Monday, September 18, 2006

Tenacious Neanderthals

Neanderthals, our closest relatives in the fossil record, might have survived for longer, and co-habited with modern humans more extensively, than previous studies have proposed, suggests a paper recently published online by Nature.

This year marks the sesquicentennial of the discovery of the fossilised remains of an individual that would become the prototypical, or type, specimen of a new species of human, Homo neanderthalensis. In 1856, lime quarry workers in the Neander Valley in western Germany recovered a number of fossils, including fragments of the cranium, of a human skeleton that was initially thought to be a diseased modern human. Later, as similar remains were found in other parts of Europe, the Neanderthals, as they were named in honour of their place of discovery, became accepted as a distinct, and extinct, type of human.

Neanderthal bones or their associated technology (including hand axes fashioned out flint) have since been found across Europe and into western Asia. Fossil remains bearing at least some of the distinctive features of Neanderthals are seen in bones 600,000 years old, although the full range of Neanderthal features don’t come together into the ‘classic’ Neanderthal form (such as the type specimen found in the Neander Valley) until about 100,000-200,000 years ago. The ancestors of Neanderthals (and of modern humans), H. heidelbergensis, lived in Africa, and about 600,000 years expanded into Europe and western Asia. This can be inferred from the trail of artefacts that they left across the newly inhabited continent. Stone tools, such as hand axes, first appear in archaeological sites around 1.7 million years ago in Africa, but then 500,000 years ago are seen all over the place in large areas of Europe. These technology-wielding people in Europe subsequently evolved into the Neanderthals (the stone tools used by the Neanderthals are called ‘Mousterian’ after the Le Moustier site in France).

The early humans that remained in Africa after the diaspora that led to the colonisation of Europe and the emergence of the Neanderthals later ventured out of Africa themselves, and into the Neanderthals’ range, sometime around 40,000-60,000 years ago. Anatomically modern humans had evolved in Africa roughly 160,000 years ago, but these later pioneers were also recognisably behaviourally modern, and the artefacts they made suggest a more complex culture and sophistication than either previous humans or Neanderthals. In fact, the presence of modern humans, or Cro-Magnons, in Europe is often inferred from the sorts of artefacts that are found in archaeological digs (and the same goes for the Neanderthals – it’s not just about bones). By about 45,000 years ago, modern humans lived in Europe and Asia.

Then something dramatic happened. Sometime within the past 35,000 years, after a good evolutionary innings, the Neanderthals stepped out of the evolutionary game, and modern humans went on, at least temporarily, to become masters of the sport of global dominance. Why the Neanderthals failed where the Cro-Magnons succeeded is a matter of intense debate. Potential explanations include a genocide by the Cro-Magnons against the Neanderthals, hostile climate, and the cultural superiority of the Cro-Magnons that gave them an edge in competition for resources and habitats. The notion that climate was the decisive blow to the Neanderthals has been re-asserted recently, but a problem with this idea is that Neanderthals seemed to have coped pretty well with climatic conditions that varied widely over a geologically rapid time frame (as quick as 1,000 years), and which could rapidly change glacial regions in to much warmer environments. Neanderthals managed with the challenges of climate for perhaps 30,000 years, so why did they stop coping after modern humans turned up?

One important issue to look at in exploring these explanations is whether Neanderthals and modern humans co-habited, and what the effects were of living together. The Neanderthals certainly disappeared after modern humans arrived on the scene, but how long was the overlap? A long period suggest that the modern humans didn’t suddenly wipe the Neanderthals through mass murderer, and could suggest that another factor did them in.

Like practically every question in human palaeoanthropology, this is a vexatious issue. Depending on which dates you rely on, and which sites you look at, the speed at which Neanderthals died out ranges from just 2,000 years in some places to 10,000 in others. These estimates are derived from the age of the earliest modern human remains (bone or artefact) and the latest Neanderthal in a similar location – both of which have potentially significant error bars. This makes the discovery of Neanderthal remains that are younger than previous finds very interesting, because it suggests that the Neanderthals persisted for longer and therefore died out more slowly – and so perhaps modern humans weren’t such a potent force after all (although you could perhaps draw the same conclusion about climate, and assume that any Neanderthals that survived later coped with the climate until modern humans came along).

Writing in Nature Clive Finlayson and colleagues report just such remains – of Mousterian artefacts, if not bones, in Gorham’s Cave in Gibraltar that they claim are at most 28,000 years old, and perhaps as young as 24,000 years. As the authors admit, this only allows a reasonable inference that Neanderthals inhabited the cave. But if this assumption is correct, and the dates accurate (which is contentious), then these findings push the date of the most recent Neanderthals a couple of thousand years nearer the present. The authors interpret this as evidence that Cro-Magnons were not such a potent evolutionary poison to the Neanderthals.

It is not clear, however, how significant these dates are, even if accurate. Perhaps modern humans weren’t in this outpost of southern Europe at the time, and the Neanderthals were surviving there just as they had done elsewhere in Europe, against the slings and arrows of outrageous climate change (perhaps Gibraltatr was a more hospitable locale, and that’s why they went there), until modern humans arrived and spoiled the party. Even if the general pattern of modern human contact with Neanderthals was rapid extinction of the latter, it seems reasonable to suppose that some areas would remained free of modern humans for longer than others, and that the dynamics of elimination of Neanderthals would have differed from region to region and across time. And so this find, remarkable as it is, seems to be compatible with the general idea that the evolutionary death knell for the Neanderthals was sounded by modern humans.

Future work will have to resolve the dating issues (John Hawks has a detailed discussion of some of the technical aspects of the find here), and assess the validity of the conclusions tentatively drawn from this study. But the contents of Gorham’s Cave are likely to provide another chapter in the increasingly long and complex book of human evolution.

For another good article on Neanderthals, this time their genome, see here.


Anonymous Stephanie said...

For Neanderthals to go extinct their death rate exceeded their birth rate for a while. This can't have been due solely to Cro-Magnons, but is the result of every factor (including Cro-Mags).
So did Cro-Mag numbers suffer at the same time, e.g. poor climate, and they simple filled the Neanderthal niche in more clement times, or did they overrun Neanderthals killing them directly or with germs? If they were in contact there must have been interbreeding; hybrids probably would have been less fit but is there any evidence of this?

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