Mammoth DNA in Ancient Dirt

December 16, 2009

What can a pinch of dirt from the Alaska permafrost tell us about the extinction of mammoths and prehistoric horses? An awful lot, says an international team of researchers headed by James Haile, a geneticist at the University of Copenhagen. By sequencing ancient mitochondrial DNA from soil samples and dating the soil, Haile and his colleagues concluded that both mammoths and ancient horse species were still grazing Alaskan meadows some 7600 to 10,500 years ago–at least 2500 later than other research suggests. Their findings have just appeared in an online paper in the Proceedings of the National Academy of Science

Haile and several of his colleagues have a long-standing interest in the subject of ancient DNA, and whether this fragile molecule can really survive degradation over thousands of years in geological layers. Remember, we are not talking about ancient DNA encased in animal teeth or bone: Haile and his colleagues are searching for ancient molecules from urine and faeces in the soil.

Here’s what Haile’s team did in this new study. They collected permafrost core samples from the tundra near Stevens Village, and dated the layers in the core by two methods: C14 dating and optically simulated luminescence. Then the team sequenced the ancient mitochondrial DNA in the layers. In the stratum dated between 7600 and 10,500 years ago, they discerned the ancient DNA of woolly mammoth, prehistoric horse, moose, and snowshoe hare. In upper layers dated to more recent times, they found moose, hare, and the like, but no trace of woolly mammoth or prehistoric horse.

Team member Eske Willerslev, an ancient DNA expert at the University of Copenhagen, sees this as the beginning of a whole new era in our studies of the ancient megafauna and their mysterious demise. “With ancient DNA analysis,” Willerslev said in a prepared statement, “we are completely independent of skeletons, bones, teeth, and other macrofossil evidence from extinct animals. This greatly increases the possibility of finding evidence of the existence of a species through time.” Indeed, the team has coined a new term for this: they now talk of identifying “ghost ranges” for the animals.

All this sounds extremely interesting and exciting. And if Haile, Willerslev and their colleagues have it right, researchers will definitely need to rethink their theories about the demise of the mammoths and other large megafauna. The team’s new proposed extinction dates would not mesh in any way with the arrival of human hunters in the Americas or with a proposed comet strike.

But I confess I am skeptical. The validity of dirt DNA, for example, still seems to be a hotly contested issue among ancient DNA experts. Researchers are still debating, for example, the authenticity of ancient human DNA extracted from fecal material found Paisley Cave in Oregon, evidence that was used to advance the case of Pre-Clovis humans in the New World.

Big claims require big evidence. Haile and his colleagues have now put out the idea that researchers can abandon the quest for skeletal evidence and simply take soil samples to pinpoint the demise of the mammoths. Let’s see other research teams duplicate his findings.

Iron from the Sky

December 11, 2009

Serge Lebel’s discovery of small meteorites in a 200,000 year-old site in France has got me thinking once again about the critical role that other such space debris has played in human history. So I took another look last night at a wonderful paper that Robert McGhee, a former curator at the Canadian Museum of Civilization, and one of the world’s great experts on ancient human cultures in the Arctic, wrote about the influence of the Cape York meteorite on Arctic history.

According to Danish metallurgist and meteorite expert Vagn Buchwald, the Cape York meteorite produced the largest shower ever recorded. Falling to earth in northern Greenland, the ten known fragments littered a strewnfield measuring 100 km NW-SE. The largest of the iron-rich masses weighed 30 tons, the smallest some 250 kg.

When Europeans first arrived in northern Greenland, they learned of these meteorite chunks from indigenous Inuit hunters of the region. The Inuit regularly travelled to the fragments to break off pieces of iron, which they then cold-hammered into a host of immensely valuable tools, including chisels, blades, gravers and pegs. The Cape York fragments were their sole source of iron, and they so treasured them that they gave them names such as Ahnighito, an Inuit word meaning “Tent”, and incorporated them into their mythic tales. (Ahnighito is now in the collection of the American Museum of Natural History.)

