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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?
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.
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.
Grouping living things in species allows biologists to study aspects of life ranging from our own evolutionary history to the conservation of rain forests in the Amazon.
Today we still recognise each species by its own special features.
The physical traits of Homo sapiens include a high and rounded ('globular') braincase, and a relatively narrow pelvis.
Measurement of our braincase and pelvic shape can reliably separate a modern human from a Neanderthal - their fossils exhibit a longer, lower skull and a wider pelvis.
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.
Pronounced differences in the braincase, ear bones and pelvis can still be recognised in fossils of Neanderthals and modern humans from 100,000 years ago. This suggests a separate evolutionary history going back much further - so far so good for differentiating H. neanderthalensis from H. sapiens.
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.
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.
We now know from the same kind of genomic research that many other species of mammal interbreed with each other - for example different kinds of baboons (genus Papio), wolves and wild dogs (Canis), bears (Ursus) and large cats (Panthera). In addition, one recent estimate suggests that at least 16% of all bird species interbreed with each other in the wild.
Thus the problem is not with Neanderthals and modern humans and all the other species that interbreed with each other, but with the biological species concept itself. It is only one of dozens of suggested species concepts, and one that is less useful in the genomic age, with its profuse demonstrations of inter-species mixing. The reality is that in most cases in mammals and birds, species diverge from each other gradually. It may take millions of years for full reproductive isolation to develop, something that clearly had not yet occurred for H. neanderthalensis and H. sapiens.
In my view, if Neanderthals and Homo sapiens remained separate long enough to evolve such distinctive skull shapes, pelvises, and ear bones, they can be regarded as different species, interbreeding or not.
Humans are great classifiers, and we do like to keep things orderly. But we should not be surprised when the natural world (past and present) does not match up to our neat and simple schemes.
But what about the archaeological evidence that is also commonly cited in favour of uniting the Neanderthals with us as Homo sapiens - that they had 'cultural' behaviours such as burying their dead and painting designs on the walls of caves?
Well, interesting as that is, it should be excluded from the biological classification of species, since behaviours are potentially more plastic, evolve more quickly, and spread more easily within and between species than traits based on anatomy and DNA.