Anatomy, Evolution, and the Role of Homologous Structures

X-rays showing stages of human evolution (B&W, negative)
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If you've ever wondered why a human hand and a monkey's paw look similar, then you already know something about homologous structures. People who study anatomy define these structures as any body part of one species that closely resembles that of another. But you don't need to be a scientist to understand how homologous structures can be used not just for comparison's sake, but for classifying and organizing the many different kinds of animal life on the planet.

The Definition of Homologous Structure

Homologous structures are parts of the body that is similar in structure to other species' comparative parts. Scientists say these similarities are evidence that life on earth share a common ancient ancestor from which many or all other species have evolved over time. Evidence of this common ancestry can be seen in the structure and development of these homologous structures, even if their function is different.

Examples of Organisms

The more closely organisms are related, the more similar the homologous structures between organisms. Many mammals, for example, 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, with a large upper arm bone (the humerus on the human). 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 bone structure may be very similar, function varies widely. Homologous limbs can be used for flying, swimming, walking, or everything humans do with their arms. These functions evolved through natural selection over millions of years.

Homology and Evolution

When Swedish botanist Carolus Linnaeus was formulating his system of taxonomy to name and categorize organisms in the 1700s, how the species looked was the determining factor of the group in which the species would be placed. 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.

Linnaeus's taxonomy system places species into broad categories. The major categories from general to specific are kingdom, phylum, class, order, family, genus, and species. As technology has evolved, allowing scientists to study life at the genetic level, these categories have been updated to include domain in the taxonomic hierarchy. Domain is the broadest category, and organisms are grouped primarily according to differences in ribosomal RNA structure.

Scientific Advances

These changes in technology have altered the way scientists of Linnaeus's generation once categorized species. For example, whales were once classified as fish because 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. Further genetic research has demonstrated that whales may be closely related to hippos.

Likewise, bats were originally thought to be closely related to birds and insects. Everything with wings was 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 batwing resembles the human arm structure wise, the bird wing is very different, as is the insect wing. Therefore, scientists realized, 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 has become widely accepted as evidence for evolution. Not until the latter half of the 20th century, when it became possible to analyze and compare DNA, were researchers able to reaffirm the evolutionary relatedness of species with homologous structures.