... mammals ... They—or rather, we—actually originated around the same time as the dinosaurs, over 200 million years ago, when all land was gathered together as one supercontinent, scorched by vast deserts. Those first mammals had an even deeper legacy, tracing back to about 325 million years ago, to a humid realm of coal swamps, when the ancestral mammal lineage split from the reptile line on the great family tree of life. 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 metabolism, a distinctive lineup of teeth (canines, incisors, premolars, molars), mammary glands that mothers use to nourish their babies.