ONE of the benefits of knowing the complete genetic sequences of humans and other animals is that it becomes possible to compare these blueprints. You can then work out what separates man from beast—genetically speaking, at least.
The human brain sets man apart. About 2m years ago it began to grow in size, and today it is about three times larger than that of chimpanzees, man's closest relative. Human intelligence and behavioural complexity have far outstripped those of its simian cousins, so the human brain seems to have got more complex, as well as bigger. Yet no study has pinpointed the genetic changes that cause these differences between man and chimp.
Now a group of scientists believe they have located some interesting stretches of DNA that may have been crucial in the evolution of the human brain. A team led by David Haussler of the Howard Hughes Medical Institute in California, compared the human genome with that of mammals including other primates. They reported the results in Nature.
The researchers looked at the non-human genomes first, seeking regions that had not changed much throughout evolutionary history. Regions that are untouched by normal random changes typically are important ones, and thus are conserved by evolution. Next the researchers found the equivalent regions in the human genome to see if any were very different between humans and chimps. Such a sudden change is a hallmark of a functional evolutionary shift.
They found 49 regions they dubbed “human accelerated regions” (HARs) that have shown a rapid, recent evolution. Most of these regions are not genes as commonly understood. This is because they code for something other than the proteins that are expressed in human cells and that regulate biological processes. A number of the HARs are portions of DNA that are responsible for turning genes on and off.
Intriguingly, the most rapidly changing region was HAR1, which has accumulated 18 genetic changes when only one would be expected to occur by chance. It codes for a bit of RNA (a molecule that usually acts as a template for translating DNA into protein) that, it is speculated, has some direct function in neuronal development.
HAR1 is expressed before birth in the developing neocortex—the outer layer of the brain that seems to be involved in higher functions such as language, conscious thought and sensory perception. HAR1 is expressed in cells that are thought to have a vital role in directing migrating nerve cells in the developing brain. This happens at seven to 19 weeks of gestation, a crucial time when many of the nerve cells are establishing their functions.
Without more research, the function of HAR1 remains mere speculation. But an intriguing facet of this work is that, until now, most researchers had focused their hunt for differences on the protein-coding stretches of the genome. That such a discovery has been made in what was regarded as the less interesting parts of the human genome is a presage of where exciting genomic finds may lie in the future.
This article appeared in the Science & technology section of the print edition under the headline "Man, deconstructed"