This is worth a quick note, both for the new methodology and the mildly interesting results. A new article in Proc. Nat. Acad. Sci. USA reports the use of sedimentary ancient DNA (“sedDNA”) analysis to show that both the wooly mammoth and the horse survived in North America several thousand years later than previously thought. (Horses, which evolved largely in North America, went extinct around time that humans arrived here from Asia, and were later re-introduced in domesticated form by colonizing Europeans.)
Using fossils to determine when a species went extinct always yields a date older than the target date, for it’s unlikely that the latest fossil is from the last remaining population (this phenomenon is known in paleobiology as the Signor-Lipps Effect).
To get a better handle on extinction of the North American megafauna, Haile et al. simply extracted DNA from the permafrost at a site on the Yukon River floodplain in central Alaska, and, by sequencing, assigned it to various animals groups that became extinct. By correlating the DNA sequences with the date of the permafrost layer from which they were taken, the authors could determine the most recent time the carriers of that DNA lived. (Sediment dating was done by the measurement of optically stimulated luminescence [OSL], a recently devised way of using the luminescence of minerals to determine when they were last exposed to sunlight.)
Halle et al. found this: both horses and mammoths persisted in Alaska up to a time between 10,500-7,600 years b.p.
This is several thousand years later than the previous “extinction dates” based on fossils themselves, about 13,000 and 14,000 years b.p. for mammoths and horses, respectively.
What does this mean? Well, the earlier coincidence between fossil-based extinction of these large beasts with the arrival of humans in North America (about 14,000 years b.p.) had led to the conclusion that humans quickly hunted these beasts to death, or that they went extinct due to rapid climatic changes during the late Pleistocene. It could still be true that the extinction of these beasts was promoted by humans, or by climate, but that this didn’t take place so quickly after human contact. As the authors say,
. . .the sedaDNA evidence for mammoth and horse persisting into the Holocene in interior Alaska is incompatible with such rapid extinction and indicates that late-surviving mammoths in the New World were not confined to islands in the Bering Sea that might have afforded protection from human hunters (10, 34). The protracted survival of mammoth and horse is also inconsistent with the hyperdisease hypothesis (5) (which requires their swift demise following human contact) and with megafaunal extinction due to end-Pleistocene environmental changes associated with abrupt climatic events (35), altered vegetation patterns (2), or intense wildfires sparked by a presumed extraterrestrial impact (6, 7).
Here’s a nice graphic from the paper:
Fig. 1. (caption taken from the paper): Stratigraphic profile and location (see inset map) of the Stevens Village site. Elevation is height in meters above river level, and age ranges (in calendar years) are shown at the 95 and 68% confidence intervals for radiocarbon (14C) and OSL, respectively. OSL ages were obtained from quartz sediments and 14C ages from plant macrofossils. Inset photo shows detail of buried vegetation (with arrow at shrub root) and lateral continuity of paleosol at 5 m elevation. The mammalian taxa identified from sedaDNA sequences are shown by symbols, with the scientific names given in Table 1.
I’m not a paleobiologist, nor an expert in dating, but the authors did deal with several possible problems, for example the objection that older DNA could simply have been washed into younger sediments.
Haile, J. et al. 2009. Ancient DNA reveals late survival of mammoth and horse in interior Alaska. Proc. Nat. Acad. Sci. USA. 106:22352-22357.