18) The first mammals

A Tupaia or tree-shrew. Perhaps similar to the earliest mammal, tree shrews are arboreal insectivores found in india and South East Asia. Image credit: © Tenzin Alex Anderson.

225 million years ago, the first mammals, small and nocturnal, jump, climb, swing, and swim through a world of giants. Some rodent-sized insect-eaters evolve lactation, enabling mothers to spend more time in the nest keeping their young both fed and warm.