[Paleopsych] NS: Did humans evolve in fits and starts?

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Did humans evolve in fits and starts?
http://www.newscientist.com/article.ns?id=dn7539&print=true

      * 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
    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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Weblinks

Eichler Laboratory, University of Washington, Seattle, US
http://eichlerlab.gs.washington.edu/
Laurence Hurst, Bath University, UK
http://www.bath.ac.uk/bio-sci/hurst.htm
Genome Research
http://www.genome.org/