Anatomy, Evolution, and the Role of Homologous Structures

Animal classifications are now based on structural similarities

X-rays showing stages of human evolution (B&W, negative)
Nicholas Veasey / Getty Images

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 a body part of one species that closely resembles that of another. But you don't need to be a scientist to understand that recognizing homologous structures can be useful not just for comparison, but for classifying and organizing the many different kinds of animal life on the planet.

Scientists say these similarities are evidence that life on earth shares 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 functions are different.

Examples of Organisms

The more closely organisms are related, the more similar the homologous structures are. 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 in humans) and a lower part made of two bones, a larger bone on one side (the radius in humans) and a smaller bone on the other side (the ulna). These species also have a collection of smaller bones in the "wrist" area (called carpal bones in humans) that lead into the "fingers" or phalanges.

Even though the 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 was placed. As time passed and technology advanced, homologous structures became 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 evolved, allowing scientists to study life at the genetic level, these categories have been updated to include domain, the broadest category in the taxonomic hierarchy. Organisms are grouped primarily according to differences in ribosomal RNA structure.

Scientific Advances

These changes in technology have altered the way scientists categorize species. For example, whales were once classified as fish because they live in the water and have flippers. After it was discovered that those flippers 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.

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. After more research and the discovery of homologous structures, it became apparent that not all wings are the same. Even though they have the same function—to make the organism able to get airborne—they are structurally very different. While the bat wing resembles the human arm in structure, the bird wing is very different, as is the insect wing. Scientists realized that bats are more closely related to humans than to birds or insects and moved them to a corresponding branch on the phylogenetic tree of life.

While the evidence of homologous structures has long been known, it has just recently been widely accepted as evidence of evolution. Not until the latter half of the 20th century, when it became possible to analyze and compare DNA, could researchers reaffirm the evolutionary relatedness of species with homologous structures.