29 March 2010
Overkill or Overchill?
15,000 years ago the northern regions of the planet, from the coast of Portugal, right across Eurasia, and into the New World, were inhabited by a diverse group of large mammals. This included now extinct animals such as the woolly mammoths and rhinos, as well as others that we still find today, such as horses, bison and reindeer. While many of these species vanished forever, and others, like the reindeer survived relatively intact, some, like the musk ox, held on, but only just. Scientists have argued about the causes of these extinctions and population reductions for over a hundred years. Was it climate change, wiping out those that couldn't cope? Or was it the arrival of humans into these regions, with their voracity for killing, that pushed the lost species over the edge? An international team of scientists led by the University of Copenhagen, using advanced computational techniques to analyse a large 'ancient DNA' dataset recovered from ancient musk ox bones - some spanning back over 60,000 years in age, have come closer to helping us understand the causes. In particular, they show that at least for the musk ox, we humans aren't to blame.
DNA studies have matured
Associate Professor Tom Gilbert, and Dr Paula Campos, from the Centre of Excellence in GeoGenetics at the Natural History Museum led the international team of scientists responsible for the findings.
- "Over the past decade ancient DNA studies have matured, moving away from simple questions of identity - what species is this bone? Or where do we find species X? - to actually being able to give us insights into the population size and dynamics of animals, stretching back over the last 100,000 years", says Dr Gilbert. "Thanks to significant computational developments made by colleagues of ours, this gives us the fantastic opportunity to literally watch what happened to the ancient populations. When did they increase, or decrease, and at what rate?"
Why the musk ox?
Use of these methods is not trivial - good quality reconstructions rely on the recovery of DNA from a suitably large number of ancient samples, in particular bone, distributed back in time. DNA is a fragile molecule, and rapidly degrades away, so many ancient samples can't be studied in this way. However, DNA is preserved better in cold environments, so species that inhabit the northern extremes of the world can offer the possibility. And hence why the group focussed on the musk ox.
- "Our work was very much inspired by two previous studies, on the bison and mammoth" says Dr Campos. "These studies represented two key groups of large herbivores - those like the bison that survived in relatively large numbers, and those like the mammoth that died out completely. What we wanted to do was investigate the middle case - a species that used to be very common, and that survived, but only by the skin of its teeth". She adds. "This, is the musk ox". While once common all over Eurasia and the North of the Americas, they are only naturally found now in the Canadian Arctic and Greenland. Why?
Human or climate influence?
"We know that palaeo-eskimo cultures were fond of them" says Dr Gilbert "In a culinary manner" he adds with a grin. Archaeological sites, such as those from the Greenlandic Saqqaq culture, are full of butchered musk ox remains. So was their loss elsewhere due to this hunting pressure? To find out, Dr Campos and key collaborators, Professor Eske Willerslev (Copenhagen) and the late Professor Andrei Sher, Russian Academy of Sciences, scoured museums for remains, bringing back hundreds of bones from all over the northern hemisphere.
And then the hard work began. Dr Campos had to analyse the remains, to find out which contained DNA, eventually teasing out 150 ancient DNA sequences. Subsequently, she, and key collaborator Dr Beth Shapiro, at Penn State, could model the population history, noting it to be significantly different from the previous mammoth and bison data. Could any link be drawn between the pattern, and either climatic records or evidence of humans?
- "For this species at least, the data is pretty conclusive" says Dr Campos. "It wasn't humans driving the population changes. This is the first direct evidence that humans were not to blame for the extinctions". Not only did the team find that some regional populations of musk ox vanished at least twenty thousand years before humans even entered the region, but elsewhere, humans arrived without denting the population at all. Probably the most striking finding, however, is that as the environment changed to enable it, humans and musk ox entered Greenland almost simultaneously about 5000 years ago. "In go the humans, and up rockets the local musk ox numbers" says Dr Gilbert. "While they may have been edible, the first Greenlanders certainly weren't eating enough of them to matter".
Given that humans weren't to blame, the team explored various other options. "Climate seems the most likely reason" says Dr Gilbert. "In particular, climate instability. We know that musk ox do well in the cold - one might call them the only large herbivore that is left, that is truly adapted to the far North" adds Dr Campos. "But even from historical records we can see that what they don't cope well with, is relatively rapid climate change. Over past periods of climatic instability, their populations were seen to decline, for example."
"Given the current climatic instability, we wonder how this will effect their survival in the near future" adds Dr Gilbert. "There's lots in the news about the plight of polar bears, but musk ox may be similarly at risk".
The authors acknowledge that this is just a single data point - one species, perhaps with particular biological characteristics that rendered it particularly sensitive to such change. "The results are markedly different to the bison and the mammoth, and as future studies are published on other large mammals, we will see whether there are any patterns" says Dr Gilbert. "It would be nice to say that humans aren't always the bad guys - but I guess only time will tell if this is the case!"
The results of the team's research is published in a leading American scientific journal Proceedings of the National Academy of Sciences of the USA
- Tom Gilbert, Paula Campos, Eske Willerslev - Centre for GeoGenetics, University of Copenhagen, Denmark
- Beth Shapiro, Penn State University, USA
- Andrei Sher, Russian Academy of Sciences
- Ludovic Orlando, University of Lyon 1
- Marc Suchard, UCLA, USA
- Philippe Lemey, Catholic University, Leuven, Belgium
Associate Professor Tom Gilbert, tel.: +45 5189 1330 or
Dr Paula Campos tel.: +45 5057 0928