The subject of anatomy has been studied by humans since the beginning of existence. Differences in anatomy have been cataloged and studied by scientists for hundreds of years. Even though there is great diversity in species on Earth, there are still many similarities between their anatomical structures.
What is a Homologous Structure?
Homologous structures are parts of the body that are similar in structure to other species' comparative parts. These similarities are evidence that life on Earth has a common ancient ancestor that the diverse species have evolved from over time. The common ancestry of the species can be seen in the structure and development of these homologous structures, even if their function is different.
The more closely the organisms are related, the more similar the homologous structures between organisms. Most examples of homologous structures revolve around the limbs of the species being compared. The bone structure within those limbs are similar between closely related species.
Examples of Homologous Structures
Many mammals have similar limb structures. The flipper of a whale, the wing of a bat, and the leg of a cat are all very similar to the human arm. All of the mentioned species have a large upper arm bone (the humerus on the human) and the lower part of the limb is made up of two bones - a larger bone on one side (the radius in humans) and a smaller bone on the other side (the ulna in humans). All of the species also have a collection of smaller bones in the "wrist" area (these are called carpal bones in humans) that lead into the long "fingers" or phalanges.
Even though the bone structure in these limbs of the mammals are very similar, the function of the limb itself is very different. The homologous limbs can be used for flying, swimming, walking, or everything humans do with their arms. These functions evolved through natural selection as the common ancient ancestor underwent speciation to make all of the diversity we have on Earth today.
Homology Determines Phylogeny
Originally, when Carolus Linnaeus was formulating his system of taxonomy to name and categorize organisms, how the species looked were the determining factor of which group they would be placed in. As time went on and technology became more advanced, homologous structures became more and more important in deciding the final placement on the phylogenetic tree of life. Whales were once classified as a fish since they live in the water and have flippers. However, after it was discovered that those flippers actually contained homologous structures to human legs and arms, they were moved to a part of the tree more closely related to humans. In fact, it seems whales are much more closely related to hippos than fish.
Likewise, since bats fly, they were originally classified as closely related to birds and insects. Everything with wings were put into the same branch of the phylogenetic tree. However, after much more research and the discovery of homologous structures, it was apparent that not all wings are the same. Even though they have the same function, to make the organism be able to get airborne and fly, they are structurally very different. While the bat wing resembles the human arm structure wise, the bird wing is very different, as is the insect wing. Therefore, bats are more closely related to humans than birds or insects and were moved to their corresponding branch on the phylogenetic tree of life.
While the evidence of homologous structures has been known for quite some time, it was only fairly recently that it was widely accepted as evidence for evolution. It took the back up of evidence from DNA to reaffirm the evolutionary relatedness of species with homologous structures.