers 
will have little or no opportunity to mate with individuals remaining on those 
continents. Mountains, rivers, lakes, and other natural barriers also account 
for geographic separation between populations that once belonged to the same 
species.  Once isolated, geographically separated groups of individuals become 
genetically differentiated as a consequence of mutation and other processes, 
including natural selection. The origin of a species is often a gradual 
process, so that at first the reproductive isolation between separated groups 
of organisms is only partial, but it eventually becomes complete. Scientists 
pay special attention to these intermediate situations, because they help to 
reconstruct the details of the process and to identify particular genes or sets 
of genes that account for the reproductive isolation between species.

A particularly compelling example of speciation involves the 13 species of 
finches studied by Darwin on the Galápagos Islands, now known as Darwin's 
finches. The ancestors of these finches appear to have emigrated from the South 
American mainland to the Galápagos. Today the different species of finches on 
the island have distinct habitats, diets, and behaviors, but the mechanisms 
involved in speciation continue to operate. A research group led by Peter and 
Rosemary Grant of Princeton University has shown that a single year of drought 
on the islands can drive evolutionary changes in the finches. Drought 
diminishes supplies of easily cracked nuts but permits the survival of plants 
that produce larger, tougher nuts. Droughts thus favor birds with strong, wide 
beaks that can break these tougher seeds, producing populations of birds with 
these traits. The Grants have estimated that if droughts occur about once every 
10 years on the islands, a new species of finch might arise in only about 200 
years. The following sections consider several aspects of biological evolution 
in greater detail, looking at paleontology, comparative anatomy, biogeography, 
embryology, and molecular biology for further evidence supporting evolution.

The Fossil Record

Although it was Darwin, above all others, who first marshaled convincing 
evidence for biological evolution, earlier scholars had recognized that 
organisms on Earth had changed systematically over long periods of time. For 
example, in 1799 an engineer named William Smith reported that, in undisrupted 
layers of rock, fossils occurred in a definite sequential order, with more 
modern-appearing ones closer to the top. Because bottom layers of rock 
logically were laid down earlier and thus are older than top layers, the 
sequence of fossils also could be given a chronology from oldest to youngest. 
His findings were confirmed and extended in the 1830s by the paleontologist 
William Lonsdale, who recognized that fossil remains of organisms from lower 
strata were more primitive than the ones above. Today, many thousands of 
ancient rock deposits have been identified that show corresponding successions 
of fossil organisms.

Thus, the general sequence of fossils had already been recognized before Darwin 
conceived of descent with modification. But the paleontologists and geologists 
before Darwin used the sequence of fossils in rocks not as proof of biological 
evolution, but as a basis for working out the original sequence of rock strata 
that had been structurally disturbed by earthquakes and other forces.  In 
Darwin's time, paleontology was still a rudimentary science. Large parts of the 
geological succession of stratified rocks were unknown or inadequately studied.

Darwin, therefore, worried about the rarity of intermediate forms between some 
major groups of organisms.  Today, many of the gaps in the paleontological 
record have been filled by the research of paleontologists. Hundreds of 
thousands of fossil organisms, found in well-dated rock sequences, represent 
successions of forms through time and manifest many evolutionary transitions. 
As mentioned earlier, microbial life of the simplest type was already in 
existence 3.5 billion years ago. The oldest evidence of more complex organisms 
(that is, eucaryotic cells, which are more complex than bacteria) has been 
discovered in fossils sealed in rocks approximately 2 billion years old. 
Multicellular organisms, which are the familiar fungi, plants, and animals, 
have been found only in younger geological strata. The following list presents 
the order in which increasingly complex forms of life appeared:



Life Form Millions of Years Since First Known Appearance (Approximate) 
Microbial (procaryotic cells) 3,500 Complex (eucaryotic cells) 2,000 First 
multicellular animals 670 Shell-bearing animals 540 Vertebrates (simple fishes) 
490 Amphibians 350 Reptiles 310 Mammals 200 Nonhuman primates 60 Earliest apes 
25 Australopithecine ancestors of humans 4 Modern humans 0 .15 (150,000 years)

So many intermediate forms have been discovered between fish and amphibians, 
between amphibians and reptiles, between reptiles and mammals, and along the 
primate lines of descent that it often is difficult to identify categorically 
when the transition occurs from one to another particular species. Actually, 
nearly all fossils can be regarded as intermediates in some sense; they are 
life forms that come between the forms that preceded them and those that 
followed.  The fossil record thus provides consistent evidence of systematic 
change through time--of descent with modification. From this huge body of 
evidence, it can be predicted that no reversals will be found in future 
paleontological studies. That is, amphibians will not appear before fishes, nor 
mammals before reptiles, and no complex life will occur in the geological 
record before the oldest eucaryotic cells. This prediction has been upheld by 
the evidence that has accumulated until now: no reversals have been found.

Common Structures

Inferences about common descent derived from paleontology are reinforced by 
comparative anatomy. For example, the skeletons of humans, mice, and bats are 
strikingly similar, despite the different ways of life of these animals and the 
diversity of environments in which they flourish. The correspondence of these 
animals, bone by bone, can be observed in every part of the body, including the 
limbs; yet a person writes, a mouse runs, and a bat flies with structures built 
of bones that are different in detail but similar in general structure and 
relation to each other. Scientists call such structures homologies and have 
concluded that they are best explained by common descent. Comparative 
anatomists investigate such homologies, not only in bone structure but also in 
other parts of the body, working out relationships from degrees of similarity. 
Their conclusions provide important inferences about the details of 
evolutionary history, inferences that can be tested by comparisons with the 
sequence of ancestral forms in the paleontological record.

The mammalian ear and jaw are instances in which paleontology and comparative 
anatomy combine to show common ancestry through transitional stages. The lower 
jaws of mammals contain only one bone, whereas those of reptiles have several. 
The other bones in the reptile jaw are homologous with bones now found in the 
mammalian ear. Paleontologists have discovered intermediate forms of 
mammal-like reptiles (Therapsida) with a double jaw joint--one composed of the 
bones that persist in mammalian jaws, the other consisting of bones that 
eventually became the hammer and anvil of the mammalian ear.

