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The Miocene epoch extends from about 23.03 to 5.33 million years ago. It is the first and longest epoch of the Neogene period. Being relatively recent in geological history, much of the Miocene flora and fauna was similar to today's, except it was somewhat more primitive near the beginning and more diverse throughout, being that human beings have quickly killed off many of the unique animals that evolved in the last 20 million years.
Since 49 million years ago, the Earth had been substantially cooling, but this phenomenon accelerated during the Miocene. At the start of the period, South America separated from Antarctica, allowing a cold Antarctic current -- the Antarctic Circumpolar Current -- to circulate around Antarctica endlessly, without any land barrier. This prevented ocean thermal mixing, causing extreme cooling in Antarctica and causing glaciation there to become continent-wide. The white ice reflected back thermal energy into space, further lowering the temperature.
Because of the relatively low temperature throughout the Miocene period, it was dominated by grasses, just like today. Therefore, the primary herbivores were those that adapted to life on the steppes, including the ancestors of horses, rhinos, and the highly successful ruminants, which were already outnumbering their competitors. Many modern animals existed as early as the start of the period, including raccoons, wolves, camels, deer, and whales. Marine mammals were somewhat less numerous than today, and whales were smaller. Dolphins evolved near the beginning of the period, along with the technique of echolocation, while porpoises evolved at its middle.
During the Miocene, South America and Australia were isolated from every other continent, developing their own unique and widely divergent fauna. South America developed an entire unique class of ungulates which evolved to fill the niches occupied by ungulates on other continents, including those corresponding to rabbits, hippos, and chalicotheres (odd large animals that walked on their knuckles). Many of these animals are now extinct, outcompeted when North America merged with South America three million years ago and introduced the latter to the mammalian fauna of the rest of the world.