It was climate that killed many of the large mammals after the latest Ice Age, including horses in North America.
But what more specifically was it with the climate that led to this mass extinction?
Scientists believe they have found the answer, hidden in a large number of sediment samples from around the Arctic and in the gut content from permafrozen woolly rhinos, mammoth and other extinct Ice Age mammals.
Danish researcher Eske Willerslev and his colleagues, in findings published in this week’s edition of the journal, Nature, have concluded that the common image of a light-brown grass-steppe dominating the northern hemisphere during the Ice Age does not hold any longer.
The landscape was far more diverse and stable than today, and big mammals fed on grasses, but particularly forbs – protein-rich herbaceous flowering plants.
But at the Last Glacial Maximum between 25,000 and 15,000 years ago, at a time when the climate was at its coldest and driest, a major loss of plant diversity took place.
The animals barely survived.
After the Ice Age ended about 10,000 years ago it became warmer again. After the large reduction of plant diversity during the Last Glacial Maximum, another kind of vegetation began to dominate – grasses and woody plants. One of the key food sources of the large mammals – the forbs – did not fully recover to their former abundance.
This likely proved fatal for species like woolly rhino, mammoth, and horses in Asia and North America, the researchers said.
Even though it became warmer again after the end of the Ice Age, the old landscapes did not return.
“We knew from our previous work that climate was driving fluctuations of the megafauna populations, but not how,” said Willerslev, an ancient DNA researcher and director of the Centre for GeoGenetics at the Natural History Museum of Denmark.
“Now we know that the loss of protein-rich forbs was likely a key player in the loss of the ice age megafauna.
“Interestingly, one can also see our results in the perspective of the present climate changes. Maybe we get a hold on the greenhouse gases in the future. But don’t expect the good old well-known vegetation to come back when it becomes cooler again after the global warming.
“It is not given that the ‘old’ ecosystems will re-establish themselves to the same extent as before the warming.
“It’s not only climate that drives vegetation changes, but also the history of the vegetation itself and the mammals consuming it.”
Professor Christian Brochmann, a botanist at the Natural History Museum at the University of Oslo in Norway, said the permafrost contained a vast, frozen DNA archive left as footprints from past ecosystems.
The researchers were able to could identify the different plant species that co-occurred with extinct ice age mammals.
Dr Mari Moora and Professor Martin Zobel, vegetation ecologists from the University of Tartu, Estonia, said ecologists had been able to piece together the characteristics of more complete plant communities occurring in the Arctic during the last 50,000 years for the first time.
The new information showed clearly that the vegetation of the Late Pleistocene was rich in forbs, but lost considerable diversity at the peak of the ice age.
Different plant communities, with grasses and woody plants prevailing, then started to develop during the Holocene.
Dr Pierre Taberlet, an ecologist at the CNRS in France, said the results presented in this paper would never have been obtained without a broad collaboration, which involved 30 teams from 12 countries. It involved the scientific areas of ancient DNA, palaeo-ecology, taxonomy, molecular ecology, community ecology, zoology, bioinformatics, molecular genetics, and geology.
“Whereas competition among scientists often is believed to be the main stimulus promoting global scientific output, this study clearly demonstrates that extensive collaboration is a viable alternative.”
The article in Nature elaborates on the Willerslev group’s results from 2011, where the researchers pointed at climate as the culprit for the mass extinction of some of the large mammals.
But in 2011 the researchers lacked a “smoking gun”. They now believe they have it. The 242 permafrost sediment samples and eight fossil samples from large mammals from around the Arctic have been dated and analysed for DNA.
The result shows that the likely main reason for the mass extinction of the large mammals after the latest Ice Age is changes in the vegetation.