The Distribution of Species

Biogeography also has contributed evidence for descent from common ancestors. 
The diversity of life is stupendous. Approximately 250,000 species of living 
plants, 100,000 species of fungi, and one million species of animals have been 
described and named, each occupying its own peculiar ecological setting or 
niche; and the census is far from complete. Some species, such as human beings 
and our companion the dog, can live under a wide range of environments. Others 
are amazingly specialized. One species of a fungus (Laboulbenia) grows 
exclusively on the rear portion of the covering wings of a single species of 
beetle (Aphaenops cronei) found only in some caves of southern France. The 
larvae of the fly Drosophila carcinophila can develop only in specialized 
grooves beneath the flaps of the third pair of oral appendages of a land crab 
that is found only on certain Caribbean islands.

How can we make intelligible the colossal diversity of living beings and the 
existence of such extraordinary, seemingly whimsical creatures as the fungus, 
beetle, and fly described above? And why are island groups like the Galápagos 
so often inhabited by forms similar to those on the nearest mainland but 
belonging to different species? Evolutionary theory explains that biological 
diversity results from the descendants of local or migrant predecessors 
becoming adapted to their diverse environments. This explanation can be tested 
by examining present species and local fossils to see whether they have similar 
structures, which would indicate how one is derived from the other. Also, there 
should be evidence that species without an established local ancestry had 
migrated into the locality. Wherever such tests have been carried out, these 
conditions have been confirmed. A good example is provided by the mammalian 
populations of North and South America, where strikingly different native 
organisms evolved in isolation until the emergence of the isthmus of Panama 
approximately 3 million years ago. Thereafter, the armadillo, porcupine, and 
opossum--mammals of South American origin--migrated north, along with many 
other species of plants and animals, while the mountain lion and other North 
American species made their way across the isthmus to the south.

The evidence that Darwin found for the influence of geographical distribution 
on the evolution of organisms has become stronger with advancing knowledge. For 
example, approximately 2,000 species of flies belonging to the genus Drosophila 
are now found throughout the world. About one-quarter of them live only in 
Hawaii. More than a thousand species of snails and other land mollusks also are 
found only in Hawaii. The biological explanation for the multiplicity of 
related species in remote localities is that such great diversity is a 
consequence of their evolution from a few common ancestors that colonized an 
isolated environment. The Hawaiian Islands are far from any mainland or other 
islands, and on the basis of geological evidence they never have been attached 
to other lands. Thus, the few colonizers that reached the Hawaiian Islands 
found many available ecological niches, where they could, over numerous 
generations, undergo evolutionary change and diversification. No mammals other 
than one bat species lived in the Hawaiian Islands when the first human 
settlers arrived; similarly, many other kinds of plants and animals were 
absent.  The Hawaiian Islands are not less hospitable than other parts of the 
world for the absent species. For example, pigs and goats have multiplied in 
the wild in Hawaii, and other domestic animals also thrive there. The 
scientific explanation for the absence of many kinds of organisms, and the 
great multiplication of a few kinds, is that many sorts of organisms never 
reached the islands, because of their geographic isolation. Those that did 
reach the islands diversified over time because of the absence of related 
organisms that would compete for resources.  Similarities During Development 
Embryology, the study of biological development from the time of conception, is 
another source of independent evidence for common descent. Barnacles, for 
instance, are sedentary crustaceans with little apparent similarity to such 
other crustaceans as lobsters, shrimps, or copepods. Yet barnacles pass through 
a free-swimming larval stage in which they look like other crustacean larvae. 
The similarity of larval stages supports the conclusion that all crustaceans 
have homologous parts and a common ancestry. Similarly, a wide variety of 
organisms from fruit flies to worms to mice to humans have very similar 
sequences of genes that are active early in development. These genes influence 
body segmentation or orientation in all these diverse groups. The presence of 
such similar genes doing similar things across such a wide range of organisms 
is best explained by their having been present in a very early common ancestor 
of all of these groups.

New Evidence from Molecular Biology

The unifying principle of common descent that emerges from all the foregoing 
lines of evidence is being reinforced by the discoveries of modern biochemistry 
and molecular biology.  The code used to translate nucleotide sequences into 
amino acid sequences is essentially the same in all organisms. Moreover, 
proteins in all organisms are invariably composed of the same set of 20 amino 
acids. This unity of composition and function is a powerful argument in favor 
of the common descent of the most diverse organisms.

In 1959, scientists at Cambridge University in the United Kingdom determined 
the three-dimensional structures of two proteins that are found in almost every 
multicelled animal: hemoglobin and myoglobin. Hemoglobin is the protein that 
carries oxygen in the blood. Myoglobin receives oxygen from hemoglobin and 
stores it in the tissues until needed. These were the first three-dimensional 
protein structures to be solved, and they yielded some key insights. Myoglobin 
has a single chain of 153 amino acids wrapped around a group of iron and other 
atoms (called "heme") to which oxygen binds. Hemoglobin, in contrast, is made 
of up four chains: two identical chains consisting of 141 amino acids, and two 
other identical chains consisting of 146 amino acids. However, each chain has a 
heme exactly like that of myoglobin, and each of the four chains in the 
hemoglobin molecule is folded exactly like myoglobin. It was immediately 
obvious in 1959 that the two molecules are very closely related.  During the 
next two decades, myoglobin and hemoglobin sequences were determined for dozens 
of mammals, birds, reptiles, amphibians, fish, worms, and molluscs. All of 
these sequences were so obviously related that they could be compared with 
confidence with the three-dimensional structures of two selected 
standards--whale myoglobin and horse hemoglobin. Even more significantly, the 
differences between sequences from different organisms could be used to 
construct a family tree of hemoglobin and myoglobin variation among organisms. 
This tree agreed completely with observations derived from paleontology and 
anatomy about the common descent of the corresponding organisms.

