The Eocene epoch (which lasted from 56 million ago to 34 million years ago) was hot! Temperate forests ran all the way to the poles. Steamy tropical jungles grew in the latitude where Maine is now and the equatorial regions of earth were (probably) sweltering. Tropical reefs formed in the coastal waters around a heavily forested and ice-free Antarctica. Since there was not year-round ice at each pole, the sea levels were much higher.
The Eocene was a time when most of the contemporary mammalian orders first appeared. The earliest artiodactyls, perissodactyls, rodents, bats, probiscideans, sirenians, and primates all originated during this time. Of course mammals were not the only story: the Eocene was also a time of great diversification for birds and many familiar orders of avians developed then. Reptiles begin to put the setbacks which marked the end of the Cretaceous behind them and several giant new species emerged including an immense tropical ur-python and a host of crocodiles and turtles. It is harrowing to think that the first wee dawn horses and cute little early atiodactyls were forced to contend with a 13 meter long super snakes and giant crocodilians (which flourished in the great hot swamps of Alaska), but such is the case.
The high temperatures of the Eocene are perplexing to scientists. By contrast, the temperatures of the Paleocene (which was the first era of the Cenezoic and had directly preceded the Eocene) were much more temperate. In fact the temperature spike of 56 million years ago seems to have ended the Paleocene and brought about the diversification of Eocene life. The rapid warming is known as the Palaeocene-Eocene thermal maximum and scientists have been vigorously debating what caused the climate change. An immense amount of carbon seems to have entered the atmosphere at this time, which in turn led to greenhouse warming. It remains controversial as to how such a large quantity of carbon got into the atmosphere. Comet/meteorite impact, massive peat fires, and volcanic activity have been suggested as triggers, however supporting evidence is lacking. The release of globally significant quantities of hydrocarbons–which had been trapped in undersea clathrates seems like a more feasible hypothesis, as does the idea that the earth’s orbit brought the planet closer to the sun for a time.
The end of the Eocene was also linked to the carbon cycle. Reduced carbon dioxide in the atmosphere seems to have led to global cooling and newly evolved varieties of grasses began to invade large swaths of the world. Additionally two massive meteor strikes in Siberia and Maryland combined with substantial volcanic activity to finish off the long hot summer. But during the Oligocene, the era which followed the Eocene, the world was a much more familiar place inhabited by orders of animals which are still here with us today (or are us–since primates first evolved during the Eocene).