[Paleopsych] NS: Did humans evolve in fits and starts?
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Tue Jul 12 18:58:50 UTC 2005
Did humans evolve in fits and starts?
* 17:30 17 June 2005
* Gaia Vince
Humans may have evolved during a few rapid bursts of genetic change,
according to a new study of the human genome, which challenges the
popular theory that evolution is a gradual process.
Researchers studying human chromosome 2 have discovered that the bulk
of its DNA changes occurred in a relatively short period of time and,
since then, only minor alterations have occurred.
This backs a theory called punctuated equilibrium which suggests that
evolution actually occurred as a series of jumps with long static
periods between them.
Evolutionary stages are marked by changes to the DNA sequences on
chromosomes. One of the ways in which chromosomes are altered is
through the duplications of sections of the chromosomes. These DNA
fragments may be duplicated and inserted back into the chromosome,
resulting in two copies of the section.
Evan Eichler, associate professor of genomic sciences at the
University of Washington in Seattle, US, and colleagues looked at
duplicated DNA sequences on a specific section of chromosome 2, to
compare them with ape genomes and Old World monkey genomes. They
expected to find that duplications had occurred gradually over the
last few million years.
Instead, they found that the big duplications had occurred in a short
period of time, relatively speaking, after which only smaller
rearrangements occurred. Eichler found the bulk of the duplications
were present in the genomes of humans, chimpanzees, gorillas and
orang-utans, but were absent in Old World monkeys - such as baboons
An analysis of the degree of chromosomal decay for this section showed
that the major duplications occurred in the narrow window of
evolutionary time between 20 million and 10 million years ago, after
human ancestors had split from Old World monkeys, but before the
divergence of humans and great apes.
It is unclear why [these duplication] events occurred so frequently
during this period of human and great ape evolutionary history. It is
also unclear as to why they suddenly cease, at least in this region of
chromosome 2, Eichler says.
Other regions may show different temporal biases. The important
implication here is that episodic bursts of activity challenge the
concept of gradual clock-like changes during the course of genome
evolution, he says.
Since duplications are important in the birth of new genes and
large-scale chromosomal rearrangements, it may follow that these
processes may have gone through similar episodes of activity followed
Laurence Hurst, professor of evolutionary biology at the University of
Bath in the UK, says the study was very interesting, although he would
like to see this punctuated evolution demonstrated for other
chromosomes, to be more confident that this is a general pattern.
There is growing evidence that evolutionary processes may occur in
bursts. We now know, for example, that 50 million years ago there was
a burst of activity that resulted in lots of new genes being produced,
he told New Scientist.
It is unknown what effect the sudden duplication activity may have had
on chromosome 2. Eichler theorises that it may have resulted in genes
for increased brain size or pathogen evasion. If specific regions of
chromosomes can have very punctuated events, it means our models based
on gradual evolution are probably wrong, he says.
The group will continue looking at the chromosome duplications to try
and correlate them with changes in gene function or expression.
Journal reference: Genome Research (vol 15, p 914)
Hominid inbreeding left humans vulnerable to disease
25 January 2005
Jumping genes help choreograph development
29 May 2004
Eichler Laboratory, University of Washington, Seattle, US
Laurence Hurst, Bath University, UK
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