Similar family histories have been obtained from the three-dimensional 
structures and amino acid sequences of other proteins, such as cytochrome c (a 
protein engaged in energy transfer) and the digestive proteins trypsin and 
chymotrypsin. The examination of molecular structure offers a new and extremely 
powerful tool for studying evolutionary relationships. The quantity of 
information is potentially huge--as large as the thousands of different 
proteins contained in living organisms, and limited only by the time and 
resources of molecular biologists.  As the ability to sequence the nucleotides 
making up DNA has improved, it also has become possible to use genes to 
reconstruct the evolutionary history of organisms. Because of mutations, the 
sequence of nucleotides in a gene gradually changes over time. The more closely 
related two organisms are, the less different their DNA will be. Because there 
are tens of thousands of genes in humans and other organisms, DNA contains a 
tremendous amount of information about the evolutionary history of each 
organism.

Genes evolve at different rates because, although mutation is a random event, 
some proteins are much more tolerant of changes in their amino acid sequence 
than are other proteins. For this reason, the genes that encode these more 
tolerant, less constrained proteins evolve faster. The average rate at which a 
particular kind of gene or protein evolves gives rise to the concept of a 
"molecular clock." Molecular clocks run rapidly for less constrained proteins 
and slowly for more constrained proteins, though they all time the same 
evolutionary events.  The figure on this page compares three molecular clocks: 
for cytochrome c proteins, which interact intimately with other macromolecules 
and are quite constrained in their amino acid sequences; for the less rigidly 
constrained hemoglobins, which interact mainly with oxygen and other small 
molecules; and for fibrinopeptides, which are protein fragments that are cut 
from larger proteins (fibrinogens) when blood clots. The clock for 
fibrinopeptides runs rapidly; 1 percent of the amino acids change in a little 
longer than 1 million years. At the other extreme, the molecular clock runs 
slowly for cytochrome c; a 1 percent change in amino acid sequence requires 20 
million years. The hemoglobin clock is intermediate. The concept of a molecular 
clock is useful for two purposes. It determines evolutionary relationships 
among organisms, and it indicates the time in the past when species started to 
diverge from one another. Once the clock for a particular gene or protein has 
been calibrated by reference to some event whose time is known, the actual 
chronological time when all other events occurred can be determined by 
examining the protein or gene tree. An interesting additional line of evidence 
supporting evolution involves sequences of DNA known as "pseudogenes." 
Pseudogenes are remnants of genes that no longer function but continue to be 
carried along in DNA as excess baggage. Pseudogenes also change through time, 
as they are passed on from ancestors to descendants, and they offer an 
especially useful way of reconstructing evolutionary relationships.  With 
functioning genes, one possible explanation for the relative similarity between 
genes from different organisms is that their ways of life are similar--for 
example, the genes from a horse and a zebra could be more similar because of 
their similar habitats and behaviors than the genes from a horse and a tiger. 
But this possible explanation does not work for pseudogenes, since they perform 
no function. Rather, the degree of similarity between pseudogenes must simply 
reflect their evolutionary relatedness. The more remote the last common 
ancestor of two organisms, the more dissimilar their pseudogenes will be.

The evidence for evolution from molecular biology is overwhelming and is 
growing quickly. In some cases, this molecular evidence makes it possible to go 
beyond the paleontological evidence. For example, it has long been postulated 
that whales descended from land mammals that had returned to the sea. From 
anatomical and paleontological evidence, the whales' closest living land 
relatives seemed to be the even-toed hoofed mammals (modern cattle, sheep, 
camels, goats, etc.). Recent comparisons of some milk protein genes 
(beta-casein and kappa-casein) have confirmed this relationship and have 
suggested that the closest land-bound living relative of whales may be the 
hippopotamus. In this case, molecular biology has augmented the fossil record.

Creationism and the Evidence for Evolution

Some creationists cite what they say is an incomplete fossil record as evidence 
for the failure of evolutionary theory. The fossil record was incomplete in 
Darwin's time, but many of the important gaps that existed then have been 
filled by subsequent paleontological research. Perhaps the most persuasive 
fossil evidence for evolution is the consistency of the sequence of fossils 
from early to recent. Nowhere on

Earth do we find, for example, mammals in Devonian (the age of fishes) strata, 
or human fossils coexisting with dinosaur remains. Undisturbed strata with 
simple unicellular organisms predate those with multicellular organisms, and 
invertebrates precede vertebrates; nowhere has this sequence been found 
inverted. Fossils from adjacent strata are more similar than fossils from 
temporally distant strata. The most reasonable scientific conclusion that can 
be drawn from the fossil record is that descent with modification has taken 
place as stated in evolutionary theory.  Special creationists argue that "no 
one has seen evolution occur." This misses the point about how science tests 
hypotheses. We don't see Earth going around the sun or the atoms that make up 
matter. We "see" their consequences. Scientists infer that atoms exist and 
Earth revolves because they have tested predictions derived from these concepts 
by extensive observation and experimentation.  Furthermore, on a minor scale, 
we "experience" evolution occurring every day. The annual changes in influenza 
viruses and the emergence of antibiotic-resistant bacteria are both products of 
evolutionary forces. Indeed, the rapidity with which organisms with short 
generation times, such as bacteria and viruses, can evolve under the influence 
of their environments is of great medical significance. Many laboratory 
experiments have shown that, because of mutation and natural selection, such 
microorganisms can change in specific ways from those of immediately preceding 
generations.  On a larger scale, the evolution of mosquitoes resistant to 
insecticides is another example of the tenacity and adaptability of organisms 
under environmental stress. Similarly, malaria parasites have become resistant 
to the drugs that were used extensively to combat them for many years. As a 
consequence, malaria is on the increase, with more than 300 million clinical 
cases of malaria occurring every year.

