Figure 1.

Complex speciation between humans and chimpanzees. After an initial separation 5–7 Mya, the two lineages subsequently formed a third hybrid population. Strong selection against hybrid incompatibilities eliminated nearly the entire X chromosome from the hybrid population. In the case shown, the X chromosome from the ancestral chimpanzee lineage and autosomes mostly from a now extinct ancestral hominin lineage survived in the human descendants of the hybrid population. (The divergence data cannot distinguish if humans or chimpanzees descended from the hybrid population, but humans are arbitrarily shown as the descendants here, as in Ref. [1].) Under this scenario, the human-chimpanzee divergence time at autosomal loci is t_A (red), whereas that for X chromosome loci is t_X (blue).