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5. Eocene, 56 million to 34 million years ago

A cast of Uintatherium anceps, a rhino-like mammal, at Paris’s National Museum of Natural History.
A cast of Uintatherium anceps, a rhino-like mammal, at Paris’s National Museum of Natural History. Image: Wikimedia Commons

Great times. Basically the prequel to the Pleistocene in terms of mammalian domination, the Eocene was the middle epoch of the Paleogene, which came on the heels of asteroid impact that did away with nearly all dinosaurs. It saw the rise of ungulates (toed and hoofed mammals) like tapirs, rhinoceroses, horses, elephants, and deer. Cetaceans like Pakicetus (the most famous ancient walking whale) also enter the fossil record. Temperatures were hot and humid, and mammals diversified across Earth. For much of the Eocene, there wasn’t even ice on the planet’s poles. But the Eocene ranks lower than the Pleistocene for sheer lack of woolly mammoths and Neanderthals.

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