Small DNA changes separate chimp and human brains

Modern humans share about 95% of their genetic code with chimpanzees. Yet human brains, and what we do with them, are vastly different.

In the decade since the human genome was mapped, researchers have identified hundreds of small regions that differ between humans and fellow primates. Many show evidence of accelerated changes that might offer evolutionary clues to such fundamental differences as skeletal structure, motor skills and cognition since our human ancestors parted ways with chimpanzees some 6 million years ago.

A new study suggests that just 10 differences on one particular strand of human DNA lying near a brain-development gene could have been instrumental in the explosive growth in the human neocortex.

The DNA region, containing just 1,200 base pairs, is not a gene. But it lies near one that is known to affect early development of the human neocortex, according to the study, published online Thursday inCurrent Biology.

Researchers showed that the region, known as HARE5, acts as an enhancer of the gene FZD8. Embryos of mice altered with human HARE5 developed significantly larger brains and more neurons compared with embryos carrying the chimp version, according to the study.

"It could contribute to making us unique, and making our brains unique, said Duke University developmental neuroscientist Debra Silver, coauthor of the study. Were seeing the human enhancer turn on gene activity right at the onset of when a population of cells called neural stem cells are rapidly proliferating. They shift within a day or two to making neurons.

The type of neurons and the timing of their development are significant -- these excitatory neurons arise later in utero, which is consistent with human fetal brain development patterns driven by the gene in question.

So have they found the genetic missing link between chimp brains and human brains?

"We think its likely that theres many additional accelerated regions that are contributing to human brain development, and they may be impacting other aspects that make our brains unique, Silver said.

It still would be a pretty big gap to go all the way to: Oh, and thats why we have a spoken language, or different types of fine muscle movements or different cognitive abilities,'" said Katherine Pollard, an evolutionary genomics researcher at UC San Francisco's Gladstone Institutes, who was not involved in the study. "But this is certainly a big step in that direction."

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Small DNA changes separate chimp and human brains