Carl Zimmer in his excellent blog, The Loom:
Scientists call this unity “homology.” The British anatomist Richard Owen coined the term in 1843, sixteen years before Charles Darwin published The Origin of Species. Owen defined homology as “the same organ in different animals under every variety of form and function.” For example, a human arm, a seal flipper, and a bat wing all have the same basic skeletal layout. They consist of a single long bone, a bending joint, two more long bones, a cluster of small bones, and a set of five digits. The size and shape of each bone may differ, but the pattern is the same regardless of how mammals use their limbs–to swim, to fly, or to wield a hammer.
Darwin argued that homology was the result of evolution. The common ancestor of humans, seals, bats, and other mammals had a limb which became stretched and squashed in various contortions. And over the past 150 years, paleontologists have found a wealth of fossils that help document how the tiny paws of Mesozoic mammals diversified into the many forms found in mammals today.
But Darwin wasn’t just out to explain the evolution of mammals. He saw a kinship across the entire living world. And that’s where things got complicated. Anatomists in Darwin’s day could find no clear counterparts to many of the traits in our own bodies in distantly related animals.
It turns out the homology is there, but you just need the right eyeglasses to see it.
More here.