Thursday, July 17, 2008

Neandertals and moderns may not have mated

That, at least, is the implication of a new ancient DNA study my partner on Science's anthropology beat, Ann Gibbons, reports about today on our ScienceNOW online news service. The link is free for four weeks from today, so please click on it right away. The gist of the story is that an Italian team has sequenced mitochondrial DNA (mtDNA) from 28,000 year old Cro-Magnon bones, and that it does not show any similarities to Neandertal mtDNA previously analyzed. Cro-Magnons were modern humans, Homo sapiens, and lived in Europe around the time that Neandertals were on their way to extinction. Nevertheless, there were thousands of years of overlap between the two groups (most experts today consider them separate species), and it is possible that some of them interbreeded. While Neandertal DNA does not resemble that of people living today it is always possible that Neandertal sequences in the modern human genome were lost over time.

Here is the key paragraph:

In the new study, Caramelli's group isolated mtDNA from a different set of Cro-Magnon remains that had been found in the same cave in 2003. This time, only seven people handled the fossils, and the researchers verified that their DNA did not match that of the purported Cro-Magnon sequence. "In this case, we knew all the people that touched the bones," says team member Guido Barbujani, a population geneticist at the University of Ferrara, Italy. The team also had the work independently replicated, asking a lab in Spain to extract and analyze mtDNA from different splinters of skull and long bones. The upshot is that the Cro-Magnon mtDNA matches that of modern humans and does not contain patterns found in Neandertal mtDNA, the team reports online today in PLoS ONE. That result argues against the inbreeding hypothesis, says Barbujani. [[Note that the paper is available free at the "team reports" link]]

But the jury is still out, as Ann says. She quotes an ancient DNA expert who is still concerned about the pervasive problem of modern contamination in ancient DNA sequences.

Credit: Illustration by Knut Finstermeier; Neanderthal reconstruction by the Reiss-Engelhorn-Museum Mannheim


Don said...

Yes, of course it's possible Michael. It's also possible that hell might freeze over and that pigs might fly. It is even possible there is a God. But don't bet on it, because it is not probable. It is not probable that given no match between these seven people who handled the Cro-Mag specimen, the DNA of that specimen and Neanderthal DNA, that other modern humans will still be a match Neanderthal DNA. Or that precisely those sections of Neanderthal DNA code examined were the same as those deleted in the modern human population by genetic drift.

You need to take a reality check as to what evidence you would accept to convince you that Neanderthals were a distinct species to Cro-Mag and modern humans. The evidence is there in this paper and in several others. If you cannot accept evidence then you are not thinking rationally, you are in the realm of belief and faith: not science.

Cheers, Donald McMiken

Michael Balter said...

This would seem to be an overreaction on Don's part. While there is no real evidence for interbreeding and a lot of reasons to suspect it did not happen, there is no slam dunk case. Many reputable anthropologists continue to think it is indeed a possibility, which is exactly why studies such as this continue to be performed. Henrik Poinar, who Ann quotes about contamination concerns, is a noted expert in ancient DNA and if he is concerned then that should be taken seriously.

Also, I stated clearly that Neandertals and moderns were most likely distinct species; that does not bear directly on whether they could produce children together, as the definition of species is still very fluid.

Michael Balter said...

Further to this discussion, here is a short piece by Ann Gibbons on the subject published in Science. It is dated, published in 2001, but gives the basic outlines of the debate. More recently I wrote about work by Bruce Lahn at the U of Chicago suggesting that the ancestors of Neandertals and moderns might have interbreeded in Africa, although this too is open to interpretation.

But Did They Mate?
Ann Gibbons

If Neandertals and modern humans lived in close proximity for thousands of years (see main text), the obvious question is, did they mate· Novelists like Jean Auel, with steamy sagas of brute Neandertals and lissome moderns, have tended to answer with a resounding yes. And some scientists agree: "I think that one thing that was going on was sex," says Fred Smith, a paleoanthropologist at Northern Illinois University in DeKalb. But there's no way to track these Paleolithic trysts--unless they created offspring. Thus, for scientists, the possibility of children is the key issue. Successful reproduction would imply that Neandertals and humans were part of the same species and shared a recent evolutionary history. "I'm not interested in whether Neandertals and modern humans had sex, but whether Neandertals contributed genes to modern humans," says geneticist Svante Pääbo of the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

But even this seemingly more tractable question is hard to answer. Indeed, the species question is so tricky--and the field of paleoanthropology so divided (see sidebar on p. 1728)--that most researchers avoid it, thus creating some nomenclatural chaos. Paleoanthropologist Milford Wolpoff of the University of Michigan, Ann Arbor, who believes that Neandertals and moderns were members of the same species, advises against using those names (although most anthropologists do), because it "makes them separate." Others, such as Harvard University paleoanthropologist David Pilbeam, suggest simply calling them separate populations.

