Monday, December 3, 2012

Ancient Lemming DNA Reveals Local Extinctions

Ancient lemming DNA from the North-West European tundra reveals a series of apparently climate change driven regional extinction events in the same time period that modern humans were expanding out of Africa.  It also provides better resolution of what was going on in terms of the survival of smaller animals during the era of megafauna extinction in Europe.
The Late Pleistocene global extinction of many terrestrial mammal species has been a subject of intensive scientific study for over a century, yet the relative contributions of environmental changes and the global expansion of humans remain unresolved. A defining component of these extinctions is a bias toward large species, with the majority of small-mammal taxa apparently surviving into the present.  
Here, we investigate the population-level history of a key tundra-specialist small mammal, the collared lemming (Dicrostonyx torquatus), to explore whether events during the Late Pleistocene had a discernible effect beyond the large mammal fauna. Using ancient DNA techniques to sample across three sites in North-West Europe, we observe a dramatic reduction in genetic diversity in this species over the last 50,000 y. We further identify a series of extinction-recolonization events, indicating a previously unrecognized instability in Late Pleistocene small-mammal populations, which we link with climatic fluctuations.  
Our results reveal climate-associated, repeated regional extinctions in a keystone prey species across the Late Pleistocene, a pattern likely to have had an impact on the wider steppe-tundra community, and one that is concordant with environmental change as a major force in structuring Late Pleistocene biodiversity.
Via John Hawks referencing Brace, S, Palkopoulou, E, Dalén, L, Lister, AM, Miller, R, Otte, M, Germonpré, M, Blockley, SPE, Stewart, JR, Barnes, I, "Serial population extinctions in a small mammal indicate Late Pleistocene ecosystem instability." (2012).

Implications For Our Understanding Of The Upper Paleolithic Megafauna Extinctions

This study disfavors the crude "overhunting hypothesis" of megafauna extinction as a sole explanation for it.  Instead, it suggests that modern human predator activity may have been just one factor that lead to megafauna extinctions of species already weakened by climate change driven impairments of the extinct species environments, which smaller species survived to recolonize their old range in refugia which were not available to larger species for some reason.

On the other hand, the abstract for this study, at least, doesn't discuss the role that predators play in helpfully regulating prey species population levels.  So, a scenario in which ecological collapse works its way from the top of the food pyramid to the bottom driven by climate change, rather than from the bottom up, isn't excluded.

A Technological Miracle

As an aside, the ability of the researchers to secure and sequence ancient lemming DNA from tens of  thousands of years ago is itself a technological marvel.  This is one of a number of ancient animal DNA studies that while not providing direct evidence regarding modern human population history, does provide very solid evidence to distinguish between plausible and implausible inferences from the limited available ancient hominin DNA evidence about what happened in human prehistory.

Many magical legends about people who could learn things about the past from touching objects that were there have been less ambitious.

The Relevance Of Ancient Animal DNA More Generally

Animals have always outnumbered humans and all but a few animals have shorter lifespans on average.  So, it is very likely that there are more recoverable ancient animal DNA sequences out there than there are ancient hominin DNA sequences.

Ancient animal DNA studies have focused on domesticated animals, such as pigs and cattle, and this has provided valuable insights about human prehistory since the Neolithic revolution.   But, those studies can't inform the tens of millenia during which modern humans were exclusively hunters and gatherers.  Wild lemmings, in contrast, can tell us quite a bit about the environment in which pre-Neolithic revolution modern humans lived.

For example, this study would suggest that any model of modern human demographic prehistory that assumes stability or steady, slow expansion, as opposed to instability with periods of regional extinctions followed by recolonizations (possibily in addition to the one known case at the last glacial maximum) may be deficient in accurately modeling human prehistory.

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