Thursday, August 18, 2011

Evolutionary accident? larger brain, due to deletion of 583 genomes half a million years agao??

Most male mammals wield a penis covered with spines made of keratin, the same material that forms fingernails, to sweep out competitors' sperm and irritate a female into ovulating. You can add humans' lack of penile spines to the list of ways we are misfits among primates, along with our absence of tails and fur. Even chimpanzees, our closest relatives, have penile spines. A new study suggests that this feature disappeared due to a chunk of DNA that went missing after our evolutionary divergence from chimps. The researchers have identified another DNA deletion that may have contributed to humans' bigger brains.

The question of what makes us distinctly human is hardly a new one, of course, but developmental genomicist Gill Bejerano and developmental geneticist David Kingsley, both of Stanford University in Palo Alto, California, decided to look at the issue from another angle. Maybe humans don't have an advantage over chimps genetically, as we often like to think we do—maybe we've actually lost something. Bejerano and Kingsley compared the chimp genome with the human genome, looking for DNA regions that chimps had but humans did not. And rather than looking at genes, as most research in the past has done, they examined DNA regions that don't code for genes but instead regulate how nearby genes are expressed.

They found 583 deletions in the human genome, and Bejerano says choosing which to study first was a tough decision. "Each region could be its own adventure," he says. They ended up choosing two: a deleted region near a gene for male hormone response and a region close to a gene involved in brain development. The Neandertal genome also lacks these regions, indicating that these deletions occurred more than half a million years ago.