Whatever you call them, those who think the two groups did indeed mate and bear children cite as evidence a 4-year-old child buried in Lagar Velho, Portugal, about 24,500 years ago. The skeleton, says Joao Zilhão of the Portuguese Institute of Archaeology in Lisbon, is anatomically modern but has features inherited from Neandertal ancestors. The mix of inherited features--short arms and a broad trunk like a Neandertal, but a modern-looking chin and pubic bone--implies that this child was not the result of a chance affair and that Neandertals and moderns interbred extensively for many generations, according to a 1999 report in the Proceedings of the National Academy of Sciences (PNAS). Or, as Time magazine quoted co-author Erik Trinkaus of Washington University in St. Louis: "This is not one Neandertal and one modern human making whoopee in the bushes." If Trinkaus is right, Neandertals disappeared because most of their traits were swamped out when they interbred with modern humans, whose population size was much greater. But other paleoanthropologists doubt that the Portuguese boy is a hybrid. Ian Tattersall of the American Museum of Natural History in New York City and Jeffrey Schwartz of the University of Pittsburgh argued in a commentary in the same issue of PNAS that he looks simply like a "chunky" modern human child, "lacking any suggestion of Neandertal morphology."

In the "replacement" view, Neandertals became extinct without fertile offspring. But this debate may never be settled by morphology, partly because there is little consensus on the criteria used to classify Neandertal and early modern human skulls. There is some genetic evidence, however: Studies of the maternally inherited mitochondrial DNA (mtDNA) from three Neandertals show it to be distinctly different from that of living humans, suggesting that Neandertal genes do not survive today and supporting a replacement view (Cell, 11 July 1997, p. 19; Science, 11 July 1997, p. 176).

But a new genetic study of an anatomically modern man who died 62,000 years ago at Lake Mungo, Australia, raises another possibility, according to a report in January in PNAS (Science, 12 January, p. 230). The Lake Mungo man apparently possessed a now- extinct lineage of mtDNA, although this has not yet been confirmed in an independent lab, a step most ancient DNA researchers say is essential. But all researchers agree that the man is anatomically modern and therefore might have contributed genes to living people. If a mtDNA sequence present in an ancient modern human could simply become extinct, then something similar could have happened to the mtDNA of Neandertals. "Then the absence of Neandertal mtDNA in living humans does not reject the possibility of some genetic continuity with modern humans," John Relethford of the State University of New York College at Oneonta wrote in PNAS.

Further complicating the debate is the lack of any genetic yardstick for species definition and the fact that the variation between Neandertals and modern humans falls within the range of mtDNA variation between subspecies of chimpanzees, says Pääbo, whose lab sequenced the Neandertal mtDNA. Another genetic tack is to examine Cro-Magnon mtDNA to see if these modern humans who lived in Europe in the past 40,000 years are ancestral to living Eurasians. mtDNA has been extracted from two late Cro-Magnons from Gough's Cave in England, but analyses that tie them to recent Europeans have yet to be published in detail, says paleoanthropologist Christopher Stringer of the Natural History Museum in London. In the meantime, barring the discovery of a cave artist's depiction of the event, there is no consensus on whether Neandertals and modern humans mixed it up.

Don said...

It is not an over-reaction it is a reaction to irrational thinking that is endemic in science and particularly archaeology and anthropology. We are already on the wrong side of the equation in this discussion, the core issue is this:
Where is the evidence that Neanderthals and Moderns are the same species? There is none. Those you cite, despite their high profile, provide only weak supposition. There is nothing on your side of the argument with strong inference: nothing basic like the huge and distinctive skeletal differences which Chris Stringer, Mellors and others cite as evidence for distinct species.
The sources you cite, the would be emperors (of what I shall call the American school of admixture) have no clothes.

Cheers, Don

Michael Balter said...

Wow, Don is showing a serious failure to read what I am writing, a sure sign of overreaction. I have not said anywhere that Moderns and Neanderthals were the same species; what I said is that it remains possible that there was some interbreeding between them, even if they are two species. Read over what I said, and read the literature carefully. Over and out with Don, but interested in any other comments.

Anne Gilbert said...

I do not think that Neandertals and "moderns" were two separate "species", though I am willing to accept firm evidence that they were. Because of the continuing possible contamination issue, I can't accept these claims as "firm proof". Unless they can get the sequencing done totally by robots, that is. Be that as it may, even if Neandertals and "moderns" were separate species, this would not have prevented interbreeding, and it is possible that this is exactly what the Lagar Velho specimen suggests. There are any number of organisms classified as separate species, that happily interbreed --- and produce fertile offspring. It is well known that in certain parts of North America, wolves x coyotes produce fertile offspring; all of the "wolves" that live on Isle Royale, Michigan, seem to be descended from the matiing of some female coyote with some male wolf. And they act like wolves(though wolves and coyotes have behaviors as similar as those Neandertals and "moderns" seem to have had). Not only that, but if I walk some four blocks from where I live, and sit at the beach for a while, I will see gulls that are hybrids of "Western" gulls(Larus occidentalis) and "glacous winged" gulls(Larus glaucescens). All the gulls around here are considered hybrids between these two species. Based at least in part on the Lagar Velho evidence, such as it is, some paleoanthropologists and archaeologists think that the Iberina Peninsula may have been a hybrid zone(at least for a while), between incoming "moderns" and resident Neandertals, at least until enough "moderns" showed up to "swamp" any Neandertal genes, since N's were apparently a rather small population to begin with. Western Washington is similarly considered such a "gull hybrid zone", though the two species of gulls are more or less equal in size.
Anne G

terryt said...

I think Don missed this bit: "the variation between Neandertals and modern humans falls within the range of mtDNA variation between subspecies of chimpanzees". And it's impossible to get hybrid mtDNA anyway.

I agree with both Michael and Anne. We can't jump to conclusions. By the way Anne. Like your picture of Neanderthal girl on your blog. Looks very human, even attractive!