Did Humans Evolve In
Fits And Starts?

By 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 and macaques.
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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 by quiescence."
Growing evidence
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)
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