2 splits between human and chimp lines suggested

The split between the human and chimpanzee lineages, a pivotal event in human evolution, may have occurred millions of years later than fossil bones suggest, and the break may not have been as clean as human might like. A new comparison of the human and chimp genomes suggests that after the two lineages separated, they may have begun interbreeding.

The analysis, at the Broad Institute in Cambridge, Massachusetts, sets up a serious conflict between the date of the split as indicated by fossil skulls, about 7 million years ago, and the much younger date implied by genetic analysis, as late as 5.4 million years ago. The conflict can be resolved, team suggests in an article published in Thursday’s Nature, if there were in fact two splits between the human and chimp lineages, with the first being followed by interbreeding between the two populations and then a second split.

The suggestion of a hybridization has startled paleoanthropologists, who nonetheless are treating the new genetic data seriously. The earliest human-lineage fossil remains, like Sahelanthropus, seem clearly to have been bipeds, walking on two feet, but the ancestors of chimps presumably on their two feet and the knuckles of their hands, as do modern chimps.

If the earliest hominids are bipedal, it’s hard to think of them interbreeding with the knuckle-walking chimps – it’s not what we had in mind, said by a biological anthropologist at Harvard.