Now here is where McGhee’s ideas come into play. Archaeologists have long known that the ancestors of today’s Inuit originated much further to the west, quite likely along the coasts of the Bering Sea. But some time in the 12 century A.D., these ancestral Inuit, known as the Thule, migrated swiftly into the eastern Arctic. The big question has long been what drew them eastward so quickly? Over the years, researchers have proposed a variety of theories, from climate change (the migration coincided with the Medieval Warm Period) to sharp increases in Thule populations.

But McGhee proposed a very different theory. He suggested that the Thule hurried into the Eastern Arctic in order to lay their hands on a major source of precious iron. In all likelihood, he suggests, the Thule had earlier acquired bits of iron by trading across Bering Strait, but this would have been a drop in the bucket compared to the wealth of iron in the Cape York fragments. Moreover, it seems likely that the Thule learned about this iron source from an another Arctic culture, the Dorset, who were lightly scattered across the region. In McGhee’s view, this knowledge would have been sufficient to lure some bands eastward.

If the Canadian archaeologist is right, the course of Arctic history was altered forever by a hunger for iron, and metal from the sky.

Did Ancient Humans Witness An Asteroid Explosion 200,000 Years Ago?

December 10, 2009

A few weeks ago, I received a very surprising press release from a Quebec archaeologist announcing evidence from the south of France pointing to a 200,000-year-old asteroid explosion. Serge Lebel, a former associate professor at the University of Quebec at Montreal and now an independent researcher, is the principal investigator of a major Neanderthal site, Bau de l’Aubesier, in Provence. He now contends that he has found clear proof at the French rockshelter of a previously unknown extraterrestrial event.

I met Lebel in the early 1990s, when I spent a week at Bau de l’Aubesier in order to interview him and his team for a feature article. Lebel struck me at the time as a cautious researcher: he was very reluctant to speculate in any way, or to go beyond the evidence he had in hand. And since then, he has made some important contributions to science. In 2001, for example, he was the lead author of a major paper in the Proceedings of the National Academy of Science (PNAS) on Middle Paleolithic human remains excavated at Bau de l’Aubesier. The research seemed to be unfolding in a conventional way.

So I was a little taken aback by Lebel’s announcement. We have now exchanged several emails on the subject, and this, in brief, is what I have learned. The evidence for an asteroid explosion comes from H1, a black, continuous 40-cm-thick layer at the site. Lebel has long mulled over the origins of this layer: it seemed far too thick and extensive for hominin hearths or fireplaces. Now he has discovered 10 meteorites (all less than 2 centimeters in diameter), micrometeorites, and metal droplets in situ in the layer. In addition, H1 has produced flint tools coated with a silvery-colored deposit consisting of chromium, iron, aluminum and titanium. The chemistry of this silvery deposit, says Lebel, “is the same as the extraterrestrial material.” And both the limestone deposits in the shelter and artifacts in the black layer show, he says, “the effects of an intense heat (over 4000 degrees Celsius.)”

Lebel contends that this evidence points to the explosion and fragmentation of an asteroid as it entered the earth’s atmosphere, sending a hail of meteorites to the ground. And he proposes, furthermore, that “human populations of this era were witnesses to this event.” When I asked him if any researchers working on other sites dating to around 200,000 years ago had found similar evidence, he noted that Bau de l’Aubesier is currently unique. “The 180,000 to 200,000 years ago period is not well-documented around the world,” he added. “And at the geological time scale, the event is a ‘precise moment’ not always recorded in cave infilling.”

If I had never met Lebel or spent time at the excavations at Bau de l’Aubesier, I would have just deleted his first announcement. Other archaeologists have turned up evidence of another purported extraterrestrial event–the possible explosion of a comet 12,900 years ago that may have triggered global cooling, the extinction of certain species of megafauna and the demise of the Clovis culture. But other research teams now strongly contest these claims. Today, for example, an international research team led by Francois Paquay, a geologist at the University of Hawaii, casts new doubt on the comet strike theory in an online paper in PNAS. Paquay and his colleagues could not find any evidence of extraterrestrial debris in the 12,900-year-old layers they tested in North America.

All this, of course, is good science, as researchers test the comet-strike idea by searching for extraterrestrial evidence in their own data. I’d personally like to see Lebel publish his findings soon in a good journal, so that geochemists and others can take a good look at it there.

-Heather Pringle