What Makes us Different?

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And sometime in the next few weeks, a team led by molecular geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology, in Leipzig, Germany, will announce an even more stunning achievement: the sequencing of a significant fraction of the genome of Neanderthals--the human-like species we picture when we hear the word caveman--who are far closer to us genetically than chimps are. And though Neanderthals became extinct tens of thousands of years ago, Pääbo is convinced he's on the way to reconstructing the entire genome of that long-lost relative, using DNA extracted, against all odds, from a 38,000-year-old bone.

Laid side by side, these three sets of genetic blueprints--plus the genomes of gorillas and other primates, which are already well on the way to being completely sequenced--will not only begin to explain precisely what makes us human but could lead to a better understanding of human diseases and how to treat them.

FIRST GLIMMERINGS

Scientists didn't need to wait for the chimp genome to begin speculating about the essential differences between humans and apes, of course. They didn't even need to know about DNA. Much of the vitriol directed at Charles Darwin a century and a half ago came not from his ideas about evolution in general but from his insulting but logical implication that humans and the African apes are descended from a common ancestor.

As paleontologists have accumulated more and more fossils, they have compiled data on a long list of anatomical features, including body shape, bipedalism, brain size, the shape of the skull and face, the size of canine teeth, and opposable thumbs. Using comparative analyses of these attributes, along with dating that shows when various features appeared or vanished, they have constructed increasingly elaborate family trees that show the relationships between apes, ancient hominids and us. Along the way they learned, among other things, that Darwin, even with next to no actual data, was close to being right in his intuition that apes and humans are descended from a single common ancestor--and, surprisingly, that the ability to walk upright emerged millions of years before the evolution of our big brains.

But it wasn't until the 1960s that details of our physical relationship to the apes started to be understood at the level of basic biochemistry. Wayne State University scientist Morris Goodman showed, for example, that injecting a chicken with a particular blood protein from a human, a gorilla or a chimp provoked a specific immune response, whereas proteins from orangutans and gibbons produced no response at all. And by 1975, the then new science of molecular genetics had led to a landmark paper by two University of California, Berkeley, scientists, Mary-Claire King and Allan Wilson, estimating that chimps and humans share between 98% and 99% of their genetic material.

ZEROING IN ON THE GENES