So where did these mysterious giants come from? What were they like? And what drove them to extinction? Biologists have been arguing over these questions ever since Cuvier’s time. In the past few years, however, a wealth of new information has emerged, thanks in part to DNA studies.
The mammoth has one of the best fossil records, offering an incredible insight into the evolution of this lineage. “You can trace how the anatomy has changed from a general, elephant-like animal to this very specialized creature that is the woolly mammoth,” says Adrian Lister of the Natural History Museum in London and the author of “Mammoths: Giants of the Ice Age.”
By themselves, though, bones can tell us only so much. Luckily, the freezerlike conditions in which woolly mammoths lived and died have preserved not only bones but also flesh and hair. Sometimes entire animals have been found frozen, such as Lubya, a 1-month-old mammoth discovered in 2007. Thanks to hairs from two frozen specimens, around half the woolly mammoth genome has now been sequenced.
Ancient DNA has begun to fill in many gaps in knowledge about mammoths, including a sense of their movements and origins, scientists say.
It has long been clear that mammoths arose in Africa, says Lister, because fossils of ancestral mammoths dating back as far as 5 million years ago have been found there. However, it was not clear from anatomy alone whether these ancient animals were more closely related to African or to Asian elephants.
In 2006, three groups sequenced the woolly mammoth’s mitochondrial DNA, revealing the structure of the elephant family tree. The studies show that the lineage leading to African elephants split off from the common ancestor first, around 6 million years ago. Not too long after that, the mammoths forked away from what would become the Asian elephant.
These early mammoths had the spirally curved tusks characteristic of their kind but otherwise probably looked much like elephants. They remained restricted to Africa until around 3 million years ago, when they began to spread across Europe and Asia.
There is little evidence of adaptation to cold in these individuals, which makes sense because the climate was still relatively mild. But times were changing. Around 2.5 million years ago, an epoch of ice ages began. Many forests, which had provided trees and bushes for nourishment, were replaced by open grassland.
These dramatic changes led to the evolution of a new kind of mammoth, the steppe mammoth, which adapted to life in a colder world and to the changing vegetation. “The steppe mammoth’s teeth had more enamel ridges to deal with a more grassy diet, and a higher crown to tolerate greater wear,” says Lister.
Until recently, it seemed as though this stage in the mammoth story was a case of gradual evolution, with the first steppe mammoths appearing around 750,000 years ago. But this picture was based on fossils found in Europe.
Fossils recently unearthed in China show that the steppe mammoth evolved there about 1.7 million years ago and gradually spread out across the Northern Hemisphere, replacing earlier forms.
It was around this time that some mammoths crossed a land bridge joining Siberia to North America. There, mammoths evolved into distinctive North American forms and some eventually spread as far south as Central America.
Meanwhile, some steppe mammoths were becoming ever more specialized for cold climates and open grassland, giving rise to the woolly mammoth, the most famous of its kind. Again, while fossils in Europe suggest it appeared around 150,000 years ago, we now know the woolly mammoth began evolving around 700,000 years ago in northern Siberia, says Lister.
The woolly mammoth’s most distinctive feature was its long, shaggy coat. Preserved specimens have a wide range of hair color: blond, red, brown, even black. Recent analysis of a gene known to determine hair pigmentation, however, suggests most mammoths had a dark-brown coat.
Besides its long hair, the woolly mammoth had a thick layer of fat to insulate against the cold. It also had smaller ears and a shorter tail than its forebears, which would minimize heat loss. Its huge tusks may have been used like a snowplow to expose vegetation to eat or to break up ice. And its version of the blood protein hemoglobin, resurrected in a British laboratory, appears to be adapted to deliver enough oxygen to cells even when their extremities became cold.
So woolly mammoths were built for the cold, and they thrived during a series of ever deeper ice ages. The species spread west and east to occupy much of the Northern Hemisphere, including North America, while other mammoth species died out. Studies of mitochondrial DNA from 40 woolly mammoth specimens show its population and range expanded as the world entered the last ice age around 100,000 years ago and remained stable during that ice age. And then, as the ice age ended, it went extinct.
What happened? Some biologists think that the extinction took place very rapidly, triggered by a sudden, dramatic event around 12,000 years ago. One suggestion is that some kind of mega-disease wiped out the species. Another is that a meteorite impact in North America triggered catastrophic change. And then there’s the “blitzkrieg hypothesis,” which blames the mammoth’s demise on the spread of spear-wielding human hunters.
Hunting clearly did happen, as cave paintings and the occasional spearhead lodged in bone testify. But there’s growing evidence that woolly mammoths didn’t die out as suddenly as such cataclysmic visions would have us believe.
Dating of mammoth remains by Lister and others suggest that the woolly mammoth’s range had been in decline for several thousand years before they disappeared. And genetic studies show a loss of genetic diversity, a sign of a shrinking population. This was probably a result of trees’ replacing grassland as the world began to warm up again. By 12,000 years ago, woolly mammoths were restricted to the steppes of Siberia. “The original, huge unbroken range of the species in its heyday shrank and became fragmented in ways that map onto the way the climate was changing and the way the vegetation was changing,” Lister says.
However, the dating of remains can give only a very rough idea of when mammoths died out, because only a tiny fraction of the population will have been preserved. Danish evolutionary biologist Eske Willerslev has taken a more unusual approach.
“Modern elephants urinate about 50 litres a day, which is basically DNA all over the landscape,” Willerslev says. Working on the assumption that mammoths would have done the same, Willerslev and colleagues extracted sediment that could be accurately dated from the Alaskan tundra and looked for mammoth DNA. Although the most recent mammoth remains from this region are more than 13,000 years old, the team’s findings suggest that mammoths were still living there 10,500 years ago.
That’s 3,500 years after the first recorded human settlement and almost 2,500 years after the meteorite often blamed for wiping out mammoths in North America. “Our data show that neither the blitzkrieg hypothesis nor the idea of an extraterrestrial impact can be the whole story,” Willerslev says.
But climate change seems unlikely to be the whole story, either, as mammoths had survived previous warm periods. “It’s possible that you only get extinction if you get a combination of factors coming together,” says Lister.
The last stand of the mammoths appears to have taken place on Wrangel Island, in the Arctic Ocean off the coast of Siberia. Here a population of mammoths was cut off from the Siberian mainland 9,000 years ago as ice sheets melted and sea level rose. However, the climate and vegetation remained suitable for them, and they survived for 5,000 years before dying out around 4,000 years ago — around the time humans arrived.
Nobody knows what caused the extinction of the Wrangel mammoths. There is no direct evidence that humans killed them, so it is possible that the island was simply too small to support a mammoth population. However, ancient DNA collected from the remains of some of these last mammoths paints a picture of a stable population that thrived for 5,000 years and then suddenly died out, rather than a population in terminal decline, suggesting that humans had a hand in it.
Nicholls is a freelance writer based in London. This article was produced by New Scientist magazine and can be read online at www.newscientist.