The human genome is changing faster than ever
(Courtesy John Hawks)
Human evolution has been gathering speed for the past 50,000 years, and today natural selection may be changing the human genome faster than ever according to John Hawks of the University of Wisconsin and author of the best paleoanthropology blog on the web (johnhawks.net/weblog). Hawks led a study that examined genetic variability around the world and found that adaptive mutations have been accumulating at an increasing pace as the world's population grows. Hawks spoke with ARCHAEOLOGY's Zach Zorich about race, breeding with Neanderthals, and Zach's lousy education.
When I was in school I learned that human evolution had stopped because our population is too large.
Yeah, they were all wrong [laughs]. A lot of people were saying that, especially in the 1960s and 70s there were a lot of anthropologists who studied the past who said look, 30,000 years ago modern humans are essentially in place and we know that people in the world today have ancestry that goes back at least that far. Australians have ancestors 30,000 years ago in Australia...so the idea that you could have rapid global changes across the last 30,000 years seemed unlikely. What people were not considering is the demographic change. There are so many more people that the chances for really favorable strong mutations becomes very high. That's an old idea. Its something that Darwin pointed out, if you have lots of individuals you could have rare mutations that were advantageous. But people didn't put it together with the human historical and archaeological record of population growth.
How many new mutations have occurred in the human genome over the past 50,000 years?
We were able to count around 3,000 new adaptive mutations for Europeans around the same number in west Africans and east Asians.
So, it varies by ethnicity?
Just a bit. There are mutations that occurred in Europe that haven't spread everywhere else and the same is true for the other populations.
How do you figure out how old a mutation is?
Well in our case, we were studying the data from the international HapMap project, which allows us to look at linkage between sites in the human genome. Linkage between genes decays over time, so the more time passes by the less genes tend to be linked together, so we can use the number of links between haplotypes, or gene variations, to estimate how long ago they started to increase in frequency. So, that gives us a way to date how long ago selection starts.
What are the biggest evolutionary changes in humans over the last 50,000 years?
Well, there have been lots of trends. During the past 20,000 years and particularly the past 10,000, body size shrank a little bit, brain size shrank quite a lot, and tooth size reduced. The European and Asian genes that lighten skin color are pretty recent. Digestive things have changed, like the ability for adults to consume milk.
You mean the genes that control lactose intolerance?
Yeah, this is a great example because it is really well known that Europeans, as well as some west and central Asians have a really high lactose digestion capacity as adults. Those places each have separate mutations that are new within the past 10,000 years, and the rest of the world didn't get those mutations, so that explains the difference.
But it's also a great example because people have probed 5,000-year-old Neolithic skeletons from Germany, and the lactase-digesting allele isn't there. That's around 90% in Germany today; in the Neolithic it's not there at all. That means that not only is this a very recent change but it is a recent change that we can document skeletally in the archaeological record.
So, nobody was eating cheese in Germany 6,000 years ago?
Well, they may have started but the thing is that once the pressure is in place, once the diet change happens. That is when selection starts, so people are drinking milk, and they don't have the maximum capacity to do it, mutations that increase their capacity are advantageous, so selection increased these mutations. Other populations that never drank milk never needed these mutations, and they weren't selected.
Your work is mainly about advantageous mutations, but humans also carry an abnormally high number of disadvantageous mutations compared to other species. What does all of this say about human evolution?
In some respects it's two sides of the same coin. Because every time a new mutation arises that has an advantage, the old one is there and now it is considered a disadvantage. What we are seeing is there are a lot of ancestral gene variants (alleles) that used to be the common form in humans and they have recently been declining. For instance the big Alzheimer's disease risk allele is an ancestral form.
What do you mean by ancestral form?
It means that there was a time when everybody in the human population had that allele and its been reducing in frequency over time because the other alleles have some advantage. So when we look today at things that make you susceptible to long-term chronic diseases like Alzheimers and Type 2 diabetes a lot of those alleles are the ancestral forms. Ancient people just were not well adapted to today's circumstances. And the alleles aren't gone, they have been reducing but they are still here and they are causing what we consider to be problems although 50,000 years ago they weren't problems, people lived a different lifestyle.
What set the stage for this increase in the pace of evolution?
That gets into the question of modern human origins. Something changed about humans around 50,000 years ago, and we know that it was associated with technology. People in Africa and Europe were using more symbolic artifacts, projectile weapons and a broader range of resources (see "Home of the Modern Mind," March/April 2008). Everything else--the movement of people into Asia, Europe, and Australia--was driven by population growth. But, as to what really triggered everything, my current inclination is that it had to do with an increased ability to communicate.
Because that enabled technological changes, it enabled people to resist predation, it enabled a demographic shift. We know that in comparisons of archaic human to modern humans, Upper Paleolithic (modern) people lived longer, which means they had less risk. Less risk in humans is created through technology and subsistence innovations. If you don't starve, the predators don't eat you, that ties together. It seems to me that over the course of the past 2 million years, the only thing that could create a radical change like that is a change in social interaction and that means communication. But Neanderthals we now know had the FOXP2 gene that we think is correlated with language so when we say something about communication change I try to be very broad with that because some form of language must have been there in Neanderthals.
I'm thinking that what we're looking at is some kind of involved, complex system of genetic changes probably involving dozens maybe a hundred mutations that coordinated to enable cultural and ultimately demographic change. Knowing what I know, to look back at modern human origins and say that we are looking for a set of maybe a 100 genes that drove this process seems very credible to me.
What about the idea that as modern humans spread across the globe they interbred with populations of ancestral hominins triggering a leap in evolution?
Well, you know this is an idea that I've been pushing. I'm looking at complex systems like the brain. It takes 5,000 genes, at least, to develop a modern human brain, and I'm thinking that not every mutation is going to take place in any one small hunter-gatherer population. Its very plausible that you would get something in Neanderthals that was one gene or maybe ten genes out of 5,000 that would cross into modern humans with very little interbreeding.
Does that mean the language gene, FOXP2, could have originated in Neanderthals and passed to modern humans?
I think so. We know this gene is recent, we know that it is fixed in modern humans, so we know it was strongly selected for. We have found it in some Neanderthals, so where do you go with that? It seems to me we've got a 50/50 chance that it originated in modern humans and was passed to Neanderthals. The other explanation is that FOXP2 is very old and maybe we were all wrong in saying it was a recent mutation, but I'll tell you what--that's just wishful thinking.
If FOXP2 originated in Neanderthals would it have had time to disperse all the way out to modern human populations in Australia and all the other remote parts of the world that modern humans had populated?
If it came from Neanderthals we would have to propose that FOXP2 arose about 100,000 years ago when modern humans and Neanderthals were in contact in the Near East. The key is that if FOXP2 is more recently fixed than the common ancestor of humans and Neanderthals who lived 300,000 or 400,000 years ago, then it had to be transferred in one direction or the other. Given that it's there in several Neanderthal specimens and it's globally dispersed in humans, this transfer, if it happened, had to happen early enough for FOXP2 to get to where it was. So I'm thinking in the time frame of 100,000 years ago, but I'm waiting to see a date.
What about the possibility that it originated in the common ancestor of modern humans and Neanderthals?
Well, there we would expect to see more variability in humans around that gene. The reason researchers initially said that this gene evolved recently was because of the lack of variability around it. This is an area of the genome that has really good samples, so we know there is limited variation here. There is no reason to say its older, but I'm working on a better way to get a date for this. So we'll see if we get some results.
How does your study impact the way we look at ourselves today?
I think it addresses a fundamental problem in anthropology--what explains race? Now, we can say, look, there are genetic differences and they are the consequences of past environments and demographic growth. When we can explain human differences in terms of past environments that always makes more sense than explaining them in terms of big, competing racial groups and the really out-moded, nineteenth-century ideas about racial relationships. People rejected those for good reason, they don't make sense from an evolutionary point of view. So when we can say, look, there are genetic differences and they are the consequences of past environments and demographic growth, you know, the really new environment that we have created for ourselves. I think the demographic and environmental explanation gives us a way to talk about health disparities between populations and the physical differences that people notice, and give them a real context that relates to history.
You mentioned that skin color is something that was relatively recently selected in Europeans and East Asians. How far back does that go?
Well right now it looks like there are around a dozen genes in each of those populations that influences skin color variation. By and large those genes are different in the two groups, which is one indicator of how recent they are. If they weren't so recent they would be more widely shared, they would have had time to spread. But the ones that have been dated, and that doesn't include all of them it includes three or four of them, all of them are more recent than 20,000 years so we're really looking at a recent change in pigmentation and as we develop more accurate dates for some of the other ones I think that pattern will hold up.
So the idea that Neanderthals evolved light skin and passed it on to modern humans is not supported by the data you have.
We have not found any evidence for that, and the red-hair allele that Neanderthals had is not present in any living Europeans. So if they had lighter skin, which is plausible in environmental terms, so far the evidence is that it was independently acquired in them. But I may find next week that one of the genes is really old and looks like it came from a Neanderthal. This is an area of my active research.
If I were to meet somebody who lived 50,000 years ago, how would that person be different from me?
Well number one, somebody from 50,000 years ago who was transported to today in a time machine would probably be dead within two years from some infectious disease. That's the number one difference, we have resistance to all kinds of diseases that have emerged during the process of civilization that never existed before. The smallpox that killed many of the Native Americans, the susceptibility was there for genetic reasons. There would be some digestive changes. When we look at the diversity among people in the world to day in terms of Type 2 diabetes, and obesity susceptibility, some of those differences are going to be explained by recently selected genes that have to do with diet. I suspect there would be some behavioral changes. We're looking at a large category of genes that effect the central nervous system, we don't know what they do but I can expect there must be some behavioral differences that have influenced human populations across the last 50,000 years.
So we have a different nervous system than people 50,000 years ago?
There is something different about our brains, remember when we are looking at the skeleton we can show that the brain shrank. There are anatomical differences in the grossest sense, but also the central nervous system is a complex system that involves developmental processes that we really don't understand yet. There could be any number of things that are different.
Is human evolution still accelerating or have we reached our adaptive peak?
I think that we are undoubtedly continuing to evolve at the genetic level. What has changed in the past 100 years are the pressures on us. For one thing the variation in when people die has gotten much lower and that lowers the strength of selection. On the other hand we still have a huge variation in how many kids people will have over the course of their lives. So I expect that the things that are selected today and maybe in the future are mainly fertility related. If I were to speculate, I'm thinking people are having kids later, maybe something to do with late life fertility. Maybe something that has to do with fertility and diet change, you know people have trouble with obesity and fertility. But that is just speculation, really the past 200 years are too recent for us to even see genetically.
© 2008 by the Archaeological Institute of America