Science Talk

The Long and Winding Road: DNA Evidence for Human Migration; Plus July Issue Highlights

Gary Stix discusses his July Scientific American cover article on DNA evidence for the history of human migration. And editor in chief, John Rennie, talks about the neuroscience of dance, the quantum cosmos and Rubik's Cubes. Plus, we'll test your knowledge of some recent science in the news. Web sites mentioned on this episode include

Gary Stix discusses his July Scientific American cover article on DNA evidence for the history of human migration. And editor in chief, John Rennie, talks about the neuroscience of dance, the quantum cosmos and Rubik's Cubes. Plus, we'll test your knowledge of some recent science in the news. Web sites mentioned on this episode include

Podcast Transcription
Welcome to Science Talk, the weekly podcast of Scientific American. This episode is being released on July 7th; we took last week off due to an illness that swept through the entire podcast staff. For more info on that consult South Park episode number 106. Anyway, we planned to be back to the usual Wednesday release date next week.

This week on the podcast, SciAm's Gary Stix discusses his article in the July issue on human migrations and editor in chief, John Rennie, talks about some of the other highlights of the July issue, including an article on the neuroscience of dance. How do you perform a brain scan on somebody doing the tango? Stay tuned.

First up, Gary Stix. He wrote the July cover story on how DNA is revealing all kinds of new info about the history of human migration around the globe. We spoke in the library at Scientific American.

Steve: Gary, How are you doing?

Stix: Good Steve.

Steve: Traces of a distant past. This is really fascinating that each of us carries, within every cell of our body, these clues that will lead to the migration of the entire assemblage of humanity from Africa to everywhere that we live today. So how is it possible? I mean, people are used to understanding that if researchers find fossils or they find [a] civilization's artifacts, you know, pottery shards and stuff like that, but how is it possible that you take blood samples from people around the world and by looking at the DNA, you can put together this map that dates back tens of thousands of years and describe human migration?

Stix: It all has to do with the very small variations that we carry among us; 99.9 percent of human genes are identical among people, but in that tenth of 1 percent, there are variations that hold clues to where we came from.

Steve: Can you tell me about what's the nature of the clue? I mean, if I am going to take samples from a population in Japan and another population in Australia, another population of Native Americans, another population in Scandinavia, another population in Africa, and most of their DNA is going to be absolutely identical; but how is it that those little differences give me the information that I am looking for on this human migration front?

Stix: It relates to the extent of variation in the indigenous populations. There are varying theories about how this works, but the prevailing one at this point holds that humans left Africa about 50,000 years ago and the human populations in Africa contained much more variation than human populations anywhere else. So these are the indigenous populations. Obviously mixture occurs all the time, but if you look at the population in Addis Ababa and you compare it to the population, indigenous populations in a place like Buenos Aires in Argentina, there is much greater genetic variation in Africa—in Ethiopia, which is really the cradle of humanity—than there is in other places; and what population geneticists see is, is they see a gradient, a gradually decreasing amount of variation that begins from Africa and gradually decreases the farther you get from Africa. So, as I said, the amount of variations that you would find among indigenous people in Argentina is a fraction, or less than half in fact of what you would find of African populations.

Steve: Because presumably that's the last place on earth that humans colonized.

Stix: Yes, humans left Africa and gradually fanned out across Asia to Australia and then up to Europe; and then say 15,000 years ago, they crossed over what was then a land bridge to the Americas and gradually worked their way down to South America.

Steve: And that's all material that the archeologists and anthropologists had pretty well covered, but the DNA is really confirming that.

Stix: The DNA confirms that the advantage of using DNA is that there is an endless supply of DNA from human populations, and geneticists come up with new ways of exploiting that DNA. They can analyze a particular block of DNA. They can analyze individual letters in DNA. They can analyze missing sections of the genome to compare a group in, say, the America's versus a group in Asia versus a group in Africa or Europe.

