When it comes to separating humans from other animals, researchers agree that it's what's between the ears that counts most. Indeed, changes in brain-related genes appear to explain the often vast differences between human and chimp cognition. Now scientists have discovered that the spaces between these genes can be just as important.
Once thought of as junk, noncoding sequences of DNA fill in the gaps between genes and make up more than 90 percent of our genome. Recently, scientists have discovered that these stretches of DNA contain regulatory elements that control how and when nearby genes are turned on and off (ScienceNOW, 16 August). An international team led by genome researcher Edward Rubin of the Lawrence Berkeley National Laboratory in California wondered how many of these noncoding regions might play a role in human evolution.
The team looked at 110,549 human noncoding DNA sequences that seem to have been conserved during mammalian evolution. Using statistical tests, Rubin and his colleagues found 992 sequences that appeared to have undergone changes during human evolution that were not due to simple chance, suggesting that the genetic alterations were due to natural selection. The team then used two existing gene databases, called Gene Ontology and Entrez Gene, to match the noncoding sequences with the functions of the coding genes closest to them.
The strongest evidence for accelerated evolution on the human line was found in noncoding sequences next to genes involved in helping neurons adhere to each other. The team found 69 such sequences, suggesting that changes in these regulatory elements may have contributed to the evolution of uniquely human cognitive talents.
Neuronal adhesion molecules play a major role in wiring the brain, Rubin says, such as the formation of connective synapses between nerve cells. These processes, he adds, are important in early brain development and also crucial for learning, memory, and cognition in adults. For example, Rubin says, one of the noncoding sequences is next to a gene called CNTN4, which appears to be involved in the development of both verbal and nonverbal communication abilities in humans, while another is adjacent to CHL1, which is linked to cognition in both humans and mice.
The team's conclusions "sound interesting and plausible," says Ajit Varki, a molecular biologist at the University of California at San Diego. But Varki cautions that the findings should be considered tentative because the Gene Ontology and Entrez Gene databases only give broad generalizations about a gene's function and cannot pinpoint exactly what it does.