The mammoth went through three distinct stages in Europe.
The ancestral mammoth, Mammuthus meridionalis, roamed Europe during the Late Pliocene to Early Pleistocene, about 2.6 million to 700,000 years ago.
The steppe mammoth, M. trogontherii, lived about 700,000 to 500,000 years ago. It was eventually succeeded by the woolly mammoth, M. primigenius, which lived from about 350,000 to 10,000 years ago, at which point most mammoths became extinct.
At the two intervals of major transitionfrom ancestral to steppe mammoth, and steppe to woolly mammothboth species existed at the same time.
Distinctive evolutionary changes often can be linked to feeding patterns, so Lister and Sher looked for evolutionary changes in the fossil skulls and teeth of mammoths.
As the mammoth moved from ancestral to steppe to woolly forms, the skull and jaw became progressively shorter and higher. The height of the molar crowns increased, as did the number of enamel plates in the molars, and the tooth enamel thinned.
The scientists believe these changes occurred because of a shift in dietfrom soft leaves of a wooded habitat to tougher grasses that sprang up as the climate became progressively colder during periods of glaciation.
After they established the evolutionary sequence of mammoths in Europe, Sher and Lister compared their findings with the fossil record in Siberia. The Siberian fossils show morphological changes similar to those found in Europe, but the new forms in Siberia occurred much earlier than they did in Europe.
"It's been long known that Siberian mammals lived in very harsh permafrost conditions at high latitudes as many as two million years ago," said Sher. "They had to adapt to the Arctic climate much earlier than the animals in Europe."
As Europe underwent periods of glaciation, the habitat of the northern animals expanded. They gradually migrated south from Siberia into Europe, co-existing with their European cousins.
The Siberian mammoths prospered because they were better adapted to the cold and the change in diet. "They either completely replaced the ancestral forms or there was intermingling or hybridization, with the Siberian form coming to predominate," said Lister.
"What we can say," he added, "is that there were long periods of relatively no change interspersed with times of quite rapid change, and these occurred in response to both changes in climate and migration from Siberia."
The study by Sher and Lister is the first to establish such a detailed continuous fossil sequence for a large terrestrial animal. "The fossil record for the woolly mammoth is fairly extensive, both through time and in geographical sampling," said Lister.
This, combined with the refinement of dating techniques, has shown that what initially looked like a gradual change over a long period of time was much more complex.
"We're now at a point where we can observe the origin of the species, which is the title of Darwin's book," said Lister. "Darwin provided the general framework for thinking about evolution, but now we can get into the nitty-gritty of it."
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