5,300-Year-Old Mummy Has No Human Descendants
JOE PALCA, host:
This is Talk of the Nation: Science Friday from NPR News. I'm Joe Palca sitting in for Ira Flatow. You may have heard the story. It's just so amazing. That - so here at least two German tourists in the early 1990s walking around in the Alps, and they come across the body of a man sticking out of a glazier. Well, it turned out that this was a - was known as the Tyrolean Iceman and he had quite a past.
He'd been in that ice for more than 5,000 years. They found an arrow tip in his shoulder and signs of a head trauma, so they think they met a violent end. Probably the first or a bronze-age murder victim. Now, the frozen corpse has given up some more secrets. Scientists have sequenced some of his DNA, and compared his genes to those of modern people.
And it looks like the Iceman may be from a rare branch of the human-family tree with no modern relatives. Joining me to talk more about the mummy, and how he may be a one of a kind is my guest, Martin Richards. He's a professor of archeo-genetics at the University of Leeds in the U.K. Thanks for talking with me today.
Professor MARTIN RICHARDS (Archeo-genetics, University of Leeds, United Kingdom): Hello.
PALCA: Archeo-genetics, great title. I mean, it's sort of a opposites, isn't it?
Prof. RICHARDS: Yes.
PALCA: Modern and...
Prof. RICHARDS: Simply using genetics to look into archaeological questions, rather than looking for biological questions. We use it to try and just answer the sort of questions that archaeologists are trying to answer, and perhaps haven't been able to.
PALCA: Excellent. So, if you'd like to join our conversation, the number is 800-989-8255. That's 1-800-989-TALK, and if you want more information about what we're talking about this hour, go to our website, www.sciencefriday.com, where you'll find links to our topic.
So, I guess the first question, Martin Richards, is what kind of DNA were you sequencing? It wasn't the DNA from the nucleus of the cell, the whole genome which is what people usually think about when they think about inherited DNA.
Prof. RICHARDS: Yes. Yeah, that's right. It was mitochondrial DNA, which is in the cytoplasm in the mitochondria, which are the energy units, or the batteries of the cell, where the respiration processes largely go on, and the important thing about mitochondrial DNA is that it's inherited down the maternal lines.
So, you only get mitochondrial DNA from your mother. You don't get it from your father. So where the advantage of that - the disadvantages you're only learning about your maternal line of descent. But the advantage is that you get a clear picture of the maternal line of descent right away back through time, by comparing the mutational variation and the mitochondrial DNAs of different people.
PALCA: So this is - this whole notion of a mitochondrial Eve, that you could go back to the first woman by just figuring out what we're all descended from.
Prof. RICHARDS: Yes, that's right. So if you trace the variation back down the maternal line, right the way back to the common ancestor of all of the mitochondrial DNAs in the world, which is effectively the same as tracing the maternal geniality back through time, if you were to do that.
Prof. RICHARDS: You know, by genealogical methods if you could go back two and a half - to 200,000 years, then you get back to the so-called mitochondrial Eve, who lived in Africa about 200,000 years ago, you know.
PALCA: So you were able to get enough DNA from this mitochondrial - what remains of the mitochondrial cells in this guy's body, to get a - to complete sequence of the mitochondria?
Prof. RICHARDS: Yes, that's right. So that one of the other advantages that mitochondrial DNA, is because it's out in the mitochondria, and there's a lot of mitochondria in the cell compared to one nucleus than you get from many more copies of mitochondrial DNA per gram of tissue, than you would get from nuclear DNA.
So, it's easier to retrieve ancient mitochondrial DNA, than it is any other source of DNA, and a lot of the work that's been done on ancient DNA has been done firstly on mitochondrial DNA.
PALCA: So, you looked at his DNA and found what?
Prof. RICHARDS: Well, so when we analyzed the sequence of the Iceman's mitochondrial DNA sequence, which was a very sort of long-winded and difficult process, because it's - even for the Iceman who's, you know, exceptionally well preserved and not very old, so in evolutionary terms is 5,000 years ago. It's not so long ago, but he still had very little DNA left.
