[Paleopsych] AP: Genes Show Signs Brain Still Evolving

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Mon Sep 19 01:15:10 UTC 2005

Genes Show Signs Brain Still Evolving

    By LAURAN NEERGAARD, AP Medical WriterThu Sep 8, 5:01 PM ET

    The human brain may still be evolving. So suggests new research that
    tracked changes in two genes thought to help regulate brain growth,
    changes that appeared well after the rise of modern humans 200,000
    years ago.

    That the defining feature of humans -- our large brains -- continued
    to evolve as recently as 5,800 years ago, and may be doing so today,
    promises to surprise the average person, if not biologists.

    "We, including scientists, have considered ourselves as sort of the
    pinnacle of evolution," noted lead researcher Bruce Lahn, a University
    of Chicago geneticist whose studies appear in Friday's edition of the
    journal Science.

    "There's a sense we as humans have kind of peaked," agreed Greg Wray,
    director of Duke University's Center for Evolutionary Genomics. "A
    different way to look at is it's almost impossible for evolution not
    to happen."

    Still, the findings also are controversial, because it's far from
    clear what effect the genetic changes had or if they arose when Lahn's
    "molecular clock" suggests -- at roughly the same time period as some
    cultural achievements, including written language and the development
    of cities.

    Lahn and colleagues examined two genes, named microcephalin and ASPM,
    that are connected to brain size. If those genes don't work, babies
    are born with severely small brains, called microcephaly.

    Using DNA samples from ethnically diverse populations, they identified
    a collection of variations in each gene that occurred with unusually
    high frequency. In fact, the variations were so common they couldn't
    be accidental mutations but instead were probably due to natural
    selection, where genetic changes that are favorable to a species
    quickly gain a foothold and begin to spread, the researchers report.

    Lahn offers an analogy: Medieval monks would copy manuscripts and each
    copy would inevitably contain errors -- accidental mutations. Years
    later, a ruler declares one of those copies the definitive manuscript,
    and a rush is on to make many copies of that version -- so whatever
    changes from the original are in this presumed important copy become
    widely disseminated.

    Scientists attempt to date genetic changes by tracing back to such
    spread, using a statistical model that assumes genes have a certain
    mutation rate over time.

    For the microcephalin gene, the variation arose about 37,000 years
    ago, about the time period when art, music and tool-making were
    emerging, Lahn said. For ASPM, the variation arose about 5,800 years
    ago, roughly correlating with the development of written language,
    spread of agriculture and development of cities, he said.

    "The genetic evolution of humans in the very recent past might in some
    ways be linked to the cultural evolution," he said.

    Other scientists urge great caution in interpreting the research.

    That the genetic changes have anything to do with brain size or
    intelligence "is totally unproven and potentially dangerous territory
    to get into with such sketchy data," stressed Dr. Francis Collins,
    director of the National Human Genome Research Institute.

    Aside from not knowing what the gene variants actually do, no one
    knows how precise the model Lahn used to date them is, Collins added.

    Lahn's own calculations acknowledge that the microcephalin variant
    could have arisen anywhere from 14,000 to 60,000 years ago, and that
    the uncertainty about the ASPM variant ranged from 500 to 14,000 years

    Those criticisms are particularly important, Collins said, because
    Lahn's testing did find geographic differences in populations
    harboring the gene variants today. They were less common in
    sub-Saharan African populations, for example.

    That does not mean one population is smarter than another, Lahn and
    other scientists stressed, noting that numerous other genes are key to
    brain development.

    "There's just no correlation," said Duke's Wray, calling education and
    other environmental factors more important for intelligence than DNA

    The work was funded by the Howard Hughes Medical Institute.

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