Trace your genealogy back 25 million years, and you'll meet long-tailed monkey-like primates living in trees. Those primates were not just the ancestors of ourselves, but of all the other apes--chimpanzees, bonobos, gorillas, orangutans, and gibbons--along with the monkeys of the Eastern Hemisphere, such as baboons and langurs. By comparing ourselves to these other primates, scientists can get clues to our evolution over the past 25 million years. Until now, most of those clues have come from fossils and studies on the behavior and physiology of apes and monkeys. But in the past few years scientists have begun to pore over a new record: the one that is inscribed in our genome and the genomes of other apes and monkeys.
The first draft of the human genome was published in 2000, and in 2005 came the genome of the chimpanzee--our closest living relative. Scientists compared the two genomes to get a sense of what the genome of our common ancestor looked like, and how the genomes of both species have changed over the past few million years. (I wrote about the first wave of chimp/human studies here). One of the biggest surprises came when one team of researchers concluded that the ancestors of chimpanzees and humans interbred for over a million years, producing hybrid humanzees.
But there's a limit to how much you can learn from just two genomes. If you find two versions of a gene that are nearly--but not quite--identical in humans and chimpanzees, it's hard to know for sure how that difference evolved. Imagine, for the sake of brevity, that the human version of a gene is AAAT, and the chimpanzee version is AAAC. (Real genes are hundreds or thousands of nucleotides long.) It's possible that the ancestor of humans and chimps had the AAAC version of the gene, and in humans the C mutated to T. But it's also possible that humans have the ancestral version, and in chimps the T flipped to C. It's even possible that the ancestral version was neither. It might have started out as AAAA, and in humans the final A became T and in chimps A became C.