Steve: And just as a concrete example, there is a particular group in Africa that appears to be a remnant of the original humans in Africa, that did not leave the continent and the variability there, because they've been in that one place for such a long time, is very large, whereas—I mean this is what we have said, but I'm just trying to put it in concrete terms—the variability within that population in, say, Argentina, is very small. I mean, it's not that one group has evolved more than another group. It's that the variability within one group versus another is either large or small because one has been doing it for [a] hundred thousand years and one just arrived 15,000 years ago, and that's when you start the clock on that intra-group variability.

Stix: Right. So the other point is that most geneticists and anthropologists believe that it was a single group. Their estimates of how large that group was, some believe that it could have been as small as a few hundred people who left settled in a new area and then a subset of that group then split off and settled in another area, and then a subset of that subsequent group split off and this process continued until the whole world was settled.

Steve: And every time you split off, you start the clock again within that group.

Stix: Right, right. So what you have is this subset of the genome that originated in Africa and obviously the evolution continues, but you're starting the clock with the subset of that original set of genes.

Steve: Right. You said that there is an endless supply of DNA, but in a way, there isn't, because our—it's an interesting situation we're in. The very technology that allows us to do this work has also allowed [endowed] modern life in general with the ability to just fly across the world; we settle on another continent tomorrow and mix all those genes up again, so our ability to do this research could be severely compromised, let's say a hundred years from now, because there may have been so much mixing of these populations. So we really have to do this research now, don't we?

Stix: That has been the perspective of a number of population geneticists who have felt that we really need to carry out this research as soon as possible because of globalization, because of the mixing of peoples. That was the reason that in the early '90s, a very well-known population geneticists, Luca Cavalli-Sforza, suggested that at the same time that we begin a Human Genome Project, we begin something called the Human Genome Diversity Project that would go out and collect samples from many, many different indigenous populations from around the world and then have a basis for comparing that genetic diversity for researching the hypothesis that we're talking about, that humans originated in Africa and gradually spread out with this decreasing genetic diversity. There was a problem with that. Many of the people who they approached were not eager to have their blood samples [sampled] or to give samples of sputum because they felt that, one, this may be taking something that is intrinsic to their own belief systems, which is there are some groups that believe that taking the blood is in essence robbing the soul in some ways. Others had had bad experience with people coming and wanting to take plants and other types of materials that they've been using and patenting them.

Steve: Sure. There's been a lot of colonial exploitation in human history.

Stix: Yeah.

Steve: And the groups that have traditionally been exploited can be a little leery about any kind of research that involves delving into their pasts and taking actual samples of their bodies.

Stix: Absolutely, and in fact, some of the strongest resistance came from Native Americans because of their experiences.

Steve: And we saw an example of that with a Kennewick skull that was found. This goes back years now, probably about 12 to 13 years.

Stix: Right.

Steve: ... where the discovery introduced alternative possibilities to the standard story within some Native American populations about their origins and so they were resistant to scientific evaluation; and you have a similar situation now where some groups don't want to hear that they just got to this continent 14,000 years ago because within their own belief system, they might think they've always been here; and, you know, hate to tell you guys, but you weren't.

Stix: There are some groups that are concerned, there are some individuals within some groups that are concerned [about] what will be found about their heritage—that they may not be of a certain group, or that they may have ancestors who came [from] groups that they weren't aware [of].

Steve: So it's like finding out that you are adopted and your birth parents were maybe not the famous and rich movie stars that you're hoping they were.

Stix: Or that your relatives are...

Steve: ...are the very people you grew up hating?

Stix: Essentially you relate it to everyone; and this is a wonderful technique to do historical research, adding to the archeological, the anthropological and other records. And again because it's possible to look at DNA samples from people who are alive today that there is, in a way, more available than you can find of pottery shards or fossils and that's one of the reasons that people are concentrating so much on this.

Steve: So, there is a really interesting thing that you talk about a little bit in the article that's going to further inform all this; and that is everybody [has heard of] sure of the Human Genome Project, but there's a second Human Genome Project going on now, the Neandertal genome project; and it looks like it's going to be possible to just sequence the genome or at least parts of it from fossils of Neandertalsand what is that going to wind up telling us?

