In 1643, workers unearthed some huge bones in a Belgian field. The naturalists who studied them were convinced they had come from a humanlike giant. Their length, after all, tallied with a biblical reference to Og, a giant king supposedly killed by Moses.
In 1728, British anatomist Hans Sloane identified similar remains from Siberia as belonging to elephants. But what were hotclimate animals doing in Siberia? Only at the end of the 18th century did French zoologist Georges Cuvier conclude that giant bones like these were from an elephant relative that died out long ago: the mammoth.
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 specialised creature that is the woolly mammoth," says Adrian Lister of the Natural History Museum in London.
By themselves, though, bones can tell us only so much. Luckily, the freezer-like 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-monthold mammoth discovered in 2007. Thanks to hairs from two frozen specimens, around half the woolly mammoth genome has now been sequenced.
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. 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.
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 yearsago, 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.
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 specialised for cold climates and open grassland, giving rise to the woolly mammoth, the most famous of its kind.
The woolly mammoth's most distinctive feature was its long, shaggy coat. Preserved specimens have a wide range of hair colour: blond, red, brown, even black. 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 minimise heat loss. Its huge tusks may have been used like a snowplow to expose vegetation to eat or to break up ice.
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 suggests 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.
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.
The chronology of mammoths Fossil evidences show that about 5 million years ago mammoths originated from 1……………….Then some mammoths split off and migrated to 2 ………………. . In order to cope with the climate change that occurred around 2.5 million years ago, mammoths evolved into a new species, called the 3 …·…………… . About 1. 7 millions years ago, these animals gradually spread out from China across the N orthem Hemisphere, and travelled via land bridge into 4 ………………. . Some of them went further south to 5 ………………. , where they evolved to the well-known kind, named the 6 ………………. . Eventually, the mammoths went extinct around 7 . ……………… years ago.
The features of the woolly man1moths The woolly mammoths were built for the cold and thrived during the ice age. Their skin was covered with long 8 ………………. , and their 9 ………………. helped isolate from the cold. The woolly mammoths had smaller ears and shorter tails which consumed less 10 ………·……… . Their ivories were rather huge and may have been used to 11 .................... and discover 12 . . .. . . . . . . . . . . .. . . . . .