Key 'humanity' gene indentified

US researchers have identified a gene that has evolved rapidly during human evolution, a discovery they say could shed light on what makes us different from animals. The team, based at the University of California, Santa Cruz, identified the crucial brain gene by comparing the human genetic code with that of the chimpanzee, mouse and rat. The research, published online in the journal Nature, pinpointed 49 'human accelerated regions', or HARs, which appear to have undergone substantial changes during the past five million years. Of these, the new brain gene - dubbed HAR1F - showed the greatest number of changes.

The study is one of the first to build on the decoding of the entire chimpanzee genome, reported last September. The international team found that chimps share 96 per cent of their genetic material with humans, while the DNA that makes up the actual genes of the two species is almost 99 per cent identical. Scientists have long hoped that a detailed comparison of the two ape genomes would reveal key differences, which could help explain uniquely human attributes such as language, as well as predispositions to some diseases that only affect humans.

Chimps and humans last shared a common ancestor around six million years ago. The genomes of both are made up of around 2.8 billion 'base-pairs' - the chemical 'letters' of the DNA code. In the latest study, the researchers homed in on stretches of DNA that show many changes in the human version, but not in their chimpanzee, mouse or rat counterparts. HAR1F is produced by cells that regulate how six layers of the cortex are laid down during brain development. It does not code for a protein, but instead makes an RNA (a close chemical relative of DNA) molecule thought to play a role in controlling the activity of other genes. It may interact with a protein called reelin, which is known to play a vital role in the cortex layering process. However, team leader David Haussler admits that this is 'wild speculation'.

Although it is still not clear exactly what the HAR1F gene does, Gerton Lunter of Oxford University thinks that because it has changed so dramatically only in humans, 'it might be involved in human-specific brain wiring'. He also told Nature News that to understand human evolution 'we should stop looking at proteins and start looking at non-coding DNA', adding 'everything points in that direction'.

Haussler says that the finding that HAR1F is switched on at the same time as the reelin gene during development is 'very suggestive', as 'this gene is important in the development of the cerebral cortex, and that's exciting because the human cortex is three times as large as it was in our predecessors'. He points out that something caused the human brain to evolve to be much larger, and have more functions than the brains of other mammals.