Page last updated at 01:05 GMT, Tuesday, 11 May 2010 02:05 UK

To mate, or not to mate: The Neanderthal question

Clive Finlayson
By Professor Clive Finlayson
Director, Gibraltar Museum

Neanderthal skull from Forbe's Quarry, Gibraltar (Clive Finlayson)
Neanderthals: rumours of their demise are not exaggerated

There are moments in science when spectacular new evidence stops us in our tracks and makes us think and rethink.

But it is best to wait and let the dust settle to allow a good process of digestion that will not give us unwanted ulcers in the future.

That process has to sift out the news-grabbing headlines from the reality of the discovery, since the natures of science and journalism are different and need not follow similar agendas.

This is not to say that one is right and the other is wrong but simply that the focus is different.

Let me take an example: When genetic revelations of Neanderthal hair and skin colour were published in 2007, the headlines were dominated by red-haired images of celebrities.

I guess the reason for the mania is that we have, for far too long, considered the Neanderthals to have been so different from us

And the usual, boring and predictable comments about us and the Neanderthals followed. Sure, some Neanderthals were red-haired but the results spoke of much more.

Last week the excellent paper in Science by Richard Green et al on the draft sequence of the Neanderthal genome naturally grabbed the "they had sex with us" headline that seems to have been a popular obsession for decades.

I guess the reason for the mania is that we have, for far too long, considered the Neanderthals to have been so different from us.

For the supporters of the sophisticated and cultured modern humans, how could ape-like brutes have ever been allowed anywhere near us?

For others, the possibility of Neanderthals mating with us probably satisfied some bizarre angle of man and ape getting together in a Pleistocene cave while mammoths, cave bears and sabre-tooths roamed icy wastes outside.

None too different

Some of us have, instead, been advocating the closeness of Neanderthals and us for over a decade now.

I have referred to them in my latest book as the humans who went extinct. The entrenched positions of how Neanderthals gave way to the arrival of our African ancestors have seemed increasingly untenable for a while.

But many archaeologists still talk of cultural revolutions associated with our kin (and not the others) and many palaeoanthropologists have continued to insist that we are a different species (Homo sapiens) from them (Homo neanderthalensis).

TV representation of a Neanderthal male (BBC)
Neanderthals were wrongly thought of as brutes, says Professor Finlayson

This latter view must surely now be removed from text books: the one thing that was in the way of deciding whether Neanderthals and modern humans were the same or different species was our inability to apply the biological species concept to the problem.

This concept, applicable to all sexually-reproducing life forms, simply states that populations that mate freely in the wild and leave viable offspring are of the same species. That must now apply to modern humans (Homo sapiens sapiens) and Neanderthals (Homo sapiens neanderthalensis).

So what of all these great differences in anatomy and behaviour which are supposed to separate Neanderthal and modern human?

They are clearly within the range of natural variation of a geographically widespread (probably polytypic) species. At the same time we must guard against going overboard in the other direction.

Hanging around?

The argument that, because some Neanderthal genes survive among Eurasians, the Neanderthals are still among us is heavily flawed and sensationalist.

The physical characteristics that defined the Neanderthals of Europe and Asia are gone: The Neanderthals went extinct and the causes of the extinction of the last populations scattered across southern Eurasia could have been varied.

They might even have included gene swamping from modern humans even though the latest paper is unable to confirm this.

Instead, it seems that the direction of gene flow was from them to us, which is not too surprising as the small pioneering modern human populations entering Eurasia must have come across a lot of resident Neanderthals along the way.

Infographic (BBC)

But all this happened a long time ago or else the Neanderthal gene traces would not have been present in such widespread Eurasians as Europeans, Chinese and Papua New Guineans.

It must date to the very original modern human expansions in the direction of Australia and Central Asia.

The irony is that there is an alternative explanation to all this and it is recognised by the authors of the paper in their penultimate paragraph. It has not received so much attention from the news media.

Vindija bone
DNA from Neanderthal bones shed new light on our relationship to them

The alternative is that Neanderthals and modern humans did not necessarily mate with each other.

If the Neanderthal genes had been present in the ancestral African population - which they could have been as the further back we go in time, the closer the lineages were - and were exported into Eurasia, then the pioneers were simply carrying genes that were also present among the Eurasian Neanderthals, derived from an earlier common ancestry.

If those Neanderthal genes were then lost from the great African genetic pool, then they would not be present among Africans today. They would instead only survive among non-Africans.

Until such time as we can discriminate between these two alternatives, the Neanderthal-modern human mating option can only be regarded as a best-case scenario, based on the principle of parsimony.

Clive Finlayson is director of the Gibraltar Museum and adjunct professor at the University of Toronto in Canada. He is the author of The Humans Who Went Extinct: Why Neanderthals Died Out and We Survived, in which the theme of this article is expanded.

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