Scientists have identified perhaps the most crucial genetic region that makes us human. Comparing human DNA with that of chimpanzees and other animals, they have found the region of the genome subject to the strongest natural selection since our common ancestry with chimps.
The 108-letter stretch of DNA contains two genes that apparently control brain development. The researchers speculate that the speed of evolution indicates they may have been crucial in the rapid increase in brain size and complexity in hominids. Our brains are three times larger than those of chimps.
Most of the 15-million differences between the chimp genome and our own are random, inconsequential changes. Professor Katherine Pollard of the University of California looked further down the evolutionary tree to find useful DNA regions.
She and her colleagues first looked for bits of the genome nearly identical in the mouse, rat and chimpanzee, which shared a common ancestor 80-million years ago. The scientists reasoned that any DNA region that had not changed much in this time must be crucial for survival. They then trawled these regions for cases where the human equivalent had changed significantly.
Top of the list is a sequence called HAR1 (Human Accelerated Region 1). This differs by just two changes between chimps and chickens, but by 18 between humans and chimps.
By adding colour-labelled chemicals that would stick to the RNA product produced by the genes to brain tissue from human foetuses, Pierre Vanderhaueghen at the Free University of Brussels showed that one of the genes is expressed strongly in the developing neocortex during weeks seven to nine of pregnancy. — Â