Stix: Well, it will tell us something about the origins of the whole human line, not just Homo sapiens—our species—but others as well; but what it will really tell us is a lot about ourselves. It will be the closest comparison that we can make of a closely related species.

Steve: Because right now the closest related species are the chimps and gorillas, and so we may do with that but to have the genome of an actual human species that is not us...

Stix: Absolutely.

Steve: ... is a fascinating thing.

Stix: Right, and one of the major questions that people are going to be looking at is whether such a closely related species actually had interaction and actually made it at any point with humans. Now there are varying theories about that and there is some evidence in the genetic record that that may have happened, but it's very sparse evidence and this Neandertal genome project should give us a much more definitive answer to that.

Steve: So we might actually discover that 40,000 years ago, we might have acquired some genes that are now part of the Homo sapiens genome that we got from Neandertals.

Stix: Yes, and if we got them from Neandertals, it's quite likely that natural selection would have favored those genes, otherwise over a period of 40,000 years they would have disappeared.

Steve: That's good stuff. When you're writing this, did it make you feel closer to everybody else? I mean we've got millions of people in New York City, and they come from all over the world; and did you feel like a new kind of kinship with everyone?

Stix: Absolutely. One of the data points that I would have loved to put in the article, but I wasn't able to because of spaces [is] that New York city has the most diverse population in the world in terms of different groups and so that's something that I am constantly aware of when I am on the subway or when I am walking to work.


Steve: Next up, John Rennie on some of the other material in our July issue. We also spoke in the Scientific American library.

Steve: Happy July, John.

Rennie: Happy July to you, Steve.

Steve: This is another fun-looking issue, something of a lot of interest to people who are trying to figure out just where they fit in the big picture the self-organizing quantum universe.

Rennie: I know; could we have jammed more buzz words into the title of that? But that's actually what it's about. This is about a proposal some physicists have made to try to come up with a theory of quantum gravity. Listeners who follow physics at all may know theories of quantum gravity is an attempt to try to pull together general relativity with quantum theory. This is one of the big. elusive goals for physicists over the past century.

Steve: In the big unified field theory, [the] theory of everything ever.

Rennie: Exactly right. You know, people are looking for one theory that would give you a way of describing the universe as we know it. One of the best approaches to that in the past has been something like superstring theory and that's had some success; the problem is that in a sense it is too successful. You can come up with superstring theories that do describe the universe that we would see, but it also comes up with models that would describe versions of the universe that we don't see. So it's not very restrictive that way. But there is this proposal that these physicists have of an entirely novel take on this and it is based on geometry. Their idea—and watch how I strip away all the physics that tail on this—is that if you imagine that spacetime, instead of thinking of it as a continuum, if you imagine that it consisted of lots of very, very tiny little pieces of spacetime, almost like atoms at [of] spacetime; and that you then imagine that they interacted in a certain way based on the rules that we know of from relativity and quantum theory, what these physicists have shown is that you end up with a spacetime geometry that is just like the one that we do observe. And what's really interesting about that is that, unlike the superstring theories we're [where], depending on very tiny changes in your starting suppositions, you end up with completely different types of universes, lots of different starting conditions all bring you back to [a] universe that looks just like this one. So it's a very new idea. Who knows whether it will turn out to be the big elusive quantum gravity theory that people want, but it is an interesting, new idea which our readers would like to know more about it.

Steve: And once we get that Large Hadron Collider up running we might actually get some real clues that point in one direction or another as to who is on the right track.

Rennie: Yeah, I think that['s] the idea—that you look for more sorts of clues that would drive you in the right direction.

Steve: We've got a very interesting article on dancing.

Rennie: Dancing; so you think you can dance, Steve?

Steve: I actually know that I can't.

Rennie: Well, you're sort of wrong about that Steve, because the fact is all human beings—even you—have an unusual dancing ability; at least we have an unusual relationship to music, which is that human beings seem to be the only animals that will spontaneously, when they hear music start to move their bodies in rhythmic time with that. It's very odd; you don't see it in any other mammals.

