The passing of the age of the dinosaurs allowed mammals to become ascendant. But mammals have a much deeper history. They - or, more precisely, we - originated around the same time as the dinosaurs, over 200 million years ago; mammal roots lie even further back, some 325 million years.



Over these immense stretches of geological time, mammals developed their trademark features: hair, keen senses of smell and hearing, big brains and sharp intelligence, fast growth and warm-blooded met...