Lost giant: mammoth
In 1643, workers unearthed some huge bones in 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 suppodly killed by Mos.
In 1728, British anatomist Hans Sloane identified similar remains from Siberia as belonging to elephants. But what were hot climate animals doing in Siberia? Only at the end of the 18th century did French zoologist Georges Cuvier conclude that giant bones like the were from an elephant relative that died out long ago: the mammoth.
So where did the mysterious giants come from? What were they like? And what drove them to extinction? Biologists have been arguing over the questions ever since Cuvier倾世皇妃主题曲’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 evol
ution 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 Muum in London.
By themlves, though, bones can tell us only so much. Luckily, the freezer-like conditions in which woolly mammoths lived and died have prerved 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. 汉朝都城>男士祛痘印Thanks to hairs from two frozen specimens, around half the woolly mammoth genome has now been quenced.
It has long been clear that mammoths aro in Africa, says Lister, becau fossils of ancestral mammoths dating back as far as 5 million years ago have been found there. In 2006, three groups quenced 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. 感应头灯
There is little evidence of adaption to cold in the individuals, which makes n becau 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.
The dramatic changes led to the evolution of a new kind of mammoth, which adapted to life in 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 tolerant 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 crosd 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 clima
tes and open grassland, giving ri to the woolly mammoth, the most famous of its kind.
The woolly mammoth’s most distinctive feature was its long, shaggy coat. Prerved specimens have a wide rang of hair color: blond, red, brown, even black. Besides its long hair, the woolly mammoth had thick layer of fat to insulate against the cold. It also had smaller ears and shorter tail than its forebears, which would minimize heat loss. Its huge tusks may have been ud like a snowplow to expo vegetation to eat or to break up ice.
So woolly mammoths were built for the cold, and they thrived during a ries of ever deeper ice ages. T上课英语he 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 the ice age. And then, as the ice age ended, it went extinct.
What happened? Some biologist think that the extinction took place very rapidly, triggere
d by a sudden, dramatic event around 12,000 years ago. One suggestion is that some kind of mega-dia wiped out the species. Another is that a meteorite impact in North America triggered catastrophic change. And then there’s the “blitzkrieg” hypothesis,阿奇霉素分散片说明书” which blames the mammoth’s demi on the spread of spear- wielding human hunters.
H艺术摄影摄影unting 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 other suggests that the woolly mammoth’s range had been in decline for veral thousand years before they disappeared and genetic studies show a loss of genetic diversity, assign of a shrinking population. This was probably a result of trees’ replacing grassland as the world began to warm up again. B关于运动的英语作文y 12,000 years ago, woolly mammoths were restricted to the steppes of Siberia.
