What Makes us Different?

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In fact, even the most ardent proponents of genome-comparison research acknowledge that pretty much everything we know so far is preliminary. "We're interested in traits that really distance us from other organisms," says Wisconsin's Carroll, "such as susceptibility to diseases, big brains, speech, walking upright, opposable thumbs. Based on the biology of other organisms, we have to believe that those are very complex traits. The development of form, the increase in brain size, took place over a long period of time, maybe 50,000 generations. It's a pretty complicated genetic recipe."

But even the toughest critics acknowledge that these studies have enormous potential. "We will eventually be able to pinpoint every difference between every animal on the planet," says Lovejoy. "And every time you throw another genome, like the gorilla's, into the mix, you increase the chances even more."

Some of the differences could have enormous practical consequences. Since his discovery that human cells lack one specific form of sialic acid, which was accomplished even before the human genome was decoded, Varki and his collaborators have determined that 10 of the 60 or so genes that govern sialic-acid biology show major differences between chimps and humans. "And in every case," says Varki, "it's humans who are the odd one out." Such revelations could probably lead to a better understanding of such devastating diseases as malaria, AIDS and viral hepatitisand likely do so faster than by studying the human genome alone.

For most of us, though, it's the grand question about what it is that makes us human that renders comparative genome studies so compelling. As scientists keep reminding us, evolution is a random process in which haphazard genetic changes interact with random environmental conditions to produce an organism somehow fitter than its fellows. After 3.5 billion years of such randomness, a creature emerged that could ponder its own origins--and revel in a Mozart adagio. Within a few short years, we may finally understand precisely when and how that happened.