Neanderthals & humans evolved by accident via genetic drift

Neanderthals & humans evolved by accident via genetic drift

A lovely, accidentally evolved Neanderthal skull

We often think of evolution as driving creatures to become better adapted to their environment. However, this isn’t always the case. Sometimes traits evolve for no real reason.

Chance, random events, and genetic drift can cause some traits to evolve even if they don’t offer a real advantage. As long as they don’t result in a disadvantage.

Recently, researchers speculated that this may have been why neanderthals evolved. They noted that Neanderthals were living in relatively small populations; which can amplify the effect of genetic drift.

But they weren’t able to conclusively prove genetic drift explains the evolution of Neanderthals. They also concluded that humans – living in larger groups – probably evolved for a reason.

However, follow-up research has found proof that this sort of genetic drift may have played a role in the evolution of Neanderthals. What’s more, it turns out that humans may have also evolved “by accident” after all.

What is genetic drift?

Before getting into the nitty gritty of this new discovery, it’s worth going over a few key points.

“Evolution” is simply change over time. If these changes are beneficial, an animal is more likely to have offspring so the changes become more common in the population. This is natural selection. However, there are many factors that can result in neutral changes becoming common as well. Genetic drift is one of the most prominent of these factors.

If you have two mutations that offer no advantage then one shouldn’t become more common than the other. It has no advantage over it. But your kids will only inherit half your genes. So by chance, one of those neutral mutations could be left behind. Now, it’s present in more individuals so will become more common overtime. That’s genetic drift. A pretty powerful factor that can result in neutral changes evolving.

You understand it now? Probably not. Trying to explain it with words is rather tricky, so this handy diagram should help.

Half the population has one neutral mutation, half has another. By chance some of the red mutations aren't passed on; allowing the blue one to become more common even though it has no advantage. This is genetic drift. 
Half the population has one neutral mutation, half has another. By chance some of the red mutations aren’t passed on; allowing the blue one to become more common even though it has no advantage. This is genetic drift.

Of course, there are many other reasons why the blue mutation might become more common. Maybe by a fluke of luck some blue guys find a new habitat full of resources they can exploit. Or maybe a freak disaster kills off some red dudes.

The point is some evolutionary changes don’t have a real reason behind them.

Neanderthals evolved by accident

Genetic drift is a more prominent force in populations with small population sizes. Since Neanderthals appear to have lived in small groups, some had speculated that many aspects of their anatomy (particularly the skull) were actually the result of genetic drift. They didn’t evolve because they conferred an advantage.

This latest research finally confirms that – for the skull at least – this seems to have been the case. The scientists examined recent human groups to figure out how much the skull changes between generations. Armed with this information they could figure out how long it would take the changes between human and Neanderthal skull to accumulate.

These estimates were then compared to genetic estimates over how long those changes actually took to accumulate.

Bits of the skull that separate humans (right) from Neanderthals (left).
Bits of the skull that separate humans (left) from Neanderthals (right).

If the skulls changed for a reason then these two estimates should not match up. The advantages of these traits would mean they should spread through the population faster than by chance. This would appear to “accelerate” evolution. On the other hand, if the initial form was beneficial then they should be locked in place over this time, appearing to slow down the speed of evolution.

Yet the skulls indicated Neanderthals and humans split 300 – 400 thousand years ago. This matches up very closely with the genetic dates for the split. So this is fairly strong evidence that these skulls were evolving for no real reason.

So did humans

Of course, the other upshot of this conclusion is that the same is also true of humans. We have a skull that largely developed by chance. Influenced more by genetic drift than natural selection.

Personally, I find this a tad troubling as there’s not much that makes modern humans unique. And a lot of what little there is can be found in the face. We have a forehead, a chin and now browridges (unlike Neanderthals). Yet it might be the case that there’s no real reason for our special face.

Now, this is where some errors might creep into this research. Say Neanderthals started off with a beneficial face, so it didn’t change much. Humans went and evolved one, so it changed a lot. Between these two factors it might appear like there’s only been a random amount of change when you compare the two. However, the odds of it so neatly aligning with the genetic data makes this a low probability. There’s also the fact that they’re applying data derived from relatively recent populations to really old ones (and assuming that the rates all remain consistent). In short, take this data with a bit of skull-salt, but it can’t be thrown out.

Chimps have purpose

They also applied this methodology to chimpanzees. Surprisingly (if you’d like to think of us as more highly evolved) they found that the chimpanzee face does appear to have a benefit.

Chimp skulls have evolved a lot less than they should have. This strongly suggests that their current form is beneficial, preventing random mutations from building up. After all, they’d probably change the form which would apparently remove the advantage. So, not only is the chimp skull really well adapted to being a chimp. So well adapted that even small changes could “break” it.

And then there’s us humans, sitting around with whatever face chance gave us.

Perfect chimps with their perfect skull

tl;dr

Human and Neanderthal skulls appear to have evolved by “chance”, developing due to genetic drift rather than natural selection.

References

Pearson, O. M. (2013). Hominin Evolution in the Middle-Late Pleistocene. Current Anthropology, 54(S8), S221-S233.

Weaver, T. D., & Stringer, C. B. (2015, October). Unconstrained cranial evolution in Neandertals and modern humans compared to common chimpanzees. In Proc. R. Soc. B (Vol. 282, No. 1817, p. 20151519). The Royal Society.

Source: Neanderthals & humans evolved by accident via genetic drift

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