A mammoth divide

A new study of preserved woolly mammoth hair confirms that there were once two distinct clans of the extinct creature in Siberia. Comparisons of the genome sequences of the animals show that the mammoth groups diverged more than 1 million years ago and that one group went extinct long before the other.

“It’s pretty standard dogma in mammoth circles that there was only one species of mammoth in Siberia,” says Thomas Gilbert, a paleontologist at the University of Copenhagen. “DNA is hinting this might not be so.”

Tufts of wooly mammoth hair hold the key to understanding the life and death of the last Ice Age’s most iconic species. Scientists can now read the complete DNA sequence of the mammoth’s mitochondria – the energy factories of animals’ cells – from relic hair follicles, which were recovered from the Siberian permafrost.

By studying these sequences, Gilbert and his colleagues separated the remnant DNA into two distinct evolutionary groups, called clades. The team sequenced five new mitochondrial genomes and analyzed 13 existing mitochondrial genomes from samples from 18 individual mammoths. The data show that the clade II mammoths vanished from Siberia 30,000 years before the members of clade I.

The findings appear this week in the online edition of the Proceedings of the National Academy of Sciences. The work marks the first time that researchers have successfully extracted and read a complete mitochondrial sequence from the ancient, frozen sample of mammoth hair and then applied the data to draw conclusions about the populations of an extinct creature.

Studies on the population dynamics of animals from the Pleistocene period have been limited because DNA sequences from samples were short, too short to draw conclusions about population dynamics and divergence, says Michael Knapp, an evolutionary geneticist at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

“In terms of understanding the mammoth’s biology and extinction, the paper is a significant step forward,” Knapp says. Gilbert and his colleagues demonstrate that, with new sequencing technology, it will soon be possible to reconstruct population dynamics of extinct and extant animals and trace their population development, he adds.

Based on the locations where fossils from each mammoth were found, Gilbert and his colleagues hypothesize that the clade II mammoths had a much more restricted geographic range compared with the clade I animals. Clade I members roamed from western Russia, across Siberia, to North America, an area with a span greater than 6,000 kilometers, the scientists say. But clade II mammoths appeared to roam only on an east to west range of about 1,100 kilometers in northern Siberia, the team reports.

The team also reports that the genomes of clade I and clade II mammoths show no functional differences. But the researchers were able to analyze the genomes in terms of evolutionary development and phylogeny. The statistical data show that the clades diverged 1 million years ago, but that the groups appeared to have coexisted some 50,000 years ago.

Gilbert says, in addition to having a larger range, clade I could have outlasted clade II because of population size — if more members of the first group existed and left more descendents than the clade II mammoths. Or, he says, clade I might have had some genetic advantage over the other group.

“The reasoning is open to speculation right now,” Gilbert says. But, if the groups were different subspecies, “we can at least say clade II were not driven extinct by humans. The extinction was way before humans arrived” in this region of Siberia.

This theory is of interest because many paleontologists argue that the mammoths and other large megafauna of the late Pleistocene, the last Ice Age, went extinct, in part, because of hunting pressure from humans.

“If these mammoths in clade II are a subspecies,” Gilbert says, “it clearly couldn’t be humans” who killed all of them.

Knapp also commends Gilbert’s team for presenting clear, testable hypotheses to explain their observations for why there appears to have been two groups of mammoths.

“We want to explain why they might have been living together,” Gilbert says. “It might just be that one group was much older and thus different. Or alternatively the groups might have been two different subspecies.”

Or, he says, the observation could be telling scientists that there are two groups of mammoths that evolved in different places and then came back together to make one population. The scientists cannot draw any conclusions until they do further testing, and “we need nuclear DNA to answer this,” Gilbert says.

Nuclear DNA has 100,000 times more information than mitochondrial DNA. That is why scientists want to study the nuclear material. But, despite the information it holds, nuclear DNA is also harder to obtain and accurately sequence from extinct species.

Testing these hypotheses, Knapp says, should soon be possible because the amount of available sequence data and the technology is rapidly increasing. Having that information, he says, will “greatly contribute to our understanding of the life and death of this iconic Ice Age species.”