THURSDAY, Sept. 8, 2005 (HealthDay News) -- The evolution of the human brain is not quite a done deal, say researchers who've uncovered genetic evidence that man's mysterious gray matter is still undergoing beneficial change.
The scientists make their claim based on the recent evolutionary history of two genes -- microcephalin and abnormal spindle-like microcephaly-associated (ASPM) -- which appear to regulate brain size.
Over thousands of years, both genes seem to be generating new and improved versions of themselves -- beneficial mutations that are spreading rapidly among the human population to reshape and strengthen brain capacity.
"I think a lot of people might consider humans to be at the pinnacle of evolutionary lineage -- that we have achieved an advanced state as a species, and we have basically become the end-game," said study co-author Bruce T. Lahn, a Howard Hughes Medical Institute investigator and assistant professor of human genetics at the University of Chicago. "But what we found indicates that the species -- particularly when it comes to the brain, which is perhaps our most defining feature -- is still evolving."
In the Sept. 9 issue of Science, Lahn and his colleagues report on the results of two genetic analyses -- the first conducted among 90 men and women and a chimpanzee, and the second among almost 1,200 men and women. The participant pools were drawn from 59 ethnic groups from all over the world.
The researchers initially focused on identifying all the variations -- or mutations -- of microcephalin and ASPM genes present among the participants. They then honed in on one particular class of each of the two genes that appeared more abundant and genetically younger than the rest.
This class of microcephalin mutations first emerged approximately 37,000 years ago, while the ASPM variant class was estimated to have arisen about 5,800 years ago.
These time frames are a blink of the eye in evolutionary terms, the researchers point out, noting that the type of Homo sapien existent in the world today emerged only about 200,000 years ago.
Both gene groupings appeared to be nearly identical among those participants in which it was identified. According to the researchers, that may be because new mutations have simply not yet had time to diversify in this relatively short time span.
Lahn and his team observed that each of the gene classes were present among a significant number of the scanned subjects -- 30 percent in the case of the ASPM mutation, and 70 percent in the case of the microcephalin mutation.
The ASPM mutation was more prevalent in European-related populations such as Iberians, Basques, Russians, North Africans, Middle-Easterners, and South Asians, and less often found among sub-Saharan Africans, East Asians, and Native American Indians.
The microcephalin variant was also found with higher frequency outside of sub-Saharan Africa.
The researchers stressed, however, that no inferences could yet be drawn about the relative genetic evolution of one ethnic group over another, since many as yet unidentified genes probably play a significant role in brain development.
Nevertheless, given the relative youth of the mutations, the Chicago team believes the widespread presence of each gene among the study group cannot be viewed as a genetic accident.
Their findings suggest that each of the two brain gene mutations thrived and spread due to an active "selection" process, with the mutations conferring an evolutionary advantage to each carrier.
"The frequency of the gene increases because people who carry these genes are more fit and they have more kids," said Lahn. "We think each gene conveys some sort of fitness advantage in brain biology. It could be an improved cognitive function or a personality trait. We're not sure. But we know whatever consequence they render is highly favored by selection."
Lahn and his colleagues believe that, over time, human behavioral and cultural developments might go hand in hand with this type of genetic selection. The microcephalin mutation's first appearance coincided with the beginnings of man's development of art, music, religious practices, and complex tool-making techniques, the researchers point out. Similarly, the launch of the ASPM mutations occurred with the spread of agriculture, urban settlements, and the first record of written language.
The researchers concluded that the overall genetic history of these two brain gene mutations is evidence that the brain is most likely still evolving in terms of size and complexity.
"In another 10,000 or 20,000 years, I think the human brain may acquire a form that is quite different than the human brain today," Lahn said. "Not necessarily in its shape, which may remain relatively the same. But the function may be different. It may be, on average, a little smarter. Or it may acquire certain skills that in its current form it isn't well-equipped to handle -- advanced cognitive abilities such as abstract reasoning. We don't really know for sure how -- but we are still evolving."
"It's really an exciting piece of work," said Paul R. Sanberg, director of the Center for Aging and Brain Repair at the University of South Florida College of Medicine, in Tampa. "And it's not surprising, because we know that the brain is very plastic. As we learn about brain sciences, we know there's recovery following brain damage, we know there are brain cells that allow the brain to repair, we know that the brain keeps growing -- and it's interesting that there's a natural selection that allows for that over a number of years."
Sanberg was also enthusiastic about the medical innovations the findings may ultimately produce.
"There are medical conditions such as microcephaly, in which children are born with reduced brain size," he noted. "And in theory you could manipulate these genes, and this could lead to therapeutic benefits. So, as they move forward to understand the genes and the mechanisms behind their development, it could lead to some more very important research."
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