Chimps and early humans continued to mate and produce hybrid offspring for a million years.

A comparison of the human and chimp genomes by David Reich, Nick Patterson and colleagues at the Broad Institute in Cambridge, Mass.suggests that after the two lineages separated, they may have begun interbreeding.

This may help explain the 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.

Hybrid populations often go extinct because the males are sterile, so hybrid females may have mated with male chimps to produce viable offspring.

A principal finding is that the X chromosomes of humans and chimps appear to have diverged about 1.2 million years more recently than the other chromosomes. Females have two X chromosomes, males one X and one Y chromosome.

The human lineage finally emerged from this hybrid population,

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