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Genes specifying architecture of the human brain may have started evolving faster some 20 million years ago

University Of Chicago : 09 July, 2007  (Technical Article)
Genes that specify the architecture of the human brain seem to have started evolving faster some 20 million years ago, when the great apes split off from Old World monkeys.
The genes then doubled their speed of evolution after the human lineage parted ways with that of chimpanzees five million years ago.

This is the inference drawn by Dr. Bruce T. Lahn and colleagues at the University of Chicago after comparing the DNA sequence of brain genes in several species of primate. Their report is published in last month's issue of Cell.

Researchers have long hoped to pinpoint the genes that make us human by analyzing the differences between human and chimp genes, particularly those in genes that govern the assembly of the brain.

Because the chimp and human genomes are so similar, the statistical tests used to reveal evolution's shaping hand are unable to pick out significant changes, a problem that researchers circumvent by bringing in a third species as a basis of comparison.

Dr. Lahn used the DNA sequences of the macaque, an Old World monkey, as a base line to measure changes in both human and chimp genes. He found that certain human brain genes, particularly those involved in development of the nervous system, tended to change much faster than their counterparts in the chimpanzee.

Some researchers have suggested that changing a chimp brain into the human version required very few genetic changes. For instance, a single gene that allowed just one or two extra doublings of the neurons to take place in the developing brain would upgrade a chimp brain to human size.

Dr. Lahn says his finding shows the opposite is the case: many different genes are involved in constructing the special features of the human brain. This is what might be expected, he said, given that the human brain is characterized by its greater complexity, not just larger size. New neurons have to grow in the right place and connect in the right manner. 'A lot of genes have to participate in getting it right,' he said.

The finding of changes in a large number of genes is an example of evolution's propensity for tinkering, for reconfiguring old components in order to build something new, said Dr. Gary Marcus, author of 'The Birth of the Mind,' a recent book on genes and the brain. This supports the idea of evolutionary psychologists that the mind is designed to accomplish specific tasks, in Dr. Marcus's view. 'If you believed the mind is a blank slate these results would come as a surprise,' he said.

Dr. Lahn said he believed the accelerated evolution he saw in human brain genes after the split with chimps might have been driven by a snowball effect: the more important that cognitive abilities became for function in a human society, the greater the evolutionary pressure to improve still further on the brain's capabilities. This process has continued to the present day, in his view. 'There is no reason to think that evolution of these genes has stopped,' he said.

A similar comparison of human and chimp genes was conducted a year ago by Dr. Andrew G. Clark of Cornell University. He and his colleagues compared genes of all kinds, not just those active in brain tissue. They found some human brain genes involved in development showed signs of having changed in response to evolutionary pressures, but not as many as suggested by Dr. Lahn.

'We think the big thing that sets us apart from other organisms is our brain,' Dr. Clark said. 'But if you look at the whole genome the brain genes don't stand out.' He suggested the Lahn team might have biased their study by selecting brain genes first instead of screening the whole genome for evolutionary change and seeing which genes popped out. Dr. Lahn, however, ascribed the difference between the two studies to the fact that he had used the macaque as a basis for comparison, which was in his view more suitable than the mouse, Dr. Clark's yardstick.
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