Monday, October 19, 2009

Why did the Neanderthals die out?

Yesterday's entry reminded me of a Scientific American article last summer about Neanderthals. Reading it I was reminded again of the curious resistance that anthropologists seem to have towards accepting lessons from biology.

But before I get into that, I need to give some background. The theory of punctuated equilibrium, put forth in the early 70s by Niles Eldredge and Stephen J. Gould. This theory held that new species to develop fairly quickly in small isolated regions, then if successful spread over a wide range and change only slightly until they are replaced by another species. Biologists long ago accepted this theory, and it is backed up with evidence ranging from observations of speciation in action in fish in Africa, to Eldredge and Gould's initial example of a type of trilobite that went through 2 speciation episodes in several million years, both of which we are lucky enough to have fossils from the speciation episode in a single mine each. So in general this is what we expect speciation to look like - something that happens in a small area somewhere which bursts out.

Anthropologists have traced a number of types of humans that existed in the past. Of particular interest are modern humans. Two theories existed on that. One is that we evolved in Africa, then spread. The other is the multi-regional hypothesis, which is that we evolved in several regions at once, and possibly mixed in other kinds of humans. Thus leading to side questions such as, "Did we interbreed with the Neanderthals?"

There is no question which version evolutionary theory supports. Punctuated equilibrium unambiguously says that we should expect our species to have started in a small area and then spread without significant (if any) mixing. I remember reading an essay by Gould many years ago that laid this out pretty bluntly. However anthropologists don't like hearing things like, "We can tell you what probably happened with humans based on general principles established with trilobites and fish." And so the active debate lasted for decades on this point.

Reading the Scientific American article I found that the question has now been tackled with the aid of DNA analysis. Based on genetics we now know that the evolutionary biologists were right, and the multi-regional hypothesis is wrong.

But now we have a new question that anthropologists are putting energy in. What killed the Neanderthals?

First let's list some facts. The Neanderthals survived for over 150,000 years. They lived through multiple ice ages. We have evidence of them hunting both large and small prey, with a preference for big game. We coexisted with Neanderthals for thousands of years. There are minor differences in technology (for instance our ancestors had better stitching on their clothing), but nothing specific. Our ancestors specialized in smaller prey (though we were happy to chase big game after the Neanderthals died out). Yet by the end of the ice age the Neanderthals had died out, both in places where our ancestors were found, and in places we weren't.

Hence the mystery. Why did the Neanderthals die? The long (apparently peaceful) coexistence suggests that we didn't directly kill them. They survived previous ice ages. We survived in the same places at the same time. And they should have been able to adapt and use our strategies - after all they did it in previous ice ages. With such small differences, why did they die out?

The article was very good. This was all laid out very well. Along with lots of detail about relative energy requirements, technology, and so on. But I found myself asking whether they had bothered asking the ecologists.

If you study ecology, one of the basic principles is that related species avoid directly competing. How so? Well when the related species is not there they broaden their niche, and when the related species is then each species defines its ecological niche closely enough that they don't directly compete.

Let me give an example. Originally I read this with 3 species of birds, but that was many years ago so I'll simplify to 2 species of birds that I'll call A and B. These birds all hunt insects, and have the choice of hunting them in the forest or by lakes and streams. When only one species is there, they can be found in both places. But if both species coexist in a region, species A does all of the hunting by lakes and streams, while B hunts only in the forest, and so they avoid directly competing.

Why do they do this? Well A is better than B at hunting in open water. B is better than A at hunting in the forest. When only A is around, therefore, the insect population in the forest rises until a given bird is equally well off in the forest and by the water, so you find birds in both places. With B the reverse happens. But when both species are present, then the A birds lower the population of insects near rivers and streams to the point where the B birds will go hungry if they try to feed there. And conversely the B birds keep the insect populations in the forest low enough that the A birds will go hungry if they try to feed there. So A and B coexist, and specialize.

This coexistence can last indefinitely. However there is very real competition here. The presence of the other species narrows the niche the birds live in, and result in fewer of A and B. Each population is now more precarious, and less able to adapt to environmental changes. Thus the general principle that related species tend to coexist and compete through specialization.

Now let us return to the mystery of the Neanderthal extinction. In the absence of our ancestors, the Neanderthals were willing and able to use a combination of food gathering strategies. However once our ancestors arrived, both groups specialized. The two had no difficulty coexisting for thousands of years, but both populations were more precarious than they had been. In particular the Neanderthals were no longer able to use our niche because our ancestors were occupying it more efficiently than they could. Not much more efficiently, perhaps, but even a small difference is the margin between survival and death.

And so it went. Where we were, our presence forced the Neanderthals to specialize. Where we weren't, our absence was evidence that our niche wasn't a good fit to that area, so the Neanderthals there would be unlikely to benefit much from being able to adapt freely to what worked best. Which left the Neanderthals unable to adapt to climate change, and eventually left them dead.

Based on the history of the multi-region hypothesis, I predict that anthropologists will eventually acquire overwhelming evidence for a detailed version of that scenario. But they are likely to take decades to get their. And when they do they won't notice that they could have gotten there faster if they were willing to accept general principles that are already well established in birds, salamanders and the like.

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