The continents continued their drift and moved closer to their current positions. Africa’s collision with Europe formed the Mediterranean, and sea-level changes exposed the land-bridge between Alaska and Asia.
By the start of the Pliocene the trend towards a cooler, drier global climate had produced climatic conditions quite similar to today’s. Ice sheets grew on Antarctica and an Arctic ice cap formed around 3 mya. The formation of the Panamanian isthmus had a major effect on global temperatures. The direct connection between the warm equatorial Pacific currents and the Atlantic waters was severed and the Atlantic cooling cycle began (Gulf Stream), with Arctic and Antarctic temperatures dropping in the now isolated and cooler Atlantic Ocean.
Global climatic cooling had a considerable effect on vegetation. Tundra appeared inside the Arctic Circle, and below this was a band of coniferous forests. In many places, tropical forests were replaced by subtropical woodland or grassland, and this in turn had been replaced in the middle latitudes by more temperate types of woodland.
The isthmus allowed species to migrate between the two continents. Armadillos, opossums and ground sloths migrated from South to North America, while bears, cats and horses moved in the other direction. In addition, the severing of the connection between the Atlantic and Pacific allowed marine species to evolve separately on each side of the isthmus. All of the presently existing orders and families as well as many existing genera are known from this time. Rodents and elephants were successful in Asia; hyraxes migrated north from Africa; cattle and antelopes were successful and some camel species crossed into Asia from North America. Hyenas and sabre-toothed cats appeared, joining other predators such as bears, dogs and weasels. Africa was dominated by hoofed animals, and primates continued their evolution with australopithecines (‘southern apes’) appearing late in the epoch.
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