Molecular evolutionary data counter a recent proposition called "intelligent 
design theory." Proponents of this idea argue that structural complexity is 
proof of the direct hand of God in specially creating organisms as they are 
today. These arguments echo those of the 18th century cleric William Paley who 
held that the vertebrate eye, because of its intricate organization, had been 
specially designed in its present form by an omnipotent Creator. Modern-day 
intelligent design proponents argue that molecular structures such as DNA, or 
molecular processes such as the many steps that blood goes through when it 
clots, are so irreducibly complex that they can function only if all the 
components are operative at once. Thus, proponents of intelligent design say 
that these structures and processes could not have evolved in the stepwise mode 
characteristic of natural selection.  However, structures and processes that 
are claimed to be "irreducibly" complex typically are not on closer inspection. 
For example, it is incorrect to assume that a complex structure or biochemical 
process can function only if all its components are present and functioning as 
we see them today. Complex biochemical systems can be built up from simpler 
systems through natural selection. Thus, the "history" of a protein can be 
traced through simpler organisms. Jawless fish have a simpler hemoglobin than 
do jawed fish, which in turn have a simpler hemoglobin than mammals.

The evolution of complex molecular systems can occur in several ways. Natural 
selection can bring together parts of a system for one function at one time and 
then, at a later time, recombine those parts with other systems of components 
to produce a system that has a different function. Genes can be duplicated, 
altered, and then amplified through natural selection. The complex biochemical 
cascade resulting in blood clotting has been explained in this fashion. 
Similarly, evolutionary mechanisms are capable of explaining the origin of 
highly complex anatomical structures. For example, eyes may have evolved 
independently many times during the history of life on Earth. The steps proceed 
from a simple eye spot made up of light-sensitive retinula cells (as is now 
found in the flatworm), to formation of individual photosensitive units 
(ommatidia) in insects with light focusing lenses, to the eventual formation of 
an eye with a single lens focusing images onto a retina. In humans and other 
vertebrates, the retina consists not only of photoreceptor cells but also of 
several types of neurons that begin to analyze the visual image. Through such 
gradual steps, very different kinds of eyes have evolved, from simple 
light-sensing organs to highly complex systems for vision.

Human Evolution

Studies in evolutionary biology have led to the conclusion that human beings 
arose from ancestral primates. This association was hotly debated among 
scientists in Darwin's day. But today there is no significant scientific doubt 
about the close evolutionary relationships among all primates, including 
humans.  Many of the most important advances in paleontology over the past 
century relate to the evolutionary history of humans. Not one but many 
connecting links--intermediate between and along various branches of the human 
family tree--have been found as fossils. These linking fossils occur in 
geological deposits of intermediate age. They document the time and rate at 
which primate and human evolution occurred.

Scientists have unearthed thousands of fossil specimens representing members of 
the human family. A great number of these cannot be assigned to the modern 
human species, Homo sapiens. Most of these specimens have been well dated, 
often by means of radiometric techniques. They reveal a well-branched tree, 
parts of which trace a general evolutionary sequence leading from ape-like 
forms to modern humans. Paleontologists have discovered numerous species of 
extinct apes in rock strata that are older than four million years, but never a 
member of the human family at that great age. Australopithecus, whose earliest 
known fossils are about four million years old, is a genus with some features 
closer to apes and some closer to modern humans. In brain size, 
Australopithecus was barely more advanced than apes. A number of features, 
including long arms, short legs, intermediate toe structure, and features of 
the upper limb, indicate that the members of this species spent part of the 
time in trees. But they also walked upright on the ground, like humans. Bipedal 
tracks of Australopithecus have been discovered, beautifully preserved with 
those of other extinct animals, in hardened volcanic ash. Most of our 
Australopithecus ancestors died out close to two-and-a-half million years ago, 
while other Australopithecus species, which were on side branches of the human 
tree, survived alongside more advanced hominids for another million years. 
Distinctive bones of the oldest species of the human genus, Homo, date back to 
rock strata about 2.4 million years old. Physical anthropologists agree that 
Homo evolved from one of the species of Australopithecus. By two million years 
ago, early members of Homo had an average brain size one-and-a-half times 
larger than that of Australopithecus, though still substantially smaller than 
that of modern humans. The shapes of the pelvic and leg bones suggest that 
these early Homo were not part-time climbers like Australopithecus but walked 
and ran on long legs, as modern humans do. Just as Australopithecus showed a 
complex of ape-like, human-like, and intermediate features, so was early Homo 
intermediate between Australopithecus and modern humans in some features, and 
close to modern humans in other respects. The earliest stone tools are of 
virtually the same age as the earliest fossils of Homo. Early Homo, with its 
larger brain than Australopithecus, was a maker of stone tools.  The fossil 
record for the interval between 2.4 million years ago and the present includes 
the skeletal remains of several species assigned to the genus Homo. The more 
recent species had larger brains than the older ones. This fossil record is 
complete enough to show that the human genus first spread from its place of 
origin in Africa to Europe and Asia a little less than two million years ago. 
Distinctive types of stone tools are associated with various populations. More 
recent species with larger brains generally used more sophisticated tools than 
more ancient species. Molecular biology also has provided strong evidence of 
the close relationship between humans and apes. Analysis of many proteins and 
genes has shown that humans are genetically similar to chimpanzees and gorillas 
and less similar to orangutans and other primates.  DNA has even been extracted 
from a well-preserved skeleton of the extinct human creature known as 
Neanderthal, a member of the genus Homo and often considered either as a 
subspecies of Homo sapiens or as a separate species. Application of the 
molecular clock, which makes use of known rates of genetic mutation, suggests 
that Neanderthal's lineage diverged from that of modern Homo sapiens less than 
half a million years ago, which is entirely compatible with evidence from the 
fossil record.  Based on molecular and genetic data, evolutionists favor the 
hypothesis that modern Homo sapiens, individuals very much like us, evolved 
from more archaic humans about 100,000 to 150,000 years ago. They also believe 
that this transition occurred in Africa, with modern humans then dispersing to 
Asia, Europe, and eventually Australasia and the Americas.  Discoveries of 
hominid remains during the past three decades in East and South Africa, the 
Middle East, and elsewhere have combined with advances in molecular biology to 
initiate a new discipline--molecular paleoanthropology. This field of inquiry 
is providing an ever-growing inventory of evidence for a genetic affinity 
between human beings and the African apes.  Opinion polls show that many people 
believe that divine intervention actively guided the evolution of human beings. 
Science cannot comment on the role that supernatural forces might play in human 
affairs. But scientific investigations have concluded that the same forces 
responsible for the evolution of all other life forms on Earth can account for 
the evolution of human beings.

