Mammals in Motion
The earliest mammals evolved from a common ancestor—an animal with four limbs, or a tetrapod. As mammals evolved, they retained the basic skeletal pattern of their common ancestor. No matter how different mammals may look, they share an underlying body plan.
For example, compare your own arm with the flipper of a whale or the wing of a bat; at first glance, they look very different. But beneath the skin, the arrangement of the bones is startlingly similar. These similarities are called homologies.
The earliest mammals got around by walking on four feet, just like their ancient tetrapod ancestors. Later mammals evolved their own modes of locomotion. They burrow, walk, hop, gallop, swim, swing from trees, dive, glide—even fly.
In the world of mammals, being “bipedal” is extreme. Many living mammals, like meerkats and bears, stand up on two legs now and then. But only a few mammal species get around primarily on two feet. Of these, we humans are unique because we don’t hop—we walk on two legs.
Most mammals live on land, and some live in the sea. But only a few can glide through the air. And only one group—the bats—can truly fly.
Instead of just gliding on air currents, bats flap their wings to lift their bodies through the air: this is powered flight. This ability—which arose separately in birds, insects, and some extinct reptiles—has only evolved once in mammals, when bats took to the air more than 50 million years ago.
Bats are an extreme success story: among more than 5,600 living species of mammals, about 1,200 are bats!