Steve: Not even chimps do that?

Rennie: Apparently not, at least according to the authors of this article we have on the neuroscience of dance. This is really interesting for a lot of different reasons. People have always been, sort of, perplexed about why people have this sort of musical abilities of different sorts, but neuroscientists who look at motion, are particularly interested in things like this, because for years and years we've been studying what happens inside the brain that's involved with relatively simple motions—I make a fist, I move my arm, I kick my leg. We've studied a lot of those kinds of things and even relatively simple actions like, say, walking or running, we've studied what goes on inside the brain, but we also, obviously, all the time, as human beings engage in some kinds of activities that involve much more complicated nuanced arrangements of motions, things like dance. So one of the questions people in this field have wondered about is whether or not there were any other brain areas, any other kinds of activity in the brain that were associated with those more complicated motions that you didn't see if you looked at just the individual component motions. And some experiments investigating that are what Steven Brown and Lawrence Parsons, the authors of this article on the neuroscience of dance, talk about. And they did a really great, wonderful experiment—I would love to have seen this—they recruited amateur tango dancers and they did brain PET scans of them while they were dancing. And yeah, how do you do that?

Steve: Okay, right.

Rennie: Yeah, exactly. So what they did was they, you know, when people are being PET scanned they are lying on their back with their heads immobilized inside the instrument; so these people there in that position but the scientists set up sort of like a small dancer floor board underneath their feet and they were then instructed either to actually execute the leg motions that were associated with dancing the tango as they listen to music or not; or in some cases they were told just to contract the muscles in their legs but not to actually move them, again sometimes to music and sometimes not. And the results were really very interesting. They were able to subtract out a lot of the brain activity that was associated with, sort of, the simple motions and then look just for ones that seem to be associated with music-related motions and what you find is that there were several different areas of the brain that do seem to be associated with this. Some of them are in the cerebellum, the area in the back of the brain that is involved a lot with coordinating motions with sensory input from the muscles; some of them not too surprisingly are involved in the parietal lobe of the brain which is involved, sort of, where a lot of the commands to start, different motions start, but one of them was also involved in an area of the auditory pathway. So it's really very interesting because the auditory pathway is involved in just processing music and processing hearing. You wouldn't think that would ordinarily be involved with motion at all, but apparently that becomes active really only when we are listening to music and synchronizing motions with that. And in fact the authors also say that these studies have even shed some light on this phenomenon on how when we listen to music and unconsciously start to tap our fingers, or that sort of a thing; the reason we do that is because it seems like you're getting, sort of, a little bit of chatter, literally not involving the higher centers of the brain between this area in this auditory pathway and part of the cerebellum. So, [a] really cool interesting studies and sheds a lot of light on, you know, just a phenomenon of dance.

Steve: That is pretty cool. Is there any evidence as to whether our unique, kind of, dance abilities are just a byproduct of evolution or did that connection between hearing and movement really gets selected for some reason.

Rennie: Well it's hard to know very much about the, you know, sort of, the deeper evolutionary forces that would bear on that; but what is really interesting is that there are at least suggestions in some of these studies that that maybe do bolster the idea of gestures as being a very important, very old, very deep form of communication, sort of, in the same way the speech is. For example, one of the things that the authors talk about is as it has been known for a very long time in the left lobe of the brain there is an area called the Broca's region which is involved with generation of speech. And what's interesting is that in the studies it has turned out that the corresponding area in the right hemisphere which is not ordinarily involved with speech becomes active when you're involved in dance; and so given that dance does have a very strong representational communicative element to it, there is some reason to think that, you know, maybe that this really does not bolster the idea that some of the earliest language, maybe even preceding the use of speech for communication, was this idea of movement of different gestures with our hands and with our legs and moving our whole bodies to communicate things. So very interesting that, I mean that, sort of, goes to what you're saying about maybe the deep evolutionary roots of all this.

Steve: Yeah, definitely; especially with, you know, ritual dance being such a big part of pretty much every culture; I am sure it's in every culture on earth.