Conclusion

Science is not the only way of acquiring knowledge about ourselves and the 
world around us. Humans gain understanding in many other ways, such as through 
literature, the arts, philosophical reflection, and religious experience. 
Scientific knowledge may enrich aesthetic and moral perceptions, but these 
subjects extend beyond science's realm, which is to obtain a better 
understanding of the natural world.  The claim that equity demands balanced 
treatment of evolutionary theory and special creation in science classrooms 
reflects a misunderstanding of what science is and how it is conducted. 
Scientific investigators seek to understand natural phenomena by observation 
and experimentation. Scientific interpretations of facts and the explanations 
that account for them therefore must be testable by observation and 
experimentation.  Creationism, intelligent design, and other claims of 
supernatural intervention in the origin of life or of species are not science 
because they are not testable by the methods of science. These claims 
subordinate observed data to statements based on authority, revelation, or 
religious belief. Documentation offered in support of these claims is typically 
limited to the special publications of their advocates. These publications do 
not offer hypotheses subject to change in light of new data, new 
interpretations, or demonstration of error. This contrasts with science, where 
any hypothesis or theory always remains subject to the possibility of rejection 
or modification in the light of new knowledge.  No body of beliefs that has its 
origin in doctrinal material rather than scientific observation, 
interpretation, and experimentation should be admissible as science in any 
science course. Incorporating the teaching of such doctrines into a science 
curriculum compromises the objectives of public education. Science has been 
greatly successful at explaining natural processes, and this has led not only 
to increased understanding of the universe but also to major improvements in 
technology and public health and welfare. The growing role that science plays 
in modern life requires that science, and not religion, be taught in science 
classes.

Appendix Frequently Asked Questions*

What is evolution?

Evolution in the broadest sense explains that what we see today is different 
from what existed in the past. Galaxies, stars, the solar system, and Earth 
have changed through time, and so has life on Earth. Biological evolution 
concerns changes in living things during the history of life on Earth. It 
explains that living things share common ancestors. Over time, biological 
processes such as natural selection give rise to new species. Darwin called 
this process "descent with modification," which remains a good definition of 
biological evolution today.

Isn't evolution just an inference?

No one saw the evolution of one-toed horses from three-toed horses, but that 
does not mean that we cannot be confident that horses evolved. Science is 
practiced in many ways besides direct observation and experimentation. Much 
scientific discovery is done through indirect experimentation and observation 
in which inferences are made, and hypotheses generated from those inferences 
are tested.  For instance, particle physicists cannot directly observe 
subatomic particles because the particles are too small. They make inferences 
about the weight, speed, and other properties of the particles based on other 
observations. A logical hypothesis might be something like this: If the weight 
of this particle is Y, when I bombard it, X will happen. If X does not happen, 
then the hypothesis is disproved. Thus, we can learn about the natural world 
even if we cannot directly observe a phenomenon--and that is true about the 
past, too.  In historical sciences like astronomy, geology, evolutionary 
biology, and archaeology, logical inferences are made and then tested against 
data. Sometimes the test cannot be made until new data are available, but a 
great deal has been done to help us understand the past. For example, 
scorpionflies (Mecoptera) and true flies (Diptera) have enough similarities 
that entomologists consider them to be closely related. Scorpionflies have four 
wings of about the same size, and true flies have a large front pair of wings 
but the back pair is replaced by small club-shaped structures. If two-winged 
flies evolved from scorpionfly-like ancestors, as comparative anatomy suggests, 
then an intermediate true fly with four wings should have existed--and in 1976 
fossils of such a fly were discovered. Furthermore, geneticists have found that 
the number of wings in flies can be changed through mutations in a single gene. 
Something that happened in the past is thus not "off limits" for scientific 
study. Hypotheses can be made about such phenomena, and these hypotheses can be 
tested and can lead to solid conclusions. Furthermore, many key mechanisms of 
evolution occur over relatively short periods and can be observed 
directly--such as the evolution of bacteria resistant to antibiotics. Evolution 
is a well-supported theory drawn from a variety of sources of data, including 
observations about the fossil record, genetic information, the distribution of 
plants and animals, and the similarities across species of anatomy and 
development. Scientists have inferred that descent with modification offers the 
best scientific explanation for these observations.

Is evolution a fact or a theory?

The theory of evolution explains how life on Earth has changed. In scientific 
terms, "theory" does not mean "guess" or "hunch" as it does in everyday usage. 
Scientific theories are explanations of natural phenomena built up logically 
from testable observations and hypotheses. Biological evolution is the best 
scientific explanation we have for the enormous range of observations about the 
living world.  Scientists most often use the word "fact" to describe an 
observation. But scientists can also use fact to mean something that has been 
tested or observed so many times that there is no longer a compelling reason to 
keep testing or looking for examples. The occurrence of evolution in this sense 
is a fact. Scientists no longer question whether descent with modification 
occurred because the evidence supporting the idea is so strong.

Don't many famous scientists reject evolution?

No. The scientific consensus around evolution is overwhelming. Those opposed to 
the teaching of evolution sometimes use quotations from prominent scientists 
out of context to claim that scientists do not support evolution. However, 
examination of the quotations reveals that the scientists are actually 
disputing some aspect of how evolution occurs, not whether evolution occurred. 
For example, the biologist Stephen Jay Gould once wrote that "the extreme 
rarity of transitional forms in the fossil record persists as the trade secret 
of paleontology." But Gould, an accomplished paleontologist and eloquent 
educator about evolution, was arguing about how evolution takes place. He was 
discussing whether the rate of change of species is slow and gradual or whether 
it takes place in bursts after long periods when little change occurs--an idea 
known as punctuated equilibrium. As Gould writes in response, "This quotation, 
although accurate as a partial citation, is dishonest in leaving out the 
following explanatory material showing my true purpose--to discuss rates of 
evolutionary change, not to deny the fact of evolution itself." Gould defines 
punctuated equilibrium as follows:  Punctuated equilibrium is neither a 
creationist idea nor even a non-Darwinian evolutionary theory about sudden 
change that produces a new species all at once in a single generation. 
Punctuated equilibrium accepts the conventional idea that new species form over 
hundreds or thousands of generations and through an extensive series of 
intermediate stages. But geological time is so long that even a few thousand 
years may appear as a mere "moment" relative to the several million years of 
existence for most species. Thus, rates of evolution vary enormously and new 
species may appear to arise "suddenly" in geological time, even though the time 
involved would seem long, and the change very slow, when compared to a human 
lifetime.

