Genes Show Signs Brain Still Evolving
WASHINGTON - 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.
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,"
Other scientists urge great caution in interpreting the research.
Aside from not knowing what the gene variants actually do, no one knows how precise the model Lahn used to date them is,
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 ago.
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.
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