Rennie: Right, it's very fascinating sort of insight into it; and how exactly you would explain all of that, we don't really know, but the article has a lot of good insights into this.

Steve: So after a lull of I don't [know]—what, 25 or 30 years?— the Rubik's cube is making a huge comeback.

Rennie: I know, it's funny. I don't know if there is an anniversary associated with that recently that we went through but, yes the Rubik's cube is back in a big way. You see people solving it all the time and we happen to have an article here in the July issue that relates to this, to what you might not think of it first, that the Rubik's cube is a mathematical problem, but it really is. There is a mathematics of the Rubik's cube and the authors of this piece "Simple Groups at Play"discuss how the Rubik's cube is an example of a certain kind of permutation problem, and as such it represents a kind of mathematical exercise in what's called group theory, and is that they then were then able to show that they used a slightly different, related area of group theory, one in which they were talking about simple sporadic puzzles; and they developed kinds of puzzles based on different sorts of permutation. And they have examples of these that they describe in the article. And in fact if you go to Scientific American's Web site,, you can find puzzles that we have there based on the mathematics of this, and you can try them for yourself.

Steve: Pretty cool. I remember when the Rubik's cube made its first splash, and I think it was a chemist who wrote a solution and published the solution based on group theory at that time, and that became a best-selling small book, because people who read it and followed the simple directions could then very easily solve the Rubik's cube.

Rennie: Yeah, it's a fascinating thing, that it's for those of us who have never solved the Rubik's cube, and I am one of those people.

Steve: I've got half of one face.

Rennie: (laughs) You know, it turns out that fundamentally it is a problem of applying certain mathematical set of permutations to this, a certain set of movements over and over and over again until you get the desired response, but you know it's very interesting, the tales about all of this in the July issue.

Steve: Years ago I took a group theory class for chemists and the thing that I remember most clearly is that a baseball, because of the stitches, can be assigned to the point group D2D, so remember that at home because that might be on the final exam.

Rennie: Fascinating.

Steve: Now its time to play TOTALL....... Y BOGUS. Here are four science stories; only three are true. See if you know which story is TOTALL....... Y BOGUS

Story number 1: Watermelon may have a Viagra-like effect.

Story number 2: Mercury's core is shrinking.

Story number 3: Water boils faster in a copper pot that is lined with copper nanorods.

And story number 4: To save energy, new models of the hybrid Prius won't have the option of a sunroof, which when open can cause drag and make the engine work harder.

Time is up.

Story 1 is true. Watermelon does indeed appear to have similar general physiological effects as Viagra according to research out of Texas A&M. The key nutrient in question is called sertraline, which can relax blood vessels, which is how Viagra works. Most sertraline in watermelon is in the rind but researchers are trying to breed melons with higher concentrations in the flesh.

Story 2 is true. The recent flyby of Mercury gathered data that indicate that the core is shrinking. The core is mostly iron and makes up the majority of the planet. For more check out JR Minkel's July 3rd article on our Web site entitled "Mercury Flyby Reveals Active but Shrinking Core".

And story 3 is true. Water does boil faster in a copper pot lined with copper nanorods, so say researchers who published in a nano journal called Small; the rods trap tiny air bubbles and the heated water can change to gas where it makes contact with the bubbles, rather than becoming superheated and waiting for an opportunity to get to the surface to change to gas.

All of which means that story number 4, about Priuses doing away with sunroofs is TOTALL....... Y BOGUS. Because reports surfaced on July 7th that Toyota is thinking of giving the Prius the first true sunroof—a solar panel. The solar-generated wattage would only partially power the air conditioner. I am waiting for the crank powered onboard DVD player; kids in the back seat want to watch SpongeBob, they should have to work for it.

Well that's it for this edition of the weekly SciAm podcast. Visit for the latest science news, blogs and videos, and sign up for the daily digest at For Science Talk, the weekly podcast of Scientific American, I'm Steve Mirsky. Thanks for clicking on us.

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