Science Interviews


Sat, 12th May 2012

Human evolution - Professor Mark Thomas

Professor Mark Thomas, UCL

Listen Now    Download as mp3 from the show Genes and evolution - from populations to tumours

Mark -  Up until recently, when we talked about evolution, we meant genetic evolution.  Because what we were talking about was changes that are inherited.  If they're not inherited, if they're not passed on from one generation to another, then evolution can't act on them.  Processes can choose or natural selection can choose something as being a good idea or not a good idea.  But if it's not passed on from generation to generation then it's not an evolutionary process. And of course, when we think about Biology and passing on from generation to generation, we're firmly in the field of genetics there.  And in fact, you know, many people would argue that in fact evolution is just genetics plus time.  So you add genetics, the rule of genetics to time you'll get evolution.  It's just inevitable.

Kat -  And so what do we mean exactly by evolution?  What are some of the pressures that are acting to change genes over time?

Mark -  When we think about evolutionary pressures, of course the first thing that we think about is natural selection.  And of course, Darwin first proposed this is the means by which species change over time.  And natural selection is a wonderful idea.  And of course it is real, it works.  And so – but we have to remember that there are other forces that change the frequency of genetic variance over time, with different gene types over time.

And one of the really important things is just randomness, just the noisiness of inheritance.  So there are many, many different genetic variance in a population, for example, if you imagine a population of people alive today.  And some of those are getting passed on, some are not.  So of them get passed on because they're disadvantageous, because they've caused their carriers to die or fail to reproduce.  But some just won't get passed on by chance.

Kat -  So you can imagine for something like humans to be able to run away from a predatory animal, it would be advantageous to them to select for genes that make you run faster.  What are some of the other characteristics that humans have been selected for?

Mark -  Ten years ago, you would have got somebody waving their arms around saying “well I think it could be a bit of this or a bit of that”.  But now we know exactly what's been selected.  They are genes involved in environmental adaptation.  So one obvious, clear cut example of environmental adaptation you see in humans today is skin color.  And that's obviously a response to the amount of sunlight, particularly the amount of UV.

And then things involved in diet.  When you think about it, that makes a lot of sense because you don't eat or you don't eat and you don't digest or you don't deal with the food in the right way, you're not going to survive.  You need food for it's one of those things like sex that we need in order for the species to keep on going. And it turns out that in fact if we look that, say at Europeans and we dig into the genome of Europeans and look for these signatures of selection that we can see nowadays, we can actually see in the genes the signature of selection.  We can see -- we can get an idea of how strong they are.  

One of the very, very strongest signatures of selection in Europeans is actually an adaptation to be able to digest the sugar in milk.  And that only occurred in the last seven-and-a-half or 8,000 years.  That should be obvious why that's the case because prior to, when we had farming, we didn't have animals.  And if we didn't have animals, we wouldn't have a supply of fresh milk to drink.  Well, we could have got it in other ways but that would have been a bit rude.  So you know, we really need domesticated animals to get the milk.  And sure enough, that's the time when we see this adaptation to be able to digest the sugar in milk.

There are also adaptations in our immune systems.  That's exactly what you expect because of course pathogens are evolving themselves all the time.  And we are in an arms race with pathogens.  We're in this constant battle.  You know, we don't – you don't win that war.  You just keep on fighting it and you keep on doing as well as you can.  And the pathogens will try to do well as they can as well.  So the genes that are involved in our immunity to pathogens, you can see really clear cut signatures of natural selection have an action on those.

Perhaps more surprisingly, another set of genes that seemed to be under strong natural selection but it's maybe a little bit less obvious, were genes involved in sperm motility and in sperm production.  And the theory behind that, why that's the case is that, in fact sperm are competing with each other.  And if you think about it, I mean if you have a multiple mating event, then if one set of sperm could kill out the other set of sperm, they're more likely to produce offspring.  And so therefore there's going to be some – that maybe reflected some kind of competition between sperm.

Now the ones of course that everybody would really want to find out and be really excited about is, what about the ones that make us brainy?  Because of course, you know, when we require brainy species as species go but you know, you go back about five million years, I mean we the same as a chimp, with a similar size brain. So you know, the really kind of cool questions are, you know, what has changed in terms of our genes that makes us brainier?  Unfortunately, we don't really know a lot about that one.

 There are few genes that people have suggested that are involved in brain size.  There are genes that people have suggested are involved in linguistic ability, language ability. But what it beginning to look like is actually what made us start doing really smart things, was not natural selection evolving our brains to be cleverer.  I mean ultimately, they must have evolved to be cleverer at some point.  But doing the clever stuff seems to be another form of evolution.  And that form of evolution is cultural evolution.

Kat -  So you've talked about these five pressures.  We've talked about food, we've talked about sex, we've talked about the environment, our brains, and the immune system.  And you say you can see these genetic signatures of evolution.  How do you do that now?  How can we actually look in our genes now and our genes in the past and see how this has evolved?

Mark -  Imagine we’re back 10,000 years ago and some new mutation arises.  And that mutation is – when it arises, only arises in one person.  So it's very, very rare initially.  Imagine that new mutation makes absolutely no difference to anybody.  It doesn't help them survive but it doesn't lead them to dying either. By chance, that mutation may be passed on to one child or to no children or to two children or three children.  And then by chance, the next generation might be passed on again and again.  So you can imagine that the frequency of that variant might change a little bit in each generation.  It might be go up, it might go down.

Now imagine another scenario.  And so new mutation occurs 10,000 years ago and it happened in one individual but it gives a big advantage.  So it's much more likely to be passed on to two or three or four children and to increase in frequency in the subsequent generations.  So if it's increasing in frequency, then it's going to go up and up and up until it's very, very great common, right? So the difference between a gene that's been under natural selection and a gene variant that's just bouncing around, just neutral, is that the ones in the natural selection, they go from low frequency to high frequency quickly.  And it's the quickly that's the essential thing, okay?

So we can tell whether a gene has got from low frequency to high frequency, by for example, looking in a population from ancient DNA and then looking in a population today and seeing what the difference in frequency is.  And if that change is simply too quick to explain by random processes, then we have to invoke natural selection instead.


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