If humans evolved from apes, why are there still apes?

Humans did not evolve from modern apes, but humans and modern apes shared a 
common ancestor, a species that no longer exists. Because we share a recent 
common ancestor with chimpanzees and gorillas, we have many anatomical, 
genetic, biochemical, and even behavioral similarities with these African great 
apes. We are less similar to the Asian apes--orangutans and gibbons--and even 
less similar to monkeys, because we share common ancestors with these groups in 
the more distant past. Evolution is a branching or splitting process in which 
populations split off from one another and gradually become different. As the 
two groups become isolated from each other, they stop sharing genes, and 
eventually genetic differences increase until members of the groups can no 
longer interbreed. At this point, they have become separate species. Through 
time, these two species might give rise to new species, and so on through 
millennia.

Why can't we teach creation science in my school?

The courts have ruled that "creation science" is actually a religious view. 
Because public schools must be religiously neutral under the U.S. Constitution, 
the courts have held that it is unconstitutional to present creation science as 
legitimate scholarship.  In particular, in a trial in which supporters of 
creation science testified in support of their view, a district court declared 
that creation science does not meet the tenets of science as scientists use the 
term (McLean v. Arkansas Board of Education). The Supreme Court has held that 
it is illegal to require that creation science be taught when evolution is 
taught (Edwards v. Aguillard). In addition, district courts have decided that 
individual teachers cannot advocate creation science on their own (Peloza v. 
San Juan Capistrano School District and Webster v. New Lennox School District). 
(See Teaching About Evolution and the Nature of Science, Appendix A. National 
Academy of Sciences, Washington, D.C. 1998.)  Teachers' organizations such as 
the National Science Teachers Association, the National Association of Biology 
Teachers, the National Science Education Leadership Association, and many 
others also have rejected the science and pedagogy of creation science and have 
strongly discouraged its presentation in the public schools. In addition, a 
coalition of religious and other organizations has noted in "A Joint Statement 
of Current Law" that "in science class, [schools] may present only genuinely 
scientific critiques of, or evidence for, any explanation of life on Earth, but 
not religious critiques (beliefs unverifiable by scientific methodology)." (See 
Teaching About Evolution and the Nature of Science, Appendices B and C, 
National Academy of Sciences, Washington, D.C., 1998.)  Some argue that 
"fairness" demands the teaching of creationism along with evolution. But a 
science curriculum should cover science, not the religious views of particular 
groups or individuals.

If evolution is taught in schools, shouldn't creationism be given equal time?

Some religious groups deny that microorganisms cause disease, but the science 
curriculum should not therefore be altered to reflect this belief. Most people 
agree that students should be exposed to the best possible scholarship in each 
field. That scholarship is evaluated by professionals and educators in those 
fields. Scientists as well as educators have concluded that evolution--and only 
evolution--should be taught in science classes because it is the only 
scientific explanation for why the universe is the way it is today.  Many 
people say that they want their children to be exposed to creationism in 
school, but there are thousands of different ideas about creation among the 
world's people. Comparative religions might comprise a worthwhile field of 
study, but not one appropriate for a science class. Furthermore, the U.S. 
Constitution states that schools must be religiously neutral, so legally a 
teacher cannot present any particular creationist view as being more "true" 
than others.  *Adapted from Teaching About Evolution and the Nature of Science 
by the National Academy of Sciences (Washington, D.C.: National Academy Press, 
1998).

Recommended Readings

Evolution

Dawkins, Richard 1996 Climbing Mount Improbable, W.W. Norton: New York and 
London. An authoritative and elegant account of the evolutionary explanation of 
the "design" of organisms.  Fortey, Richard 1998 Life: A Natural History of the 
First Four Billion Years of Life on Earth, Alfred P. Knopf: New York. A lively 
account of the history of life on Earth. Gould, Stephen J.  1992 The Panda's 
Thumb, W.W. Norton: New York. Gould's Natural History columns have been 
collected into a series of books including Hen's Teeth and Horse's Toes, An 
Urchin in the Storm, Eight Little Piggies, The Flamingo's Smile, and Bully for 
Brontosaurus. All are good popular introductions to the basic ideas behind 
evolution, and extremely readable.  Horner, John R., and Edwin Dobb 1997 
Dinosaur Lives: Unearthing an Evolutionary Saga, HarperCollins: New York. What 
it's like to uncover fossilized bones, eggs, and more, plus Horner's views on 
dinosaurs.  Howells, W.W.  1997 Getting Here: The Story of Human Evolution, 
Compass Press: Washington, D.C. A very readable survey of human evolution by 
one of the fathers of physical anthropology.  Johanson, Donald C., Lenora 
Johanson, and Blake Edgar 1994 Ancestors: In Search of Human Origins, Villard 
Books: New York. The companion volume to Johanson's NOVA series, "In Search of 
Human Origins."  Mayr, Ernst 1991 One Long Argument: Charles Darwin and the 
Genesis of Modern Evolutionary Thought, Harvard University Press: Cambridge, 
MA. An easily understandable distillation of Charles Darwin's scientific 
contributions.  National Academy of Sciences 1998 Teaching About Evolution and 
the Nature of Science, National Academy Press: Washington, DC. An engaging, 
conversational, and well-structured framework for understanding and teaching 
evolution.  Nesse, Randolph, and George C. Williams 1996 Why We Get Sick: The 
New Science of Darwinian Medicine, Vintage Books: New York. The principle of 
natural selection as applied to modern-day health and disease. Helps to 
illustrate evolution as an ongoing phenomenon. Tattersall, Ian 1998 Becoming 
Human, Harcourt Brace: New York. A description of the current state of 
understanding about the differences between Neanderthals and Homo sapiens. 
Weiner, Jonathan 1994 The Beak of the Finch: A Story of Evolution in Our Time, 
Alfred P. Knopf: New York. Discussion of basic evolutionary principles and how 
they are illustrated by ongoing evolution on the Galápagos Islands.  Whitfield, 
Philip 1993 From So Simple a Beginning, Macmillan: New York. A large-format, 
beautifully illustrated book explaining evolution from genetic, fossil, and 
geological perspectives. A good general introduction for nonspecialists. 
Zimmer, Carl 1999 At the Water's Edge: Macroevolution and the Transformation of 
Life, Free Press: New York. Some creatures moved from water to land (the 
evolution land vertebrates) and others from land to water (the evolution of 
whales from land animals). Zimmer clearly explains these two events in the 
history of vertebrates and what might have brought them about.

