On Homology

Richard Owen, one of the great fathers of paleontology, is a curious figure in the history of biology. A contemporary of Darwin’s, he had ambiguous and sometimes self-contradictory views of evolution. Nevertheless, his most lasting contribution to modern biology is the concept of homology, one of the guiding principles of evolutionary biology.

Owen noticed that the limbs (for example) of bats, cats and seals all had a common plan – a single humerus followed by a pair of parallel bones called the radius and ulna, two rows of pebble-like bones called carpals and then a system of  long, thin bones called metacarpals and phalanges – even though they had very different functions and outward appearances. As a result, he drew a distinction between two different kinds of anatomical similarity. When two organs are similar only in function, but not in appearance, he said that their relationship was one of “analogy.” But when two organs were similar because they fit into the same plan, he called the relationship “homology.” The bat wing, cat paw and seal flipper are homologous with each other, but a bird’s wing and a fly’s wing are analogous.

homologouslimbs

The limbs of a human, dog bird and whale, color coded to show homologous bones. Courtesy Wikimedia Commons.

Oddly, although he knew Darwin’s work very well, and although the concept of homology is now explained in terms of evolutionary relationships, Owen maintained that homology only reflected the common designs in the mind of the Creator. Today, we have a different view: homology is defined as similarity as a result of separate descent from a common ancestor. The bat, the cat and the seal all had a common ancestor whose arm-bones were laid out in the same way as its descendants’.

The concept of homology is both crucial to evolutionary biology and maddeningly ambiguous (somewhat like Owen himself). It is crucial because if we want to ask essential questions such as, “how did birds evolve wings?” we must necessarily talk about homologous structures in the ancestors of the birds. But it is ambiguous for a host of practical and philosophical reasons.

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