The race is on to pinpoint how the human genome has changed since the last common ancestor of chimps and hominids. With more and more genomes being sequenzed it has become possible to compare species and locate the regions where DNA has remained static over the last ~5 million years, and where it has evolved rapidly. An extremely exiting paper reporting such a genome-wide scan in a range of animals, including humans, has been put on-line today at the Nature website. The paper identifies 49 so-called Human Accelerated Regions (HAR) where sequences are evolutionary conserved among many mammals but has diverged rapidly in humans since the last chimp-human ancestor. The fastest among these, dubbed HAR1, has accrued 18 changes in this time. As it turns out, HAR1 is not a protein-coding sequence but belongs to a non-coding RNA sequence. This is rather interesting since geneticists (and evolutionary psychologists) generally assume that adaptations work on protein-coding genes. This new result may indicate that many of the adaptations setting the human genome apart from the chimp genome can have taken place outside of the genome’s protein-coding sequences.
Equally interesting is the fact that one of the two RNA genes containing HAR1, HAR1F, is expressed in human Cajal-Retzius cells that are thought to play a crucial role in redirecting migrating neurons during development. Thus, HAR1F may possibly be linked to the expansion of the human cortex starting 2 million years ago. In a short news article published in today’s Nature Gerton Lunter speculates on what HAR1F‘s functional role might be:
What the gene does is a mystery, but there are some guesses. “Given that it’s changed so dramatically only for humans, it might be involved in humanspecific brain wiring,” says Gerton Lunter at the University of Oxford, UK. One thing is becoming clear: proteincoding genes may not be the movers and shakers of human evolution scientists once thought. “We should stop looking at proteins and start looking at noncoding DNA,” says Lunter. “Everything points in that direction.”
Now, if we could just manage to extract DNA samples from the various hominids fossils linked with major evolutionary changes (homo erectus and so on), we might some day end up with a clear picture of how the hominid brain has evolved. What an amazing triumph that would be!
By the way, in last week’s Science Paul Mellars had a nice review showing how genetical analysis also illuminates our understanding of the dispersal of modern human populations out of Africa some 60,000 years ago. You might want to read that as well.
Pollard, K. (2006): An RNA gene expressed during cortical development evolved rapidly in humans. Nature, in press.
Mellars, P. (2006): Going east: New genetic and archaeological perspectives on the modern human colonization of Eurasia. Science 313: 796-800.