Answer: Homology

If you’ve ever spent a lazy Saturday afternoon wandering about a museum of natural history, you may have noticed the striking similarity between the skeletons of different creatures and our own human skeletons. Beneath the surface of their flippers, for example, whales have what look like giant hands with long bony fingers. Similarly, the skeletons of other animals, like dogs or even birds, have remarkably similar bone structures in their forelimbs.

This similarity has interested scientists dating all the way back to Aristotle, was analyzed extensively in the 16th century by Pierre Belon, and in the 19th century was both explained and named—by Charles Darwin’s theory of evolution and by anatomist Richard Owen, respectively. The phenomenon, called homology, is when shared distant ancestry in different taxa gives rise to structures (as well as genes) that have adapted to perform different but related functions.

Our previous example of similar forelimb structures in wildly different animal species is an example of the creatures all sharing common ancestors in the animal kingdom superclass “tetrapoda”. Over millions of years, the forelimbs of all the different species in that superclass slowly evolved to support different types of bodies and help move those different bodies in radically different environments. The forelimbs of a horse support a very stout body and resist the stress of running on land while the forelimbs of a bat don’t directly support the weight of the bat on the ground, but do help power its flight.

Homological structures stand in contrast to analogous structures. A homological structure can be traced back to a common ancestor, whereas an analogous structure cannot. A good example of analogous structures are the wings of insects and birds, which evolved independently of each other and have no primordial shared ancestor to trace the wings back to.