Chimp-human differences

<p>Feb 3, 2008 10:24 PM</p>

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According to Clark, all mammals have an extensive repertoire of olfactory receptors, genes that allow specific recognition of the smell of different substances. "The signature of positive selection is very strong in both humans and chimps for tuning the sense of smell, probably because of its importance in finding food and perhaps mates," says Clark. In addition to the great departure in smell perception, <b>differences in amino acid metabolism also seem to affect chimps' and humans' abilities to digest dietary protein and could date back to the time when early humans began eating more meat, Clark speculates.</b> Anthropologists believe that this occurred around 2 million years ago, in concert with a major climate change.

"This study also gives tantalizing clues to an even more complex difference -- the ability to speak and understand language," Clark says. "Perhaps some of the genes that enable humans to understand speech work not only in the brain, but also are involved in hearing." Evidence for this came from a particularly strong sign of selection acting on the gene that codes for an obscure protein in the tectorial membrane of the inner ear. One form of congenital deafness in humans is caused by mutations to this gene, called <b>alpha tectorin.</b>

Mutations in alpha tectorin result in poor frequency response of the ear, making it hard to understand speech. "It's something like replacing the soundboard of a Stradivarius violin with a piece of plywood," Clark notes. The large divergence between humans and chimps in alpha tectorin, he says, could imply that humans needed to tune the protein for specific attributes of their sense of hearing. This leads Clark to <b>wonder whether one of the difficulties in training chimpanzees to understand human speech is that their hearing is not quite up to the task.</b> Although studies of chimpanzee hearing have been done, detailed tests of their transient response have not been carried out.</p>