[Paleopsych] NYT: Plain, Simple, Primitive? Not the Jellyfish
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Thu Jun 23 14:28:57 UTC 2005
Plain, Simple, Primitive? Not the Jellyfish
By CARL ZIMMER
Jellyfish have traditionally been considered simple and primitive.
When you gaze at one in an aquarium tank, it is not hard to see why.
Like its relatives the sea anemone and coral, the jellyfish looks like
a no-frills animal. It has no head, no back or front, no left or right
sides, no legs or fins. It has no heart. Its gut is a blind pouch
rather than a tube, so its mouth must serve as its anus. Instead of a
brain, it has a diffuse net of nerves.
A fish or a shrimp may move quickly in a determined swim; a jellyfish
pulses lazily along.
But new research has made scientists realize that they have
underestimated the jellyfish and its relatives - known collectively as
cnidarians (pronounced nih-DEHR-ee-uns). Beneath their seemingly
simple exterior lies a remarkably sophisticated collection of genes,
including many that give rise to humans' complex anatomy.
These discoveries have inspired new theories about how animals evolved
600 million years ago. The findings have also attracted scientists to
cnidarians as a model to understand the human body.
"The big surprise is that cnidarians are much more complex genetically
than anyone would have guessed," said Dr. Kevin J. Peterson, a
biologist at Dartmouth. "This data have made a lot of people step back
and realize that a lot of what they had thought about cnidarians was
Renaissance scholars considered them plants. Eighteenth-century
naturalists grudgingly granted them admittance into the animal
kingdom, but only just. They classified cnidarians as "zoophytes,"
somewhere between animal and plant.
It was not until the 19th century that naturalists began to understand
how cnidarians developed from fertilized eggs, their body parts
growing from two primordial layers of tissue, the endoderm and
Other animals, including humans and insects, have a third layer of
embryonic tissue, the mesoderm, wedged between the ectoderm and the
endoderm. It gives rise to muscles, the heart and other organs not
found in cnidarians.
Cnidarians also have a simpler overall body plan. Fish, fruit flies
and earthworms all have heads and tails, backs and fronts, and left
and right sides. Scientists refer to animals, including humans, with
this two-sided symmetry as bilaterians. In contrast, cnidarians seem
to lack such symmetry completely. A jellyfish, for example, has the
symmetry of a bicycle wheel, radiating from a central axis.
Evolutionary biologists came to view cnidarians as relics from the
early days of animal evolution. The first animals were probably
spongelike, little more than clumps of cooperative cells. Cnidarians
seemed to represent the next stage, having acquired traits like simple
tissues and nerves.
The fossil records of animals seemed to back up that hypothesis. Many
of the earliest animal fossils resembled jellyfish or other
cnidarians. The oldest known bilaterian fossils were younger,
appearing in the so-called Cambrian explosion, 540 million years ago.
Some researchers suggested that the bilaterian body plan helped set
off the Cambrian explosion. Unlike their ancestors, bilaterians had
heads, allowing them to sense their surroundings and control a
swimming, or crawling, body.
Recent research undermines that theory. The oldest fossils that can be
confidently called cnidarians are just 540 million years old. And Dr.
Peterson and his colleagues have made new estimates of the age of
cnidarians by studying their DNA.
The DNA mutates at a roughly regular rate over millions of years, a
so-called molecular clock. Dr. Peterson estimates that the common
ancestor of living cnidarians lived 543 million years ago. In other
words, cnidarians did not appear tens of millions of years before
Genetic studies have also challenged conventional theories about
cnidarians. Beginning in the 1980's, scientists studying bilaterians
uncovered a set of genes that laid out their body plan. Some of the
genes established the head-to-tail axis, others distinguished the
front from the back.
Humans and insects may look very different, but they share almost
identical versions of this genetic tool kit. And the findings
suggested that the tool kit had already evolved in the common ancestor
Dr. Mark E. Martindale of the University of Hawaii and his colleagues
decided to look for the genes that build jellyfish and other
cnidarians. It took a long time to begin generating results. The team
had to find a species that could not only survive life in the
laboratory but could produce enough embryos for research.
Dr. Martindale's group chose the starlet sea anemone, a species found
along the New England coast. Figuring out how to culture the anemone
and investigate its genes demanded great patience. "It's taken 9 or 10
years," Dr. Martindale said, "but it's turned into a gold mine."
Much to their surprise, the scientists found that some genes switched
on in embryos were nearly identical to the genes that determined the
head-to-tail axis of bilaterians, including humans. More surprisingly,
the genes switched on in the same head-to-tail pattern as in
Further studies showed that cnidarians used other genes from the
bilaterian tool kit. The same genes that patterned the front and back
of the bilaterian embryo, for example, were produced on opposite sides
of the anemone embryo.
The findings have these scientists wondering why cnidarians use such a
complex set of body-building genes when their bodies end up looking so
simple. They have concluded that cnidarians may be more complicated
than they appear, particularly in their nervous systems.
"At the molecular level, they have a lot of body regions that aren't
recognizable," said Dr. John R. Finnerty, a biologist at Boston
University who is collaborating with Dr. Martindale.
Dr. Finnerty expects that the nervous system of cnidarians will turn
out to be particularly complex. "The nervous system of a cnidarian is
described as a nerve net, but that's a textbook simplification," he
He predicts that research will show that this net is divided into
specialized regions like the human brain.
These discoveries have prompted Dr. Peterson to reconsider cnidarians'
place in the history of life. "It's changed my thinking about early
animal evolution," he said.
He now theorizes that cnidarians were not the simple forerunners of
the Cambrian explosion, but very much part of it, their evolution
driven by the rise of animal food webs.
In a paper to be published in the journal Paleobiology, Dr. Peterson
and his colleagues propose that the common ancestor of bilaterians and
cnidarians is a crawling worm. This ancient worm, which Dr. Peterson
estimates lived 600 million years ago, represented a major advance in
animal evolution. Instead of passively filtering tiny bits of food, it
was able to graze on larger prey.
"Once they're able to start grazing through those microbial mats,
there's nothing to stop them," Dr. Peterson said.
Some of these animals eventually began to eat one another. Animals
that could defend themselves were more likely to survive. One way to
avoid being eaten was to become bigger. Another way was to loft eggs
into the water column rather than leave them to be eaten on the sea
floor. Some animals even began to swim as adults in the open water.
Once the water began to fill with animals, cnidarians took on their
current form. The earliest cnidarians anchored themselves to the sea
floor and grew upward, as sea anemones and corals do today. In the
process, they abandoned the bilaterian body plan of their ancestors.
It was also at that time that cnidarians evolved their distinctive
weaponry: a cell containing a miniature harpoon called a nematocyte,
for paralyzing prey with toxins.
As new lineages of animals moved even higher into the water column,
some cnidarians evolved to hunt them as well. Jellyfish are the
product of this final stage of evolution, Dr. Peterson argues.
These new insights into cnidarians have prompted major initiatives to
understand them better. The starlet sea anemone genome is being
sequenced by the Energy Department's Joint Genome Institute, and is
expected to be complete this year.
Scientists expect a number of surprises from the genome project. They
have already discovered that a number of genes once thought to be
unique to vertebrates have turned up in the genomes of cnidarians. It
is now clear that these genes did not, in fact, arise in early
They are much older, having evolved in the common ancestor of
cnidarians and bilaterians 600 million years ago. Later, they
disappeared in branches of bilaterians like insects and nematodes that
have been the focus of extensive genome research.
In some ways, cnidarians are a better model for human biology than
fruit flies. As strange as it may seem, gazing at a jellyfish in an
aquarium is a lot like looking in the mirror.
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