But when we looked at his sequence and how it fitted in with the rest of the variation of the human species, we found that although he's a member of a cluster of - genealogical cluster of lineages that is found in about six percent of Europeans today. He actually belongs to a branch of that cluster which is either not seen anymore, or at least must be very rare.
So looking at the database that we have so far of modern people, we couldn't find anybody with that branch. So most people fit into one of four different clusters within this particular lineage, which amounts to six percent of Europeans, and the Iceman is in an extra - in a further cluster which - and he seems to be on his own now.
So, one possibility is that perhaps that lineage has actually gone extinct, or another one is that it's perhaps always just been very rare and maybe, you know, somewhere out there, there are people still with that lineage today. But we haven't seen them yet.
PALCA: Yeah, yeah. I suppose that - well, those both seem like viable explanations. Why don't we take a call now from Adam in Chicago? Adam, welcome to Science Friday.
ADAM (Caller): Hello. How are you?
ADAM: I was wondering if based on the evidence that you've gathered, if it has any relevance to evolution, and perhaps maybe the reason that you can't find any links to modern genome as a means to evolution perhaps?
PALCA: Hmm, you mean - interesting question. I mean, the idea of being that - does this actually tell us anything about where humans came from. I would think this is just one example, so probably not.
Prof. RICHARDS: Not really, because the Iceman - as I said - the Iceman is so recent, so we (unintelligible) - you know, humans are - modern humans, people who look like us had been around for 200,000 years, and the Iceman's only 5,000 years ago and long after Europe had been colonized for the first time.
So, there's nothing really to be learned from just looking - you know, about general principles of human evolution, except for the one thing that - you know, that's come out of this which is that we found a lineage that may have gone extinct. And if it hasn't gone extinct, it's certainly not very common in the modern population. And that might be a kind of lesson.
So, normally because it's so difficult to get DNA out of ancient specimens, nearly all the inferences that we make in archeo-genetics are from modern samples, and this does tell us that if lineages have been going extinct over time, which, you know, theoretically we know they must have been, then we are going to miss things like this by only tracing our variation back from modern individuals.
PALCA: Are there any other examples of bodies that are preserved enough to do this kind of analysis from around this time, 5000 years ago, or is he - is the Tyrolean Iceman the only one?
Prof. RICHARD: He's certainly - in Europe at least he's fairly unique, but there have been mummies found in Siberia as well, and one of - in fact, one of those has been looked out with this - with the same sort of technique. Ad again they found a lineage there which doesn't exactly match up with any of the modern lineages from the area.
So, that might be this precedented (unintelligible) now for that kind of situation, where we're seeing something a bit new. And I would imagine that there would be a few more, but perhaps particularly in Siberia. I don't think in Europe we're going to find any more like that.
PALCA: Hmm. Any chance they'll be able to get enough of his Y-chromosome to start saying where his children, if he had any, came from, or whether there are any of those left?
Prof. RICHARDS: Well, it's possible. Yeah, it's possible. So, it's - it remains to be seen whether there's enough Y chromosomes - as I was saying, you only get one copy of Y-chromosome DNA, and most of the other genes, to ever - to very many copies, hundreds or thousands of copies of mitochondrial DNA, because of the way - where they're found, but it's still possible that it might be possible to retrieve some Y-chromosome information, and then we can do the same process. You can - because the Y chromosome is inherited down the male line of descent, it's a sort of male counterpart to the mitochondrial DNA which gives you the female line of descend.
So, it would be extremely nice to be able to also look at the Y chromosome, and then of course the - we would be able to look directly at whether there were any direct sort of links to the Iceman himself. Because he would never have passed on this mitochondrial DNA, because...
Prof. RICHARDS: But certainly inherit it.
PALCA: Got that. All right. Well, I'm afraid that's all the time we have. Thank you very much, Martin Richards, for coming on the program.
Prof. RICHARDS: Thank you very much for inviting me.
PALCA: Martin Richards is a professor of archaeo-genetics at the University of Leeds in the U.K.