Human Brain Still a Work in Progress
Genetic Changes Suggest Human Brain Continues to Evolve
Sept. 9, 2005 -- Two new studies show that despite several thousand years on the Earth, the human brain is still evolving and adapting to its surroundings.
Researchers say the findings show two genes that influence the complexity and size of the brain have changed over time and are likely continuing to do so.
"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," says researcher Bruce Lahn, PhD, assistant professor of human genetics at the University of Chicago, in a news release. "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."
Despite these potentially dramatic findings, experts say more research is needed to prove whether these changes are evidence of natural selection and, if so, what potential competitive advantage they offer for humans.
Brain Still Evolving?
Lahn, who is also a researcher at the Howard Hughes Medical Institute, supervised two related studies on the issue appearing in this week's issue of Science.
In the studies, researchers looked at changes that occurred in two genes found in the brain, microcephalin and ASPM, across human evolution from nonhuman primates to modern humans.
In both cases, researchers found evidence that variations had surfaced and spread due to changing environmental circumstances.
"We're seeing two examples of such a spread in progress," says Lahn. "In each case, it's a spread of a new genetic variant in a gene that controls brain size. This variant is clearly favored by natural selection."
Although these variations emerged 5,800 to 37,000 years ago, researchers say these time windows are very short in evolutionary terms and were likely the result of intense environmental pressure, which also caused them to spread rapidly.
For example, the microcephalin gene variant appears to have emerged along with the advent of "cultural" traits such as art, music, and religion up to 50,000 years ago. The new variant now appears in about 70% of humans.
The ASPM variant coincided with the earliest known civilization, Mesopotamia, and now occurs in about 30% of today's humans.
Genes Vary by Location
Researchers determined the frequency of the two gene variants by surveying the DNA of more than 1,000 individuals representing 59 ethnic populations worldwide.
To determine the variation frequency of the two genes, researchers surveyed more than 1,000 individuals representing 59 ethnic populations worldwide. Each of these genes has several subtypes, and the results showed that the presence of these subtypes varied geographically.
One subtype of ASPM occurred more frequently in Europeans and the surrounding populations in North Africa, the Middle East, and South Asia. While another subtype of microcephalin was more prevalent in people outside sub-Saharan Africa.
"The next step is to find out what biological difference imparted by this genetic difference causes selection to favor that variation over the others," says Lahn.
More Study Needed
But some researchers say that although the genetic changes "do bear the footprints of natural selection," it's too soon to say whether or not that selection was acting on the brain or some other function.
They point out that both microcephalin and ASPM are found in other tissues in the body besides the brain, and several other genes have also been identified as potential contributors to the early evolution of humans.
In a report that accompanies the studies, several researchers contacted by Science say the only way to answer the questions raised by this research is with more research.