It turns out that woolly mammoths were more resilient than anyone thought.
By sequencing the genomes of the remains of two mammoths that lived 40,000 years apart, researchers from McMaster University, Harvard Medical School and the Swedish Museum of Natural History have been able to determine the population endured a significant decline about 250,000 to 300,000 years ago. But these shaggy relatives of modern elephants managed to rebound to healthy levels before going extinct around 4,000 years ago.
"This gives us a timeline as to when mammoths appeared on the scene, how their population shrank and grew over the last 500,000 years," said evolutionary geneticist Hendrik Poinar, director of the Ancient DNA Centre at McMaster University and a researcher at the Institute for Infectious Disease Research. "That is key to our understanding of why they went extinct. That helps shed light on how herbivores in general are going to deal with the rapid warming climate that we are undergoing now."
One genome was from a juvenile mammoth that lived in northeastern Siberia nearly 45,000 years ago. The other - believed to be from one of the last surviving mammoth populations - lived around 4,300 years ago on Russia's Wrangel Island, located in the Arctic Ocean.
Looking for a smoking gun
This data offers the most complete picture to date of the mammoth and builds on the pioneering work Poinar and his team began in 2006, when they first mapped a partial mammoth genome using DNA extracted from carcasses found in permafrost in the Yukon and Siberia.
Still, the findings published Thursday in the journal Current Biology contain no smoking gun that would explain why the mammoths ultimately disappeared. Originating in Siberia around 800,000 years ago, they reached North America about 400,000 years ago. They died out almost everywhere 10,000 years ago, except for a few high latitude islands north of Siberia and in the Bering Strait. The animal went completely extinct 3,000 to 4,000 years ago.
Among the theories put forth in the past were that they suffered at the hands of human hunters, were driven to extinction by climate change or shifting habitats or, more controversially, that an asteroid hit the planet and caused a deep freeze that wiped out large animals across the planet.
What can be gleaned from the genome is that the mammoths - their populations already isolated by rising seas - suffered a population bottleneck toward the end of their reign.
"We found that the genome from one of the world's last mammoths displayed low genetic variation and a signature consistent with inbreeding, likely due to the small number of mammoths that managed to survive on Wrangel Island during the last 5,000 years of the species' existence," said Love Dalén, an associate professor of Bioinformatics and Genetics at the Swedish Museum of Natural History and a co-author on the paper.
As for the earlier population dip, there is even less to go on. Poinar said they can only rule out possibilities. For instance, it couldn't have been caused by humans, since they hadn't yet arrived on the scene.
"It must have been an environmental change that affected them," Poinar told CBS News. But what that change was is still unclear.
"Here is the interesting thing. We see this decline in the data but that time does not coordinate with one of the major glaciation events called the Eemian, which we thought must be the reason we saw this decline. The decline happened way before this occurred."
The American Museum of Natural History's Ross MacPhee questioned whether the findings can say much about the rise and fall of the wooly mammoth. With the first population dip, he pointed out that the researchers acknowledged the decline could have occurred anywhere from 189,000 to 1.6 million years ago.
Using this data to try to identify the cause of the population decline is "pretty much a stretch," MacPhee said.
"The problem here is that you don't get very precise results ... They say the range of the estimates is about 200,000 to 1.6 million which is a very lengthy time indeed to imagine that there was any single cause," he said. "It is also difficult to imagine, whenever it happened, that it was just a distinct event. It sounds like a progressive thing over a very long period of time."
A second coming?
With this complete genome, Poinar believes scientists are one step closer to a long-held goal of cloning the woolly mammoth. Scientists have for years been collecting DNA from bones and teeth from carcasses found in the Siberian permafrost and more recently from a deep red liquid, that some thought could be blood. If they can get enough undamaged DNA, they could clone a mammoth or possibly cross breed it with its closest relative, an Asian elephant.
"This basically gives you the changes that account for a mammoth being a mammoth - the changes that allowed them to have hair, tremendous amounts of fat, large tusks," Poinar said of the sequenced genome. "This then gives us this roadmap, so to speak, of what we would need to change in an Asian elephant chromosome to make them mammoth-like."
MacPhee, who has collected woolly mammoth specimens since as far back as 1998, said the findings do little to unravel the mysteries of the mammoth.
But should they be brought back to life?
MacPhee, who has collected woolly mammoth specimens as far back, as 1998, doesn't think so. There is scientific value to cloning them, he said, but there remain too many ethical concerns that have not been fully addressed.
"What does it mean to bring back an animal in the adult state somewhere around 7,000 or 8,000 pounds?" MacPhee queried in an interview with CBS News.
"Would it be alone, part of a herd? Are we talking about reconstituting a species or we just talking about another form of freak show? Is it going to be in a zoo or another enclosure? There are good ethical reason not to want to that," he said. "Would they be free ranging and, if they are free ranging, where?"
Poinar also has his concerns, though he thinks it will be done at some point. He could see the value in reintroducing mammoths into parts of Siberia, which could help bring back the grasses and slow global warming. But there are also real concerns that its reintroduction would simply serve deep-pocketed investors.
"The kid in me wants to see it of course," Poinar said. "I would love to see a mammoth population walking across the tundra in the north ... There are animal-related issues of how easy it is to carry that to term and what does that mean toward the pain of current living elephants that might have to do that ... And if the money comes in from private hands, does that mean it becomes a theme park? If these are mammoths to bring back for a zoo, I am not interested at all."