Anatomical and Embryological Evidence for Evolution

Anatomical and embryological evidence for evolution comes from comparing the body structures and early development of different species, which show similarities best explained by descent from common ancestors. In brief: homologous structures, such as the forelimb bones of humans, bats, whales and cats, have the same basic plan but different functions. Vestigial structures, such as the human appendix and the pelvic bones of whales, are reduced remnants of features that were functional in ancestors. Embryos of different vertebrates are very similar in the early stages.

Evidence is a central part of the theory of evolution, and comparative anatomy was among the first kinds of evidence that Darwin used. This guide explains homologous, analogous and vestigial structures, describes embryological evidence, compares the types of structures in a table, works through an exercise on interpreting limb bones and corrects common misunderstandings.

What kinds of evidence support evolution?

Several independent lines of evidence support the theory that species change and share common ancestors. They include the fossil record, comparative anatomy, embryology, biogeography (the distribution of species), molecular biology (similarities in DNA and proteins) and direct observation of evolution, as in antibiotic resistance. This article covers anatomy and embryology. The other lines are covered in the following articles.

The logic is that if species descended from common ancestors, they should show a pattern of shared features, with close relatives sharing more features than distant ones.

What are homologous structures?

Homologous structures are body parts in different species that have the same basic structure and origin, even if they have different functions. They result from inheritance from a common ancestor.

The classic example is the pentadactyl (five-digit) limb of vertebrates. The forelimb of a human (grasping), a cat (walking), a whale (swimming), a bat (flying) and a horse (running) contains the same set of bones in the same arrangement: one upper-arm bone (humerus), two forearm bones (radius and ulna), wrist bones (carpals), palm bones (metacarpals) and finger bones (phalanges). The proportions and shapes of the bones have been modified for different functions. This is difficult to explain if each animal was designed independently for its function, but it is expected if all are descendants of an ancestor with this limb pattern. The process in which a similar structure becomes adapted to different functions in different lineages is called divergent evolution or adaptive radiation.

What are analogous structures?

Analogous structures have a similar function, but a different origin and structure. They arise through convergent evolution, in which unrelated organisms face similar environmental challenges and evolve similar adaptations. Examples include the wings of birds (modified forelimbs with feathers), bats (skin stretched across elongated finger bones) and insects (outgrowths of the body wall with no bones). All can fly, but the structures are not inherited from a common winged ancestor. The streamlined bodies and fins of sharks (fish) and dolphins (mammals) are another example.

What are vestigial structures?

Vestigial structures are reduced or non-functional (or with a reduced function) in one species, but are homologous to functional structures in related species. They are leftovers of structures that were useful in ancestors.

• Human appendix: a small pouch off the large intestine, which in plant-eating relatives is a larger structure that helps to digest cellulose. In humans it may still have a small role in the immune system, so “vestigial” does not always mean completely useless.
• Tailbone (coccyx): the remnant of a tail, found in humans and other apes, which have no external tails.
• Pelvic bones in whales and some snakes: small bones that are not attached to the backbone, which are remnants of hind limbs of land-living ancestors. Fossils such as Pakicetus and Ambulocetus show the intermediate steps.
• Wings of flightless birds, such as ostriches and kiwis.
• Eyes in cave animals, such as blind cave fish, which have reduced or lost eyes.
• Wisdom teeth in humans, which are often too large for the smaller jaw.

What does embryology show?

Comparative embryology is the study of the development of embryos from different species. Early embryos of vertebrates (fish, amphibians, reptiles, birds and mammals) look very much alike. All have a tail, and pharyngeal pouches (which become gills in fish and parts of the ear and throat in mammals). Human embryos develop a tail and these pouches at about four to five weeks, and the tail later regresses.

These similarities are best explained by common ancestry: the genetic instructions for early development are inherited from the same ancestor and are modified later in development. They are not evidence for the 19th-century claim of Ernst Haeckel that “ontogeny recapitulates phylogeny”, which stated that embryos pass through the adult forms of their ancestors. That idea was oversimplified and is not accepted. The modern field of evolutionary developmental biology (evo-devo) shows that conserved genes, such as the Hox genes, control the body plan in animals as different as flies and mice.

Types of structures compared

Worked example: interpreting limb bones

A student compares the forelimb bones of four vertebrates: a human, a bat, a dolphin and a bird. The following observations are made.

1. All four have one upper-arm bone, two forearm bones and several wrist and hand bones.
2. In the bat, the finger bones are extremely long and support a wing membrane. In the dolphin, the bones are short and flat inside a flipper. In the bird, fingers are fused and reduced, and the feathers are attached to the forearm and hand. In the human, the fingers are free and mobile.
3. The wing of a bat and the wing of a bird both give flight, but the bat wing is formed from skin and the bird wing from feathers.

Conclusions:

• The bones in all four limbs are homologous: the same set is present in the same order, which supports the idea that these animals have a common ancestor with such a limb.
• The wings of bats and birds are analogous as flying structures, because they evolved independently from the same ancestral limb in two lineages. The bones are homologous, but the wing as a flight surface is analogous.
• The differences between the limbs show adaptation to different functions: grasping, flying and swimming.

Note that the same structure can be homologous at one level and analogous at another. A good exam answer states both the evidence (the bone pattern) and the explanation (common ancestry), not just the observation.

How did the comparative method develop?

Georges Cuvier, in the early nineteenth century, developed comparative anatomy and showed that animals can be classified by their body plans. Richard Owen defined the terms homology and analogy in the 1840s. Darwin used the similarity of homologous structures and of embryos as evidence in On the Origin of Species. Karl Ernst von Baer observed in the 1820s that embryos of different animals are more similar in the early stages. Modern genetic tools allow scientists to test the evolutionary relationships suggested by anatomy.

Which mistakes are common?

Mistake 1: Mixing up homologous and analogous

Homologous means the same origin, and analogous means the same function.

Mistake 2: Saying that vestigial structures are useless

They have lost their original function, but may have other functions. The key point is that they are reduced.

Mistake 3: Claiming that human embryos have gills

They have pharyngeal pouches, which are similar to the structures from which gills develop in fish, but they never function as gills.

Mistake 4: Saying that humans evolved from chimpanzees

Humans and chimpanzees share a common ancestor that lived several million years ago. Neither species evolved from the other.