By LEE BOWMAN
Scripps Howard News Service
September 09, 2005
More than any other physical attribute, large brains define humanity. With an average size of 1,350 cubic centimeters, the human brain is larger relative to the rest of the body than that of any other animal.
"Our studies indicate that the trend that is the defining characteristic of human evolution - the growth of brain size and complexity - is likely still going on," said Bruce Lahn, an assistant professor of human genetics at the University of Chicago and lead researcher for two related studies on the brain genes published Friday in the journal Science.
"Meanwhile, our environment and the skills we need to survive in it are changing faster than we ever imagined," Lahn said. "I would expect the human brain, which has done well by us so far, will continue to adapt to those changes.
"If our species survives for another million years or so, I would imagine that the brain by then would show significant structural differences from the human brain of today," he added.
Lahn' s team, in earlier comparative studies of other primates and humans, had shown there had been accelerated evolution in humans among numerous genes, including one called Microcephalin and another called ASPM (for "abnormal spindle-like microcephaly-associated") that are both known to regulate brain size.
Mutations in either one of these genes can cause a malfunction that results in microcephaly - a syndrome in which the brain develops to a much smaller size than normal.
In the new studies, the researchers looked at variations that have occurred in the genes since the rise of modern humans about 200,000 years ago. Evolution doesn't happen across a whole species at once, but ripples through a population at various speeds.
Some individuals acquire a certain genetic variation that gives those who have it better odds of survival. Then, over generations, the percentage of the population that carries a particular gene variant increases.
"We're seeing two examples of such a spread in progress," Lahn said. "This variant is clearly favored by natural selection."
The new variant class for Microcephalin is now present in about 70 percent of humans, and the researchers estimate it emerged only about 37,000 years ago - about the time that modern humans started painting caves in Europe and making more sophisticated tools.
The ASPM variant shows up in only about 30 percent of today's people, and appears to have arisen only in the last 6,000 years - about the time of the first city-states in Mesopotamia.
Lahn said that even though there clearly is some natural selection behind the pace of the gene variants' spread, the researchers don't know what brain improvement they might confer, or even if they have anything to do with brain function or intelligence at all.
"Just because these genes are still evolving, doesn't necessarily mean they make you any smarter," he said.
But the researchers, whose work was sponsored by the Howard Hughes Medical Institute, have taken out patents on both genes, including a test that will allow them to quickly determine if a person carries the favored variants.
Scientists know of at least a half dozen other genes that are mainly linked to brain size, and the researchers will look at them, too. "We want to know how broad a trend these two genes represent," Lahn said. "I would bet that we will find evidence of selection in a lot more genes."
The team is also collaborating with others to see if people with the two variants actually have any cognitive advantage.
Although most studies on the topic suggest that there's some link between brain size and intelligence, the differences are slight. For instance, although male brains are typically 3 percent to 4 percent larger than females', men and women score similarly on IQ tests.
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