Interspecies breeding has always happened, even among ancient hominins. That said, studying hominin hybrids is tricky, and scientists still struggle to determine whether hybridization was advantageous or disadvantageous for offspring.
Now, a new investigation in Science finds that this sort of interspecies breeding is actively selected against among baboons in Africa, potentially signaling that the same held true among ancient hominins. Flowing from a combination of field observations and genomic analyses, this finding stresses the importance of studying other primates as models for human history, particularly for aspects of human history that are impossible to observe in the present.
Ancient humans tended to intermix. In fact, Homo sapiens frequently mingled with similar species of the Homo genus, including the Neanderthals and others. This mingling is thought to have had a substantial impact on human history and variation over time. That said, scientists have previously hypothesized that this sort of admixture would’ve been actively selected against through the processes of natural selection. The offspring of such pairings would be less suited for survival. But the passage of time and disappearance of all other hominins have made this hypothesis particularly difficult to test.
One workaround is to investigate other primates. In fact, scientists say that the breeding between today’s species of primates, a selection of mammals including lemurs, lorises, monkeys and apes, in addition to humans, could illuminate interspecies breeding among ancient hominins.
Taking this comparative approach, one team of researchers analyzed the interbreeding practices of baboons in the wild. As a whole, their analysis indicated that interspecies breeding is actively selected against in baboon populations, supporting the hypothesis that hominin hybrids lacked the same fitness for survival as their non-hybrid peers. More importantly, the researchers say, their analysis also accentuates the value of other primates as proxies for retrieving inaccessible human history.
Studying Baboon Breeding
To study interspecies breeding among other primates, the researchers turned to yellow baboons and anubis baboons around the Amboseli area in Kenya. There, they analyzed over 50 years of field observations on the demographics and dynamics of these populations, as well as genomic information from the offspring of intermixing.
Though the field observations indicated that the offspring of this breeding are no less suited for survival, their genomic analysis seemingly suggested the opposite. It showed that the breeding was selected against in a way similar to the hypothesis about intermingling between humans and other hominins.
Ultimately, these findings could help scientists study human variation and history. But the researchers say that the specifics of the selection against hominin hybridization, including its mode and its mechanics, are still murky and should be subject to additional studies.