At the level of genes, it's hard to tell a human from a chimpanzee. But one look at their brains and behavior, and the difference is obvious. Now researchers have found that the evolution of a key gene involved in brain development may help explain the disparity.
In the last few years, researchers have found a handful of genes they suspect play a role in the rapid brain expansion that fostered human evolution [ScienceNOW 16 December, 2003]. Now, geneticist Bing Su of the Kunming Institute of Zoology in China and colleagues believe they have found another candidate by comparing the evolution of gene sequences across a menagerie of animals.
Su's team found that a gene called the PACAP precursor stayed substantially the same across eons, but then, sometime after humans and chimps diverged, it evolved at warp speed in the human lineage. The PACAP precursor encodes at least a few different proteins. A region of the gene that produces the PACAP38 protein has held nearly constant, even in humans, presumably because the protein plays diverse roles in neuron communication and is essential for normal development of the cerebellum, affecting brain cell migration, for example. But another sequence, the gene's so-called unidentified domain (UD), appears to have changed extremely rapidly in humans--about seven times faster than it did in other mammals--and shows signs of being positively selected for in evolution. The authors, who report their findings online this week in the journal Genetics, propose that in humans, the UD produces a new protein with an important function, possibly as a regulator of PACAP38.
The differences in the PACAP precursor between chimps and humans are "compelling," says geneticist Bruce Lahn of the University of Chicago, Illinois. "This adds one more gene to the short list of candidates important for human brain evolution," he says. Neuropeptidologist Hubert Vaudry of the University of Rouen in France is not yet convinced that the UD codes for a useful protein, but says that examining regions like it, which are usually viewed as "junk DNA" because they're not translated into protein, could be a good way of turning up previously unknown proteins.