Predicting the viability of archaic human hybrids using a mitochondrial proxy

Ancient DNA evidence has confirmed hybridization between humans and Neanderthals and revealed a complex pattern of admixture between hominin lineages. Many segments of the modern human genome are devoid of Neanderthal ancestry, however, and this non-random distribution has raised questions regarding the frequency and success of hybridisation between ancient human lineages. Here, we examine the hypothesis that hominin hybrid offspring suffered a reduction in fertility by comparing patterns of sequence divergence of mitochondrial and nuclear DNA from numerous hybridising pairs of mammals. Our results reveal a threshold separating species pairs whose divergence values fall within two categories: those whose hybrid offspring can successfully reproduce without backcrossing with their parent species, and those whose hybrid offspring cannot. Using this framework, we predict that the potential hybrid offspring of Neanderthals, Denisovans, the ancient individuals from the Sima de los Huesos and anatomically modern humans would not have suffered a reduction in fertility.