Human Brain Is Still Evolving

Two genes involved in determining the size of the human brain have undergone substantial evolution in the last 60,000 years, suggesting that the brain is still undergoing rapid evolution[1].
New versions of the genes - or alleles - appear to have spread because they enhanced the brain's size and function in some way. These new alleles improve brain function, but that would not necessarily mean that the populations where they are common have any brain-related advantage over those where they are rare. Different populations often take advantage of different alleles, which occur at random, to respond to the same evolutionary pressure, as has happened in the emergence of genetic defenses against malaria, which are somewhat different in Mediterranean and African populations.

The researchers studied study two genes, Microcephalin (MCPH1) and ASPM (Abnormal Spindle-like Microcephaly Associated), that came to light because they are disabled in microcephaly ('small brain'), now better known because Zika Virus causes it[2].

Lahn and his colleagues have studied the worldwide distribution of the alleles by decoding the DNA of the two genes in many different populations. They report that with microcephalin, a new allele arose ~37,000 years ago (between 60,000 and 14,000 years ago)[3]. Some 70 percent or more of people in most European and East Asian populations carry this allele of the gene, as do 100 percent of those in three South American Indian populations, but the allele is much rarer in most sub-Saharan Africans.

With the other gene, ASPM, a new allele emerged ~5,800 years ago (between 14,100 and 500 years ago). The allele has attained a frequency of about 50 percent in populations of the Middle East and Europe, is less common in East Asia, and found at low frequency in some sub-Saharan Africa peoples. They note that the ASPM allele emerged at about the same time as the spread of agriculture in the Middle East 10,000 years ago and the emergence of the civilizations of the Middle East some 5,000 years ago, but say any connection is not yet clear.
The Microcephalin and ASPM genes are known to be involved in determining brain size and so far have no other known function, he said. They are known to have been under strong selective pressure as brain size increased from monkeys to man, and the chances seem "pretty good" that the new alleles are a continuation of that process, Dr. Lahn said.

[1] Mekel-Bobrov et al: Ongoing adaptive evolution of ASPM, a brain size determinant in Homo sapiens in Science – 2005
[2] Evans et al: Microcephalin, a gene regulating brain size, continues to evolve adaptively in humans in Science – 2005
[3] Evans et al: Evidence that the adaptive allele of the brain size gene microcephalin introgressed into Homo sapiens from an archaic Homo lineage in PNASofUSA - 2006

No comments: