Modern Humans Still Evolving, and Faster Than Ever
Friday, September 27, 2013
For those who think the forces of natural selection no longer apply to modern humans, paleoanthropologist John Hawks would urge you to reconsider. In recent times — that's 10 to 20 thousand years, for a paleoanthropologist — Hawks says we've picked up genetic variations in skin color, and other traits that allow us to break down starch and digest cheese.
IRA FLATOW, HOST:
This is SCIENCE FRIDAY. I'm Ira Flatow. We're here at the Wisconsin Institutes for Discovery in Madison, Wisconsin. It's also the site of the Wisconsin Science Festival. Wisconsin is, of course, known for its cheese and beer, none of it which you could have if you were a strict adherent to the Paleo diet. You've heard of the Paleo diet, new sort of Paleo diet. It's the caveman diet, a menu of seeds, nuts, veggies; wild game if you can get it; free-range beef, chicken or pork, if you can't.
The theory being that, hey, we didn't evolve dining on cheese and baguettes and beer and potatoes, so we're not all that well adapted to eat them, are we? Well, the passion for Paleo doesn't end quite there. Don't forget the devotees of barefoot running who claim that our years of running across the savanna without running shoes didn't prepare our muscles and joints for jogging in today's confining footwear, and thus, running barefoot is more evolutionarily appropriate.
It all make you wonder: How evolved are we to fit our modern lifestyle? How much have our genes changed in the past, oh, 10,000 years? Is it possible to change in just 10,000 years, and are we still evolving today? Well, we're going to be trying to answer those questions and yours. If you're here in our audience, please don't be afraid to step up to the microphones we have here.
And you can tweet us @SciFri, and also go to our website at ScienceFriday.com and leave us a message there, or go to Facebook at SciFri. Let me introduce my guest. John Hawks is a professor of anthropology at the University of Wisconsin in Madison. Welcome to SCIENCE FRIDAY, Dr. Hawks.
JOHN HAWKS: Thanks. It's just great to be here.
FLATOW: It's nice to have you. Is it possible we are still evolving?
HAWKS: We are certainly still evolving. Even in America today, over the last 50 years, there are changes in our population that, if you study people longitudinally and just look at who's been having kids, it's not a random selection of the population. That's evolutionary change.
FLATOW: And can you actually see that in the genes, of the things that are passed on from generation to generation?
HAWKS: For very recent changes, where we're looking at Americans today and saying what's going on in our population now, we don't know yet what genes are associated with any changes that are happening. But over a longer time span, we can look at the way that genes have changed over time and we can show that over 5,000, 10,000, 20,000 years, we've been changing very rapidly.
FLATOW: Give us an idea of what those changes are.
HAWKS: Probably the most obvious change, the one that people talk about a lot, is lactase persistence. Milk contains a sugar called lactose. We need an enzyme, lactase, to digest it. It is very normal for mammals to turn this off after you're weaned, when you're not having breast milk anymore. You don't ever need to digest lactose again.
But in certain populations, we have a persistence of lactase. There are genetic changes. There's one that's really common in Europe. There are three different ones that are common in sub-Saharan Africa, one in Arabia. Those five changes turn that gene on and continue its activity, and they allow adults to digest milk effectively, where that's really not normal among all mammals.
FLATOW: One of the things you study is celiac disease. Is that something that's changing, evolving, too?
HAWKS: Celiac is such an interesting example for us, because lactase persistence is something that's a very simple change. It's one genetic mutation, and that has an effect that we notice in the phenotype. Celiac disease is a complex trait. It's something that is an immune response in the gut to gluten in the diet. Gluten is the major protein element of wheat.
And people who have celiac - which, in Europeans is something like 1 percent, almost 2 percent of the population over the course of their lives - people who have celiac have an inability to tolerate that in their diet.
FLATOW: So they're sort of allergic to it.
HAWKS: They're allergic to it. Exactly. It's really an immune reaction to it. It's interesting to us because it's this interaction of diet and immunity, and those are things that have changed in recent human evolution. We have new viruses and bacteria that have attacked us, and we also have new diets. Wheat, which is the source of gluten, was not a major component of human diets at all before 20,000 years ago.
So, in that sense, the condition is new. But our work on this condition, it's the genes that influence it, show that the risk has also not been constant in human evolution. Different populations today have different risks, and that's probably a function of immunity being selected in the recent past.
FLATOW: So are, really, the diets that we choose, they're affecting our evolution, what we eat?
HAWKS: Absolutely. We look across human populations today, and you see that people who have had dairying for a long time have lactase persistence. You see people who have been farming with grains, wheat, rye, rice for a long time, they have additional gene duplications of a gene called amylase, which is expressed in your saliva and breaks down starches into sugars.
Those sorts of diet changes have co-evolved with genetic changes that make people better suited to them.
FLATOW: Let's think of something that's not a diet change, things that we're doing now. I'm going to think about social communities and the Internet. Could we be evolving to better use those somehow, to be better adapted to using our thumbs on our cellphones, or something like that?
HAWKS: Adaptation, in a biological sense, in an evolutionary sense, is about how many kids you leave behind. So you could sort of imagine what's going on with the Internet, right? Is it making you more likely to have kids or less likely to have kids?
HAWKS: You know, that's really the question that evolution has to answer is, you know, it's going to do the calculations for us.
We don't have to think for it. All we have to do is ride along, and the ones that have more kids are going to be the ones that are represented in the future.
FLATOW: How many years does it take to figure that out, to know the answer to that question?
HAWKS: When we're studying genetic changes that are really evident genetically, we're able today to look at things that are longer than 2,000, 3,000 years, in terms of their time span. If we look within the population and just ask, how often do you use the Internet and how big is your family, we would have an answer. And that answer is, in a sense, the instantaneous, today version of evolution.
FLATOW: Well, could that involve something - I mean, but what about things like the ability to multitask? Because we have all these things. Could you detect that, you know, an ability to do more of that, or would that show up in having more kids, also?
HAWKS: I think it's tremendously complicated. You know, as an anthropologist, I'd say that those things which we really care about, we judge people all the time. You know, how effective are you at your job? Well, if you're a great multitasker, you know, you might be really effective at it. But those are also things that are really influenced by learning, and the more you practice, the better you are.
And those are things where genetics is not necessarily the thing that's going to make a difference.
FLATOW: Well, on the other hand, if you say, honey, I'll take out the garbage when I'm done with the computer and you do and that makes your marriage a little better and you're having more kids, it may show up that way in a sort of convoluted method, right?
HAWKS: Speaking from experience, I would say yes.
FLATOW: Yeah, I think probably a hundred different people in this room have that kind of experience. Let's talk about some of the other interesting topics. What happened - tell us who the Denisovans are, or who they were. They were interesting, and most people have not heard of them.
HAWKS: This is to me, right now, the greatest mystery in human evolution. We have been studying the genetics of ancient people, and Svante Paabo at the Max Planck Institute in Leipzig, Germany has a team who's sequencing DNA and really systematically looking at ancient sites, trying to get the ancient DNA out of bones. They worked on the Neanderthal genome, and we've been having insights about Neanderthal genetics from this place in Siberia, from Denisova Cave.
It is - you know, I was out there a couple years ago, and it's about a 10-hour journey from Novosibirsk, which is the nearest place you could fly. It - they found, in a little pinkie bone, the distal phalanx of the fifth digit, which is a pinkie, they found a complete genome of a girl who lived something like 50,000 years ago.
And that genome is genetically unlike Neanderthals, and it's genetically unlike modern humans. It is really a population that we did not anticipate existed. It's like a mystery population. What's really striking about it is that there are living descendents today of this population. They're not entirely - they have other ancestors, but a small fraction of their ancestry comes from a population like this girl. And the people today who have that ancestry live in Australia, Australian aboriginal peoples, the islands of New Guinea, Melanesia, you know, the extreme southeast part of the Asian sphere.
HAWKS: We cannot really account for it. It is - you know, we have ideas about what was going on.
FLATOW: How did they get the name for them?
HAWKS: Denisovans. It's from the cave, Denisova Cave. And the cave is a place that has a long archeological record in it. It was named after a myth, or, I think, a story that a guy lived in it named Denis.
FLATOW: Oh. Well, makes sense.
HAWKS: Exactly, yeah.
FLATOW: And were they cohabitating with Neanderthals and humans at the same time, or were humans inbreeding with them? Or...
HAWKS: In this one cave, there is evidence of Neanderthal DNA, and there's evidence of modern human artifacts. Now, we don't think that they were living there simultaneously. The dating, you know, gives a span of tens of thousands of years. But this is a very, very interesting part of the world right now. The Altai has modern humans that show up very early there, 40, 45,000 years ago. It has different groups of Neanderthals that are there.
And, at this point, they're working on the most complete Neanderthal genome that's ever been sequenced, comes from this same cave, and this Denisovan genome, which represents a previous unknown population.
FLATOW: And all we have is this one little finger bone. That's all we have.
HAWKS: There is the one finger bone that has a whole genome, and there are two teeth that have a mitochondrial genome which is the same. So that's what we've got.
FLATOW: Well, why haven't we found other evidence for it?
HAWKS: It is very possible that there are other populations that we know archeologically that will turn out to be the same population. We have skeletal evidence from China. We have a little bit of skeletal evidence from Southeast Asia, from South Asia. It may be that we already have Denisovans in our collections, and we just don't have the genetics from them yet.
Or it may be that they represent other things that we don't yet connect to what we've got.
FLATOW: Do we have any traits from the Denisovans?
HAWKS: We do not know. We don't know.
FLATOW: We don't know.
HAWKS: We don't know. There are living people who have DNA that comes from the Denisovans.
FLATOW: In certain parts of the world.
HAWKS: In certain parts of the world. It is not yet clear exactly what that DNA may do, with one exception. It's known that there's an HLA type, which is present in this ancient genome. HLA is a human leukocyte antigen. It's a major component of the immune system, and the reason why you might have heard about it is if you've been tested for tissue donation, because that's what we recognize - your immune system recognizes invasive cells from this.
This is a highly diverse system in humans, and there is one HLA type in this ancient genome which is also shared by living people in this area. So there's something that functionally might do something.
FLATOW: All right. We're going to take a break. I hope any Denisovans come to the microphone here in our audience.
HAWKS: Please, come out of the woodwork. I'm ready. We'll get the sequencers going.
FLATOW: We have our - please step up to the mic. We're actually eager to have you ask questions, and feel free. We're going to take a break. We'll be right back, talking more with John Hawks after this break. Stay with us.
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FLATOW: This is SCIENCE FRIDAY. I'm Ira Flatow. We're talking this hour about human evolution and how we're still evolving today, with my guest John Hawks, professor of anthropology at the University of Wisconsin here in Madison. Let's go to the audience. Yes, sir. Step up to the mic.
MAX: Hello. My name is Max. I have a question. It's a bit more extreme than the celiacs disease or the lactase enzyme. But I was wondering what you think about the idea that we as humans cannot digest fibrous materials such as plants because of our ancestry, if caveman's.
HAWKS: Oh yeah, that - there are lots of fibers in our diet that our digestive systems can't do very much with. And this actually is mostly due to a really distant primate legacy. The fact is that, you know, from a long history of living in trees and being adapted to fruit-eating and focusing on relatively high-energy foods, we've excluded from our gut the possibility of a longer transit time of, you know, the stuff that eventually comes out the other end and the bacterial communities that allow it to be digested.
We have some primates today, colobus monkeys that have a rich bacterial community that lets them break down cellulose. And of course lots of other kinds of mammals, you know, cattle, sheep and so on have that. But humans have committed to a higher-energy food lifestyle. And that's a reflection of this legacy of evolution.
FLATOW: Could we genetically engineer a genome to do that if we wanted to?
HAWKS: Yeah, I mean, you could imagine ways that you could either select for a very different kind of transit time that would give you a different microbial community and favor that.
FLATOW: Well, we'd have to grow a few more stomachs like the ruminants.
HAWKS: Exactly right. You've got to slow it all down. But, you know, what you'd really want to do is to look around the world at different peoples that are eating different fractions. And we discovered today metagenomic variation in those microbes that do correlate with diet.
FLATOW: So they have already thought about that.
HAWKS: Oh, yeah, yeah, yeah.
FLATOW: Yeah, and they're evolving maybe a little bit toward being able to do that or...
HAWKS: It is so complex, we don't know. But we know that there's variability that we can see that's related to it.
FLATOW: All right. Fascinating. Yes, sir.
UNIDENTIFIED MAN: From what you're saying about human evolution or advanced primate evolution, it sounds like the things may be an awful lot more complicated than we thought they were. And how far do you expect that will go as we get more and more genetic information, DNA sequencing from different populations? Do you expect there will be hundreds of groups like this?
HAWKS: You know, it is fascinating because when we look around the world at the ways that people vary and some of them are obvious, like skin pigmentation. You see it with your eyes. And some of them are less obvious, like altitude. You - living at high altitude, some people are able to breathe more easily at high altitude and some of that is because of genetic adaptation. It changes the fraction of capillaries in their cells. It changes the oxygen-carrying capacity.
I mean, there is an invisible world within our bodies that varies among people today. And we're beginning to uncover aspects of that that we didn't know about. We're also beginning to discover the genetics that underlie things that we've known about for a long time. In Asia, people tend to have dry earwax.
HAWKS: And in Europe and Africa they tend to...
FLATOW: I've seen a commercial how to get rid of that, I think...
HAWKS: Yeah, it is really weird. Dry and flaky earwax as opposed to sticky. Anthropologists have known about this for almost 100 years. And it's only in the last several years that we've discovered, oh yeah, there's a genetic change that does that. And the dry version is new and it's been strongly selected in East Asia.
HAWKS: We have no idea why.
FLATOW: I love that answer.
HAWKS: It's great.
FLATOW: It's a great answer, yeah.
HAWKS: It's great. The earwax mechanism, it is an apocrine gland which is similar to sweat glands, similar to lactation in its genetics. And so it's possible that it's some other function that's protecting your skin against something. But we really don't know. And it's great because we're discovering that there are 20,000 genes in a genome. They do all kinds of different things and we don't have a clue how three-quarters of them work.
FLATOW: Wow. That's a lot of work. A lot of jobs out there, it sounds like, if you had the money to study them.
HAWKS: I think it's cool, yeah.
FLATOW: Yeah, but you mentioned something about skin pigmentation. Now, in doing research for this I was surprised to find that this is - skin color is actually a recent genetic change, right?
HAWKS: Yeah, skin pigmentation today varies in humans. And the reasons for its variation sum up to more than 20 genetic changes in different places. We don't know the timing of all of them. We know the timing of several of them and for those it is all relatively recent. Within the past 20,000 years, genes for light skin have been selected in Europe, in Asia, in North America. And by and large these are different genes in different places. They're new.
And the reasons why Europeans are light and the reasons why North Chinese and Siberians are light, the reasons why Southern Africans are lighter than equatorial Africans, these are by and large different genetic changes. So it's a case where expanding into higher latitudes, less sunlight has really invoked changes that have happened relatively recently.
FLATOW: Has it anything to do with vitamin D or anything - production of anything like that or...
HAWKS: The best hypothesis for this is that we need vitamin D. It's produced in our skin. Ultraviolet light strikes our skin and produces it. And in equatorial latitudes you get lots of sunlight and it's no problem. But if you're wearing a lot of clothes, if you're living in a higher latitude where the sun is lower, in parts of the year there's very little sun, then you have a vitamin D deficiency. And maybe lighter skin is creating more than darker skin would.
FLATOW: So it was 10,000 years ago, everybody had the same pigments?
HAWKS: It's - I can't say that, but I can say that the differences between people were less.
FLATOW: Let's go to the audience here. Yes, ma'am.
UNIDENTIFIED WOMAN #1: So I understand you study evolution and the people with the most children, you know, win in the end. Do you mind if I ask how many children you have?
HAWKS: Oh, what a great question.
HAWKS: Oh, that's a leading question. My wife is listening and she's going to say, oh. We have four kids. Four kids at home, yeah.
UNIDENTIFIED WOMAN #1: Oh, congratulations.
HAWKS: Thank you.
FLATOW: So there's no number that you've discovered is the winning number for...
HAWKS: Four is the winning number, Ira.
FLATOW: In your family.
FLATOW: Wink, wink, Mrs. Hawks. Four is the winning number.
HAWKS: Not five. I've been reliably informed that that is not the winning number.
FLATOW: OK. I got that. Yes, ma'am.
UNIDENTIFIED WOMAN #2: You've been talking about the genes going, like, more and more different from our ancestors' genes. Is it possible it could be moving like more similar to our ancestors' genes sometimes?
HAWKS: You know, that's a really great question because what we find is that in recent humans we're living in new environments. We're eating different kinds of foods and we're living in environments that - high altitude for instance, where people didn't used to live. And in those cases we all tend to be evolving in different ways. But in cases where life sort of reverts to a different - to a form that was more common in the past, there's every reason to think that it can go the opposite way.
FLATOW: Is it possible that - thanks for that question - is it possible that Homo sapiens, we will split into separate species somewhere along the way?
HAWKS: That is, I think, a really key question. And it's one that I think about a lot in the past because people talk about Neanderthals and they talk about now Denisovans. And they interacted with modern humans. And we have some of their genes. So it's clear that even after a long history of living in different parts of the world and adapting in different ways, humans in the past could still interbreed with each other and were probably biologically the same species.
So looking forward in the future you have to think of what's the scenario where there would be an interruption. I think of things like space travel where if we sent people off to colonize another star system and they were really off on their own, then we would have speciation. But...
FLATOW: Much like the islands here on Earth.
HAWKS: Exactly, exactly. But you have to think, this has to happen over a long time and probably with some pressures that they're adapting to that are different than pressures here on Earth. I think on Earth the huge population we have, the diversity of ways of living and yet we're all interbreeding with each other. We all get along, you know, at that level. It's hard to imagine the scenario where we speciate right here where we are.
FLATOW: One last question from the audience here on this topic.
UNIDENTIFIED WOMAN #3: Hi. You've talked a lot about looking at DNA sequences and genetics but you're also talking about some fairly short time scales. Are you also looking at some epigenetic markers? Is that an up and coming area of research for you, and how we turn on and off genes rather than just ditch them and duplicate them?
HAWKS: I think that epigenetics is...
FLATOW: What is epigenetics?
HAWKS: Epigenetics is changes to not the DNA sequence but changes to the way that the sequence is marked chemically that influence the way that genes are translated and transcribed inside of cells. So they influence the reason why you have different kinds of cells, liver cells, brain cells. Even though they have the same genome is become the DNA is marked in ways that make them different.
FLATOW: Make it a liver, make it a...
HAWKS: Exactly right. And we know that epigenetic markers are responding to stresses in our environment. We know that those things are persistent in people and that, you know, when we look at humans in the past and say, well we've got different body sizes, we've got stresses in the past, diseases in the past that are different than now, there are some reasons to think that epigenetics are actually influencing the ways that our bodies are responding to this.
FLATOW: So that's how stresses - real stresses in our lives could make the genes react differently.
HAWKS: That's exactly right. When we look at long-term chronic diseases and the influence of stress, one of the major ways that those long-term things have a health effect on us is by marking our genes, this epigenetic mechanism. Yeah, I think that it's really a promising area to think about how rapid changes unfold in response to environments.
FLATOW: Thank you. Thank you for that question. Thank you, John.
FLATOW: That's fascinating. John Hawks is a professor of anthropology at the University of Wisconsin in Madison. Transcript provided by NPR, Copyright NPR.View this story on npr.org