From the shadows of dinosaurs, our small, mammalian ancestors emerged, diversified,
and evolved. Pictured here is one of our early primate relatives, Smilodectes
gracilis, that scampered around 55 million years ago. The first mammals
lived in a warm, wet world and dined on soft leafy plants. But beginning about
35 million years ago, the climate began to cool. Grasslands replaced forests.
Some animals, such as the ancestors of horses, adapted to the changes. A number
of familiar furry forms began to appear among the Oligocene mammals, 37-23 million
years ago. Then, long-limbed Miocene mammals appeared between 23 and 5 million
years ago.
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| Fossil Horse | Oligocene Mammals | Miocene Mammals |
