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 ...