Scientists have glimpsed the woolly mammoth’s last stand at the genetic level.
As big as elephants, but bearing enormous curved tusks and shaggy coats, the ice age beasts once roamed in the millions across a vast northern grassland that stretched from Scotland to the Yukon.
Now a comparison of the DNA taken from the remains of two woolly mammoths uncovered one thousand kilometres apart and separated in time by 40,000 years has provided the most detailed look yet at the population history of the long extinct species.
The analysis, published Thursday in the journal Current Biology, shows that in their final centuries, when mammoths were contemporary with the pyramid builders of Egypt but confined to an island off the coast of Siberia, they had lost a significant fraction of their genetic diversity – the hallmark of a species on the brink.
“They would look like the cheetah to us today – precariously inbred,” said Hendrik Poinar, an evolutionary geneticist at McMaster University in Hamilton, Ont., and a member of the international team that examined the mammoth DNA.
Intriguingly, the study shows that woolly mammoths experienced two population crashes over the past several hundred thousand years. The most recent one occurred at the end of the Pleistocene Epoch some 12,000 years ago. This coincides with the dramatic shift in Arctic climate at the end of the last ice age, a time when mammoths faced a changing habitat and increased exposure to human hunters.
But a much earlier crash revealed for the first time in the genetic data is a mystery. It happened about 285,000 years ago – long before Homo sapiens arrived on the scene – and it does not coincide with any obvious change in global climate. The genetic data show that the mammoth population plunged sharply at that time to about a quarter of its previous size, then bounced back. The revelation adds an interesting complication to scientists’ understanding of mammoth history and suggests the species was genetically equipped to endure a big setback.
“These were resilient creatures,” Dr. Poinar said. “Species have a measure of resilience that I think we continually underestimate.”
But the data also show that the mammoth’s resilience eventually deteriorated. The first of the DNA samples, roughly 45,000 years old, came from a well preserved juvenile mammoth found in the Yakutia region of eastern Siberia. The second was drilled out of a 4,300-year-old mammoth tooth found on Wrangel Island, believed to be one of the last redoubts of the species. For the study, researchers painstakingly reconstructed and compared the whole genomes of the two specimens.
“Without doubt, the most impressive component of the work is the quality and completeness of the data,” said Kevin Campbell, an evolutionary biologist at the University of Manitoba who was not involved in the study. “For the first time, scientists will be able to discern – at the genetic level – what set mammoths apart from their close living relatives.”
Dr. Poinar said the gap in time between the two specimens is enough to allow researchers to establish a mutation rate for the mammoth genome – the speed at which the genome changes over time. The rate is rather low, suggesting that mammoths needed a large and well mixed population to maintain a healthy genetic diversity. As the ice age ended that became a problem. Mammoths gradually disappeared from the Siberian mainland, while a small population persisted on Wrangel, which cut off by rising sea levels about 10,000 years ago.
Like humans, each mammoth has two sets of chromosomes, one paternal and one maternal. By comparing the two, scientists can look for genetic similarities and differences that reveal the likely size of the available mammoth gene pool. The Wrangel specimen showed about 20 per cent less genetic diversity The details of the similarities suggests the population there may experienced many generations of inbreeding between individuals that were related at the level of cousins or second cousins, said Love Dalén, a team member and researcher in bioinformatics at the Swedish Museum of Natural History in Stockholm. He added that limited resources would have capped the mammoth population on Wrangel at about 1,000 individuals at any one time.
Those conditions make the Wrangel Island mammoths a useful model for studying the genetics of small populations, Dr. Dalén said.
“In the long run, I think the information we get out of the Wrangel Island population will actually help us conserve endangered species today.”
Dr. Poinar said that the study is just the first taste of a multiyear effort to unravel the story of the woolly mammoth by sequencing of genomes from many more specimens across a broad span of time.Report Typo/Error