Homologies

Hands Or Wings, Flippers Or Paws?

Humans, whales, bats, eagles, lizards, frogs and chimpanzees are very different types of animals that use their forelimbs in very different ways. But beneath the skin, the forelimb bones of these animals are startlingly similar. These likenesses in structure, called homologies, are the result of descent from a common ancestor.

In related species, the same anatomical features evolved into distinct forms as they were used in different environments or for different functions. The forelimbs of chimpanzees are adapted for climbing trees, those of whales to support flippers for moving through water, and those of bats to support membranous wings for flying. But the underlying similarities of these homologous bones reveal that all these animals share a common ancestor: a four-legged animal--a tetrapod--living over 365 million years ago.

This cladogram shows the common ancestor of modern vertebrates, why they are similar, and indicates points of divergence through time.

Vertebrates are animals with backbones and include all fish, amphibians, reptiles, birds, and mammals. A backbone is made up of a series of small bones, called vertebrae. Backbones enclose and protect the spinal cord, the bundle of nerves that carries information to and from the brain.

Tetrapods are vertebrates that have, or had, four limbs and include all amphibians, reptiles, birds, and mammals. All tetrapod limbs are made up of similar sets of bones. In some species, such as whales and snakes, some limbs have been lost or radically altered as these animals evolved over time.

Mammals are tetrapods with three middle ear bones instead of just one. All mammals have three bones in their middle ear called the malleus, incus, and stapes, commonly known as the hammer, anvil, and stirrup. The middle ear bones help transmit sounds to the brain.