Cynodonts were a group of early synapsids that first appeared in the Late Permian period, about 260 million years ago. Their evolutionary line produced the first mammals during the Late Triassic period, roughly 225 to 200 million years ago. When a large asteroid struck Earth at the end of the Cretaceous period, about 66 million years ago, most large animals, including the non-avian dinosaurs, became extinct. The small early mammals that descended from cynodonts survived, largely because of their modest size, flexible diets, and ability to shelter in burrows or other protected places. Today, every living mammal, including humans, is known to be descended from cynodonts.