Homology forms the basis of organization for comparative biology.
Homologous organs can be defined as the organs of different species that have a similar basic structure but different functions.
For example, the fins of a whale, the forelimbs of a frog, and man have the same basic structures but perform different functions, therefore these are called homologous organs.
In 1843 homology was defined as: “the same organ in different animals under any variety of form and function.”
Scientists had noted that, within a group of related species, some structures shared similarities in shape.
For example, organs as different as a bat’s wing, a marine dog’s fin, a cat’s paw, and a human’s hand have a common underlying structure, with identical or very similar arrangements of bones and muscles.
According to scientific reasoning, it is said that there must be a common structural plan for all vertebrates, as well as for each class of vertebrates. He called this plan the archetype.
Similarly, a distinction was made between the homology of the analogy (also known as homoplasia ), which was defined as a “part or organ in an animal that has the same function as another part or organ in a different animal.”
Homologous structures are structures that are derived from a common ancestor , that is, they have a common evolutionary ancestor. This is not to say that homologous structures have the same function, for example, that a whale’s fin is homologous to a human arm.
These two limbs are superficially different, but their internal skeletal structure is essentially the same. Similarly, the wings of a bird and the wings of a bat are homologous structures.
Examples of homologies
Homologous structures in modern organisms may show even less similarity in shape, but it is still possible to trace their development and use them as a measure of evolutionary relationship.
- Tetrapod ossicles are homologous to bones in a fish jaw.
- Homology in the wings of birds and bats.
- Vestigial organs.
- Molecular homology.
The stirrup, anvil, and hammerhead of mammals are homologous to parts of the fish jaw and gill arches.
The earliest fish, like the ancestors of modern lampreys and lampreys, had no jaw. Their gills acted to filter food particles from the water. These gills were supported by a series of gill arches, made up of cartilage or bone.
As the earliest fish evolved jaws, the gill arches closest to the mouth were co-opted to act as jaw bones.
The first enlarged arch, called the mandibular arch, became the base of the upper and lower jaws. The second, or hyoid arch, extended from the square bone (at the back of the skull) to the angle of the mandible, and acted to support the mandible.
The hyoid arch later became the hiomandibular bone, which reinforces the quadrate bone in bony fish.
As tetrapods evolved from a group of lobe-finned fish, the square bone fused with the skull, giving it a stronger bite.
This means that the hiomandibular bone lost its function of supporting the jaw. However, its location near the ear appears to have allowed the hiomandibular to transmit vibrations to the inner ear.
The bone of the hypomandibular bones is homologous to the stapes or columella of a reptile’s ear. (Reptiles have only one bone, the stirrup, to transmit vibrations to the inner ear.)
One group of reptiles, the synapsids, evolved into mammals. Among the multitude of changes this involved was the development of the other two bones of the mammalian inner ear, the anvil and the hammer. Like the stapes, these two bones are also derived from the jaw bones.
In early synapses, two bones formed the joint between the upper and lower jaws. The square was part of the skull, while the articular bone was part of the lower jaw.
Like the hiomandibular bone (the stapes) to which they were connected, these two bones became progressively smaller and eventually completely lost their connection to the jaw. This evolutionary sequence can be traced through an excellent series of transitional fossils.
Once separated from the jaw, these three bones became the ossicles of the middle ear. The incus is homologous to the square bone, the hammer to the articular bone, and the stirrup to the hypodemandibular bone.
The skull of a mammal has a swollen area just below and behind the jaw joint. This is the auditory bulla, which contains the ossicles.
The evolution of these structures supports the hypothesis that early mammals were active at night (nocturnal), when they would have relied heavily on their senses of hearing, smelling, and touching to find food and avoid attack.
Homology in birds and bats
Birds and bats have independently developed wings from their forelimbs (an example of convergent evolution). However, while their wings look superficially quite different, examination of the underlying bones reveals that they are homologous.
The forelimb of the embryonic bird begins its development with the same structure as that of a mammalian embryo. As the bird develops, the forelimb becomes more and more wing-like and less leg-like.
Many of the bones in the hand fuse together and some are lost. On the contrary, the bat retains all the bones in its hand, but these are very elongated.
Another group of flying vertebrates, the pterosaurs, had similar modifications of the basic tetrapod forelimb. All three organisms, reptile, mammal, and bird, have the same upper and lower arm bones, although their proportions are different.
However, the bones of the hand that support the surface of the wing are quite different. In a pterosaur, the wing membrane is only supported by the fifth toe. The primary flight feathers of a bird are mounted on the 2nd finger, while in a bat, the 2nd, 3rd, 4th and 5th fingers support the wing membrane.
Some organisms have structures or organs with no apparent or predictable function.
For example, some snakes have rudiments of a pelvis and hind limbs, many flightless birds have remains, humans have a tail bone that is completely internal, and whales still have the remains of pelvis and thigh bones.
Those apparently non-functional parts are called vestigial organs or vestigial structures. Vestigial organs are often homologous to organs that are fully functional in other species, for example, the vestigial bone of the human tail (or coccyx) is homologous to the functional tail of other primates.
Very often, rudimentary organs can be detected in the embryo, but are lost later during development, for example, the teeth in the upper jaws of whale embryos or the pharyngeal clefts in the embryos of all but missing chordates in all adult forms, apart from fish.
Vestigial structures are evidence of evolution: a species with a vestigial form of an organ is related to other species where the homologous organ is fully functional.
The evolutionary history of a species leaves signs in its DNA and the proteins that DNA encodes. Two species that share a DNA base sequence (and also the specific protein encoded) probably have a common ancestor.
Usually, the DNA base sequence will be slightly different between the two, as each species will have accumulated different mutations once they have separated.
The number of mutations can be used to indicate how closely related the species are. It can also be used as an indication of how long ago they became separate species. In fact, all species share a common genetic code.
This shows that natural selection reuses genes and structures that worked well in the past.
All living organisms have their instructions for reproducing and operating encoded in a chemical language using four bases, adenine (A), cytosine (C), guanine (G), and thymine (T).
The combinations of the bases specify which amino acids the cell uses to make proteins for use in cellular functions. The fact that every living species carries the same genetic code indicates a common unique ancestor sometime in the distant past.
Characteristics of homologous organs
Homologous organs have the following characteristics:
- They differ morphologically.
- They have a similar internal structure.
- They develop in related organisms.
- The stages in development are similar.
- They perform different functions.
- They have a similar development pattern.
- Homologous organs show adaptive radiation (divergent evolution).