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 humans 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, by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass., 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.