A homologous structure is an apparatus that, according to scientists, has been derived from a common ancestor. Different from analogous structures, homologous structures do not necessarily have the same function but they do, according to biologists, have a common ancestor. When looking at anatomy from an evolutionary standpoint, it would make sense that common descent and the evolution from one animal to another gave rise to a variation on a similar structure to fit their environment. For example, the forelimbs of all mammals have the same arrangement of bones from their shoulder to their digits. The forelimb of the cat in this picture, for instance, is homologous to human arms, as are whale forelimbs and bat wings. All of these organisms have a humerus, radius, ulna, carpals, metacarpals, and phalanges bones that compose the forelimb.
According to scientists, some homologous structures are visible only in developing embryos and later develop into structures that do not look similar at all in adulthood. For example, all vertebrate embryos have a “tail” that is posterior the anus. Obviously in humans, this “tail” develops into a structure extremely different from how it first appears in the embryo. In addition to “tails”, vertebrate embryos also have pharyngeal (throat) pouches. Again, these structures later develop into very different structures in different species- In fish, the pouches develop into gills; in humans (and other mammals), the pouches develop into parts of the ear and throat (hence the name pharyngeal, which means throat). Some homologous structures are virtually useless for the organism and are considered by scientists to be “left-overs” from their ancestor. These left-overs are called vestigial structures. Snakes, for example, have vestigial structures that resemble the pelvis and leg bones. Today, these snakes have no use for walking yet a structure for walking, possibly from their ancestor, has been left in their anatomy.
In addition to structural similarities, there are also molecular similarities that point scientists to evolution. All life forms use DNA and RNA and organisms that are incredibly different from eachother (such as bacteria and humans) have similar genes.