Friday, December 18, 2009

More recent mammoth extinction

An interesting new technique indicates mammoths and other Pleistocene megafauna may have survived on the mainland much later than previously thought. The technique and especially these results are sure to be hotly debated.

Till now, the last populations of mainland mammoths, as indicated by macro-remains like bone, ivory, and hair, have been dated to around 12,900 calendar years ago. I emphasize mainland because at least two groups of mammoths are known to have survived past that date on islands--Wrangell Island north of Siberia and the Pribilof Islands in the Bering Sea. No evidence of human habitation has been found on these islands that have dates earlier than the extinction of the mammoths. This correlation supports the theory that hunting by humans was the primary cause of their extinction. A mainland date that has mammoths living side by side with humans would throw a serious monkey-wrench into the hunting hypothesis. Any date younger that 12,900 years would strike an almost fatal blow to the hypothesis that the comet believed exploded over North America at that time was the cause of extinction.

A major problem in dating the extinction of any species is that it is virtually impossible to find the remains of the very last individual of a species. Any date we come up with will be approximate. All we can say with confidence about the 12,900 year date is that mammoths went extinct not long after that. How long is guesswork. This uncertainty even has a formal name, the Signor–Lipps effect. A new technique, which involves teasing isolated strands of DNA out of wind-blown soil, is being tried out to see if it can narrow the range of Signor–Lipps uncertainty for Pleistocene megafauna. This technique has come up with dates showing mammoths and North American horses present at least two thousand years more recently than the 12,900 year date. That is a pretty long "not long" later.

Ross MacPhee, a well-known participant in the debate over the causes of mammoth extinction, along with Eske Willerslev of the University of Copenhagen, Richard Roberts of the University of Wollongong, Australia, and Duane Froese of the University of Alberta developed this new technique. They figured that, since permafrost preserves macro-remains so well, it must also preserve microscopic bits of organic matter. During its lifetime, every mammal sheds massive amounts of dander, hair, urine, and other effluvia that contain its DNA. However, it only leaves one skeleton. Statistically, it should be possible to find many more micro-remains of a species than of macro-remains. Having more samples should then allow us to narrow the range of Signor–Lipps uncertainty. Till now, no one had thought to look for mammoths, in the form of these micro-remains, by examining dust under a microscope.

To test the technique, the team went to Stevens Village, Alaska, a tiny town on the Yukon River north of Fairbanks. Because of permafrost, Arctic rivers tend to be shallow and to form very wide flood plains. To obtain samples for their test, the team needed to find a location where airborne sediments were laid down over a long period of time and not disturbed by flooding of other processes. They also needed for the location to have remained dry ever since deposition to avoid degradation or contamination of the samples by liquid water. They believe that a bluff above Stevens Village fills all of those requirements.

The sampling consisted of cores taken from fifteen levels of the bluff and numerous control samples from the surrounding area. The mineral content of the soil is consistent with dust stirred up from the flood plains along that stretch of the Yukon River. In the lab, twigs and roots were dated through radiocarbon and quartz grains were dated using optically stimulated luminescence (OSL), a method that determines when the quartz was last exposed to sunlight. The amount of DNA in the soil was amplified using the polymerase chain reaction (PCR) technique. This gave them enough DNA to determine which species deposited DNA at each level in the soil column, but did not allow for direct dating of the DNA. The dates of deposition were entirely determined from the radiocarbon and OSL tests of the surrounding strata. The results of these tests indicated that the soil column was deposited from around 11,000 years ago to a little more recently that 8000 years ago. Curiously, DNA from extinct mammoths and horses were only found in the most recent strata.

From these results, MacPhee and his co-authors conclude that at least a small population of mammoths and horses survived in central Alaska far later than previously believed. These conclusions favor a slow extinction scenario, such as succumbing to gradual environmental change possibly augmented by the introduction of a new predator (us). It argues against more sudden extinction scenarios like a bolide strike, a hunter blitzkrieg, or MacPhee's preferred explanation, a cross-species plague.

Because their data produced dates so dramatically later than any previously accepted dates, and because they challenge three of the four leading contenders to explain the end Pleistocene extinctions, they will almost certainly stir up a controversy. Not that there was not enough controversy on this topic already. I can see three areas of vulnerability. Their argument that younger strata could not have been contaminated by DNA from older sources is not bullet-proof. The dating of the soil column does not show an unambiguous procession from older to younger. The fact that the DNA itself was not dated will leave many unsatisfied.

On the other hand, if their dates survive the coming controversy, it will require rethinking common wisdom in a number of areas. The extinction narrative is only the most obvious. If islands of mammoth steppe survived two to four thousand years later that previously believed, we are going to need to rethink our ideas of early Holocene environmental history. This could even have an impact on cultural anthropology and folklore studies. If mammoths survived much closer to the present than we thought, should we take another look at those Native American legends about giant grandfather beasts?

The technique has great promise, but it also needs to get over some serious hurdles before its results should be widely accepted.

Note: Proceedings of the National Academy of Sciences paper, here. Supplemental data, here. The Academy deserves brownie points for their open access program of making certain papers available on line, free of charge. They deserve extra, super, golden, brownie points for choosing this paper to include in the open access program this week.

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