Evolution: Books for Children and Young Adults

Cole, Joanna, and Juan Carlos Barberis 1987 The Human Body: How We Evolved, 
Illustrated by Walter Gaffney-Kessell, William Morrow and Company: New York. 
This book traces the evolution of humans, from early prehistoric ancestors to 
modern tool-users. Grades 4-7.  Lauber, Patricia, and Douglas Henderson 1994 
How Dinosaurs Came to Be, Simon and Schuster: New York. A description of the 
ancestors to the dinosaurs. Grades 4-7. Matsen, Brad, and Ray Troll 1994 Planet 
Ocean: A Story of Life, the Sea, and Dancing to the Fossil Record, 10 Speed 
Press: Berkeley, CA. Whimsically illustrated tour of history for older kids and 
adults. Grades junior high to high school.  McNulty, Faith 1999 How Whales 
Walked into the Sea, Illustrated by Ted Lewin, Scholastic Trade: New York. This 
wonderfully illustrated book describes the evolution of whales from land 
mammals. Grades K-3.  Stein, Sara 1986 The Evolution Book, Workman Publishing 
Co., Inc.: New York. A hands-on, project-oriented survey of evolution and its 
mechanisms. Grades 4-8.  Troll, Ray, and Brad Matsen 1996 Raptors Fossils Fins 
and Fangs: A Prehistoric Creature Feature, Tricycle Press: Berkeley, CA. A 
light-hearted trip through time ("Good Gracious -- Cretaceous!") with similes 
kids will like ("Pterosaurs -- some as big as jet fighters."). Grades 4-6.

Origin of the Universe and Earth

Dalrymple, G. Brent 1991 The Age of the Earth, Stanford University Press: 
Stanford, CA. A comprehensive discussion of the evidence for the ages of the 
Earth, moon, meteorites, solar system, Galaxy, and universe. Longair, Malcolm 
S.  1996 Our Evolving Universe, Cambridge University Press: New York. A brief 
discussion of the origin and evolution of the universe. Silk, Joseph 1994 A 
Short History of the Universe, Scientific American Library: New York. Popular 
treatment of the evolution of the universe. Weinberg, Steven 1993 The First 
Three Minutes: A Modern View of the Origin of the Universe, Basic Books: New 
York. An explanation of what happened during the Big Bang.

Evolution and Creationism Controversy

Godfrey, Laurie, ed.  1983 Scientists Confront Creationism, W.W. Norton: New 
York. A collection of articles by scientists analyzing and refuting arguments 
of creation science.  Kitcher, Philip 1982 Abusing Science: The Case Against 
Creationism, MIT Press: Cambridge, MA. A philosophical as well as scientific 
analysis of creation science.  Matsumura, Molleen 1995 Voices for Evolution, 
National Center for Science Education, Inc: Berkeley, CA. A collection of 
statements supporting the teaching of evolution from many different types of 
organizations: scientific, civil liberties, religious, and educational. 
Numbers, Ronald 1992 The Creationists: The Evolution of Scientific Creationism, 
University of California Press: Berkeley, CA. A thorough history of the 
American creationist movement.  Pennock, Robert T.  1999 Tower of Babel: The 
Evidence Against the New Creationism, MIT Press: Cambridge, MA. A philosopher 
of science analyzes the newer "intelligent design" theory and "theistic 
science" creationism.  Skehan, James W.  1986 Modern Science and the Book of 
Genesis, National Science Teachers Association: Washington, DC. Written by a 
geologist (former Director of the Weston Seismological Observatory) and bible 
scholar, trained as a Jesuit priest.  Strahler, Arthur 1987 Science and Earth 
History: The Evolution/Creation Controversy, Prometheus Press: Buffalo, NY. A 
comprehensive analysis of creationist scientific claims.  Toumey, Christopher 
P.  1994 God's Own Scientists: Creationists in a Secular World, Rutgers 
University Press: New Brunswick, NJ. An anthropologist's view of creationism as 
a belief system upholding the moral authority of both science and religion.

Reviewers

This report has been reviewed by individuals chosen for their diverse 
perspectives and technical expertise, in accordance with procedures approved by 
the National Research Council's Report Review Committee. The purpose of the 
independent review is to provide candid and critical comments that will assist 
the authors and the National Academy of Sciences in making their published 
report as sound as possible and to ensure that the report meets institutional 
standards for objectivity, evidence, and responsiveness to the study charge. 
The contents of the review comments and draft manuscript remain confidential to 
protect the integrity of the deliberative process. We wish to thank the 
following individuals for their participation in the review of this report:

John Baldeschwieler J. Stanley Johnson Professor and Professor of Chemistry 
Division of Chemistry and Chemical Engineering California Institute of 
Technology Pasadena, California John E. Dowling Maria Moors Cabot Professor of 
Natural Science The Biological Laboratories Harvard University Cambridge, 
Massachusetts Marye Anne Fox Chancellor North Carolina State University 
Raleigh, North Carolina Wilford Gardner Dean Emeritus College of Natural 
Resources University of California Berkeley, California Timothy Goldsmith 
Professor of Biology Department of Molecular, Cellular, and Developmental 
Biology Yale University New Haven, Connecticut Avram Goldstein Professor of 
Pharmacology, Emeritus Stanford University Stanford, California Ursula 
Goodenough Professor Department of Biology Washington University Saint Louis, 
Missouri Robert Griffiths Professor of Physics Carnegie Mellon University 
Pittsburgh, Pennsylvania Norman Horowitz Professor Emeritus Division of Biology 
California Institute of Technology Pasadena, California Susan Kidwell Professor 
Department of Geophysical Sciences University of Chicago Chicago, Illinois 
David Pilbeam Henry Ford II Professor of Social Sciences Peabody Museum Harvard 
University Cambridge, Massachusetts Luis Sequeira J.C. Walker Professor 
Emeritus Department of Plant Pathology University of Wisconsin Madison, 
Wisconsin Phillip Tobias Professor Emeritus Department of Anatomical Sciences 
University of Witwatersrand Medical School Johannesburg, Republic of South 
Africa And other anonymous reviews.
 	While the individuals listed above have provided many constructive 
comments and suggestions, responsibility for the final content of this report 
rests solely with the authoring committee and the National Academy of Sciences.

Council of the National Academy of Sciences

Bruce Alberts President National Academy of Sciences Washington, DC Mary Ellen 
Avery Professor of Pediatrics Harvard Medical School Boston, Massachusetts 
Lewis M. Branscomb Professor Emeritus John F. Kennedy School of Government 
Harvard University Cambridge, Massachusetts Ralph J. Cicerone Chancellor 
University of California Irvine, California Marye Anne Fox Chancellor North 
Carolina State University Raleigh, North Carolina Ralph E. Gomory President 
Alfred P. Sloan Foundation New York, New York Ronald L. Graham Chief Scientist 
AT&T Labs Florham Park, New Jersey Jack Halpern Louis Block Distinguished 
Professor Emeritus Department of Chemistry University of Chicago Chicago, 
Illinois David M. Kipnis Distinguished University Professor Washington 
University School of Medicine Saint Louis, Missouri Daniel E. Koshland Jr. 
Professor in the Graduate School Department of Molecular and Cellular Biology 
University of California Berkeley, California Peter Raven Director Missouri 
Botanical Garden Saint Louis, Missouri Sherwood F. Rowland Donald Bren Research 
Professor of Chemistry and Earth System Science Department of Chemistry 
University of California Irvine, California William J. Rutter Chairman Chiron 
Corporation Emeryville, California Luis Sequeira J.C. Walker Professor Emeritus 
Department of Plant Pathology University of Wisconsin Madison, Wisconsin Carla 
J. Shatz Investigator Howard Hughes Medical Institute Professor Department of 
Molecular and Cellular Biology University of California Berkeley, California 
Jean D. Wilson Charles Cameron Sprague Distinguished Chair in Biomedical 
Science University of Texas Southwestern Medical Center Dallas, Texas Robert H. 
Wurtz Chief Laboratory of Sensorimotor Research National Institutes of Health 
Bethesda, Maryland

Credits

Front cover and title page: Hurricane Andrew over the Gulf of Mexico, August 
1992, NOAA.  Back cover: Map of the world by Isidore of Seville [A.D. 560-636], 
redrawn and published in 1898 in Mappaemundi: Die altesten Weltkarten, a 
six-volume work compiled by Konrad Miller. Library of Congress, Geography and 
Map Division.  page iv: Entrance to National Academy of Sciences building, 
Carol M. Highsmith, photographer.  page v: Marble seal of the National Academy 
of Sciences, David Patterson, photographer.  page vi: Marty Stouffer, 1991/PNI. 
page x: Young stars, Hubble Space Telescope, NASA.  page 3: background: Ken 
Graham/PNI; insets: photograph of Edwin Hubble: National Academy of Sciences; 
Hubble Deep Field, Hubble Space Telescope, NASA.  page 4: Young stellar disks 
in infrared, Hubble Space Telescope, NASA.  page 6: left: DNA, Dr. A. Lesk, 
Laboratory of Molecular Biology/Science Photo Library; right: RNA, Ken 
Eward/Science Source, Photo Researchers, Inc.  page 9: Charles Darwin, National 
Library of Medicine, National Institutes of Health.  page 9: Galápagos Islands, 
Archive Photos, 1994/PNI page 11: Darwin's finches. Drawing by K. Thalia Grant. 
From The Beak of the Finch by Jonathan Weiner. 1994 by Jonathan Weiner. 
Reprinted by permission of Alfred A. Knopf, Inc. page 12: Paria River, Utah. 
Grand Staircase/Escalante National Monument, Tom Till.  pages 12-13: 
Illustration of layers of sedimentary rock, Joyce Pendola, courtesy Natural 
History.  page 14: Illustration by Leigh Coriale Design and Illustration, 
adapted from Patterns in Evolution: The New Molecular View by Lewin, Scientific 
American Library. Used with permission by W.H. Freeman and Company.  page 16: 
top, Ron Sanford, 1994/PNI; bottom left, Marty Stouffer, 1991/PNI; bottom 
right, Erwin Bauer, Peggy Bauer, 1990/PNI.  page 18: Myoglobin, Irving Geis. 
page 19: Cytochrome c. Illustration by Leigh Coriale Design and Illustration, 
adapted from the Journal of Molecular Evolution, Vol. I, 37, 1971.  pages 
20-21: Drawings of Mammalian land ancestor, Balaenoptera, by N. Haver. Drawings 
of Ambulocetus, Rodhocetus, by N. Haver, Sinauer Associates.  page 22: 
Illustration adapted from The Cambridge Encyclopedia of Life Sciences. 
Reprinted with permission of Cambridge University Press.  page 24: Drawings by 
Darwen Hennings, Wadsworth Publishing Company.  page 36: Detail, Paria River, 
Utah. Grand Staircase National Momument Tom Hill.