Furthermore, increasingly sophisticated DNA studies of extinct populations have shown that, despite their general classification as the distinct species Homo neanderthalensis, Neanderthals interbred with modern human groups as they dispersed out of Africa. As a result, many people’s genomes contain roughly 1 percent Neanderthal DNA.
Given this hybridization, some researchers argue that the species boundary between modern humans and Neanderthals should be reconsidered, and that both should be classified as Homo sapiens. I believe that a morphology-based species distinction is still warranted. In fact, the scientific dispute reflects the real-world limitations of the concept of distinct biological species, which does not allow for interbreeding.
The situation is further complicated by additional instances of ancient interbreeding among native populations in Africa and Australasia (for example, Australia, New Guinea and Bougainville). In the latter case, the interbreeding derived from a population known so far from only one site — Denisova cave in southern Siberia. Fossils from there contain DNA related to, but distinct from, that of the Neanderthals.
Today, remnants of this DNA — amounting to roughly 3 percent of some people’s genomes — are present in Australasia. This indicates that the “Denisovans” must have existed not just in Siberia, but also along the southeast Asian route that early modern humans used to reach Australasia, where the hypothesized hybridization occurred.
Even Sub-Saharan Africans of today show traces of a distinct episode of interbreeding within the last 50,000 years. In this case, the source may have been as-yet-unknown remnants of the ancient species Homo heidelbergensis — a direct ancestor of both Homo sapiens and Homo neanderthalensis — surviving somewhere in southern Africa.
However, the real-world implications of the persistence of DNA from such interbreeding should not be misconstrued. The fact that paleoanthropologists commonly refer to the Neanderthals as “archaic” humans, owing to their primitive features, could lead to the semantic trap of regarding living humans with greater inputs of DNA from ancient interbreeding as somehow less “evolved” than the rest of humanity. In reality, given that most human DNA shares recent African origins, all living humans should be considered equally “modern.”
The recent discoveries cannot be ignored. The availability of commercial genetic tests that reveal roughly how much Neanderthal DNA an individual possesses provides an opportunity to appreciate the complex origins of modern humans. While continuing to highlight humankind’s shared modernity and African origins, we must also begin to accept and understand the emerging patchwork nature of our biological history.
Chris Stringer is research leader in human origins at the Natural History Museum in London, director of the Ancient Human Occupation of Britain project and a fellow of the Royal Society.
Copyright: Project Syndicate