In the 1950s, someone with microcephaly might have been a circus attraction, but today, this condition provides insight into human brain evolution.

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Genetic Evidence of Brain Evolution

One way to determine if brain evolution is in our future is to consider how our brain evolved in the past. Since scientists don't know exactly how we ended up with brains bigger than other primates, they're left looking at examples of when the brain doesn't grow to the expected size. One such condition is microcephaly, a disorder in which the brain is much smaller than normal; researchers believe that the size of a microcephalic brain is roughly similar to that of an early hominid [source: Kouprina et al.].

Microcephaly has been tied to at least two genes: ASPM and microcephalin. When mutations in these genes occur, brain size is affected. Since ASPM seems to have evolved faster in apes than in creatures such as mice, it's possible that it may have something to do with how our brains evolved. A 2004 study that compared ASPM in humans to other primates found that the sequence of the gene was roughly similar, which seems to suggest that ASPM alone wasn't responsible for differentiating humans from chimps [source: Kouprina et al.]. But ASPM could have facilitated something else in the human brain that caused our noggins to expand so dramatically.

The following year, a study led by Dr. Bruce Lahn of the University of Chicago continued tracking the presence of ASPM, as well as microcephalin, in human populations. But Lahn had noticed that these genes were changing slightly; these alternative forms of a gene are known as alleles. Lahn's group tracked the alleles in the DNA of several populations, including individuals from Europe, Africa, the Middle East and East Asia, to ensure diversity.

In the case of ASPM, a new allele emerged approximately 5,800 years ago, and is now present in about 50 percent of the populations of the Middle East and Europe [source: Wade]. It's found to a much lesser extent in the peoples of East Asia and Africa. The allele associated with microcephalin is believed to have developed about 37,000 years ago; about 70 percent of the European and East Asian populations exhibited this allele [source: Wade]. Lahn's team deemed the variations common enough to suggest that their presence was evidence of natural selection as opposed to an accidental mutation, suggesting that the brain may still be evolving [source: Associated Press].

Lahn's hypothesis that these genes have evolved as they conferred advantages to the brain comes with the same caveat as the earlier study. Scientists simply aren't sure what role ASPM plays in brain size, and it's a given that not all of the brain-size determining genes have been identified yet. African populations, who didn't appear to be carrying either gene in great frequencies, may have other genes at work on their brains, while it may turn out that ASPM and microcephalin have persisted in the other populations for some reason completely unrelated to the brain.

More work is needed on the role of ASPM, microcephalin and other genes involved in the growth of our brain, but one reason why scientists are so interested in brain size is that it has been linked with intelligence. Bigger brains might portend bigger IQs. So if the ASPM and microcephalin alleles are in fact causing our brains to evolve, what are the possible destinations? Will we be bigger-brained and smart enough to realize some amazing inventions? Or is mankind on a slippery slope down to Stupidtown? On the next page, we'll investigate what the fallout of all this evolution might be.