From the Natural History Museum of London:

Are Neanderthals the same species as us?

By Chris Stringer

First published 1 October 2019

Museum human evolution expert Prof Chris Stringer, who has been studying Neanderthals and early modern humans for about 50 years, tackles the big question of whether we belong to the same species.

Everyone on the planet today, whatever they look like and wherever they live, is classified by biologists in the species Homo sapiens. But some commentators are now suggesting that the extinct Neanderthals with their heavy brows and big noses should be classified in our species as well.

So what defines our species, and who qualifies to join the club?

An expanding family tree

When I drew up a family tree covering the last one million years of human evolution in 2003, it contained only four species: Homo sapiens (us, modern humans), H. neanderthalensis (the Neanderthals), H. heidelbergensis (a supposedly ancestral species), and H. erectus (an even more ancient and primitive species). I have just published a new diagram covering the same period of time and it shows more than double the number of species, including at least four that were around in the last 100,000 years.

Scientists currently recognise as many as nine human species from the past one million years, including the recently discovered Homo luzonensis, which was announced in April 2019.

Our species name (which means ‘wise humans’ – though we might question the wisdom of that name today) was given to us by that great Swedish classifier Carl Linnaeus in 1758. In those pre-evolutionary times, species were usually considered to be fixed identities, created by God….

Even the three tiny bones of our middle ear, vital in hearing, can be readily distinguished from those of Neanderthals with careful measurement. In fact the shape differences in the ear bones are more marked, on average, than those that distinguish our closest living relatives – chimpanzees and gorillas – from each other. …

Complications come when we consider a particular definition of species – one which Linnaeus did not develop, but which he probably would have appreciated.

The biological species concept states that species are reproductively isolated entities – that is, they breed within themselves but not with other species. Thus all living Homo sapiens have the potential to breed with each other, but could not successfully interbreed with gorillas or chimpanzees, our closest living relatives.

On this basis, ‘species’ that interbreed with each other cannot actually be distinct species.

Critics who disagree that H. neanderthalensis and H. sapiens are two separate species can now cite supporting evidence from recent genetic research. This indicates that the two interbred with each other when they met outside Africa about 55,000 years ago. As a result, everyone today whose ancestors lived outside Africa at that time has inherited a small but significant amount of Neanderthal DNA, which makes up about 2% of their genomes.

I still believe they are distinct species

In the face of this seemingly decisive evidence, why do I cling to my belief that Neanderthals and Homo sapiens are distinct species?

Well, in my view the problem is not with ancient couplings between our ancestors and Neanderthals, but with the limitations of the biological species concept. …