Drifting Continents
Since time immemorial, Earth's landmasses have been on the move. Over hundreds of millions of years, they joined, split up and reconfigured, creating first one, then another supercontinent. Early mammals were evolving when the supercontinent known as Pangaea — made up of North America, South America, Europe and Africa — existed during the early Mesozoic (about 200 million years ago). Though relatively few in number, these Mesozoic mammals were free to spread across the continents. However, it was only after the extinction of the dinosaurs 65 million years ago that the mammals began to proliferate, and by then the continents had already drifted apart.
The warm "greenhouse" climate of the Mesozoic continued well after the disappearance of the dinosaurs. Lemur-like mammals, palm trees, crocodiles and coral reefs thrived as far north as the Arctic Circle until about 50 million years ago. Then, as the continents continued to separate and drift northward, the Earth's climate cooled. Ice sheets formed on Antarctica as early as about 34 million years ago.
New land connections were made as the continents moved apart. Land bridges formed, disappeared and reformed, allowing mammals to travel between Africa and Eurasia, Asia and North America, and eventually between South America and North America.
Midway through the Cretaceous (136 to 65 million years ago), ancestors of present-day marsupials emerged in North America. Eventually they migrated southward, crossing over to Antarctica and from there to Australia before that land floated away as an island. Unique forms of marsupials evolved there in the near-absence of competition from primates, ungulates and other mammals; some, such as the Tasmanian wolf and the carnivorous kangaroo, are now extinct. They developed a range of lifestyles, duplicating forms and modes of living that existed elsewhere: Such convergent evolution produced anteaters in five different mammalian orders in the Southern Hemisphere alone.
Evolving alongside Australia's marsupials were the monotremes, which are believed to have originated on that continent — although a fossilized platypus forebear has been recently discovered in Patagonia, suggesting that at least some monotremes made the journey across Antarctica from Australia.
Land bridges continued to emerge intermittently between South America and Antarctica until the Middle Enozoic (about 32 million years ago). Fossils of extinct mammals related to present-day South American species — two marsupials and a sloth, among others — have turned up on the Antarctic Peninsula, then covered by a cool temperate rain forest.
Mammals also took advantage of land bridges connecting eastern North America and Europe via Greenland and Scandinavia and linking Alaska and Siberia. The Bering land bridge, as this latter bridge is sometimes called, was the route taken by the descendants of Darwin's fossilized horse as they left North America during the Pliocene (2 to 5 million years ago), perhaps passing the spectacled bear, Tremarctos, as it arrived from Asia. The ancestors of many present-day North American mammals, including deer, bison and voles, are known to have made this trek from Asia to North America.
Perhaps the most dramatic dispersal of mammals occurred when the Isthmus of Panama became dry land almost 3 million years ago. Called the Great American Interchange, this massive exchange involved mammals, such as the far-traveling spectacled bear, the raccoon and the llama, moving from North America to South America, and the opossum and the armadillo — to name just a few — making the journey north from South America.
