Moreover, our analyses suggest that interbreeding occurred repeatedly among our distant ancestors. The research presented here supports a similar early hybridization scenario using an entirely different approach. Although none of the subsequent studies fully rejected or confirmed the hybridization scenario, most pointed to the lack of sufficient evidence to uphold it.
This study prompted an intense debate but has remained the sole piece of evidence for such an early admixture event. In 2006, a study by Patterson and colleagues concluded that the hominin lineage first significantly diverged from the chimpanzee lineage but later hybridized back before finally diverging again. However, we have little information on the presence or prevalence of hybridization during earlier (pre-1 Ma) periods in human evolution.
These studies indicate that hybridization was prevalent during the period of the emergence of Homo sapiens and suggest that it may be the rule rather than the exception in hominin evolution. There is also evidence of a first-generation child of a Denisovan father and a Neanderthal mother and of genetic exchange between recent and ancient lineages in Africa.
For example, we now have evidence of hybridization between Neanderthals and early humans originating from Africa, between Denisovans and early humans, between Neanderthals and Denisovans, and between Denisovans and an unidentified hominin. Increasingly, the emergence and evolution of our species are being revealed as a period characterized by genetic exchange between divergent lineages. A preliminary version of this manuscript was uploaded to the preprint server BioRxiv in 2017 (10.1101/134502). Our findings suggest a complex pattern of speciation in primate/human ancestors and provide one potential explanation for the mosaic nature of fossil morphology found at the emergence of the hominin lineage. Additionally, analysis of two other NUMTs suggests that such events occur repeatedly. This large divergence suggests a distant interspecies hybridization. Our comparison of a NUMT sequence shared by humans, chimpanzees, and gorillas with their mtDNAs implies that, around the time of divergence between humans and chimpanzees, our evolutionary history involved the interbreeding of individuals whose mtDNA had diverged as much as ~4.5 Myr prior. NUMTs are considered “mtDNA fossils” as they preserve sequences of ancient mtDNA and thus carry unique information about ancestral populations. We present the following novel evidence in support of this hypothesis: the analysis of nuclear pseudogenes of mtDNA (“NUMTs”). The hypothesis that the evolution of humans involves hybridization between diverged species has been actively debated in recent years.
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