How can embryology contribute to an understanding of evolution

Looking at every level of organization in living systems, biologists see the signature of past and present evolution. Darwin dedicated a large portion of his book, On the Origin of Species , to identifying patterns in nature that were consistent with evolution. Since Darwin, our understanding has become clearer and broader. Fossils provide solid evidence that organisms from the past are not the same as those found today; they show a progression of evolution.

Scientists calculate the age of fossils and categorize them to determine when the organisms lived relative to each other. The resulting fossil record tells the story of the past and shows the evolution of form over millions of years. For example, scientists have recovered highly-detailed records showing the evolution of humans and horses. The whale flipper shares a similar morphology to appendages of birds and mammals, indicating that these species share a common ancestor.

Over time, evolution led to changes in the shapes and sizes of these bones in different species, but they have maintained the same overall layout.

Scientists call these synonymous parts homologous structures. Some structures exist in organisms that have no apparent function at all, appearing to be residual parts from a common ancestor. These unused structures such as wings on flightless birds, leaves on some cacti, and hind leg bones in whales are vestigial.

Embryology, the study of the development of the anatomy of an organism to its adult form, provides evidence for evolution as embryo formation in widely-divergent groups of organisms tends to be conserved. Structures that are absent in the adults of some groups often appear in their embryonic forms, disappearing by the time the adult or juvenile form is reached.

For example, all vertebrate embryos, including humans, exhibit gill slits and tails at some point in their early development. Echidnas, for example, develop their limbs much later than most other mammals.

But in his illustrations of echidna embryos, Haeckel deceptively omitted limb buds at early stages, despite the fact that limb buds do exist then. His followers tried to cast them as exceptions that proved the rule. But genes, it was soon discovered, controlled the rate and direction of embryonic development.

Individual genes can mutate and cause different changes to the way embryos grow — either adding new stages at any point along their path, or taking them away, speeding up development or slowing it down. Embryos do reflect the course of evolution, but that course is far more intricate and quirky than Haeckel claimed.

Different parts of the same embryo can even evolve in different directions. As a result, the Biogenetic Law was abandoned, and its fall freed scientists to appreciate the full range of embryonic changes that evolution can produce — an appreciation that has yielded spectacular results in recent years as scientists have discovered some of the specific genes that control development.

Biogeography: Wallace and Wegener. Subscribe to our newsletter. Through the process of natural selection and survival of the fittest, certain traits are more likely to be inherited than other traits. Embryology is the study and analysis of embryos. Evidence of an evolutionary common ancestor is seen in the similarity of embryos in markedly different species. Darwin used the science of embryology to support his conclusions.

Embryos and the development of embryos of various species within a class are similar even if their adult forms look nothing alike. For instance, chicken embryos and human embryos look similar in the first few stages of embryonic development.

These early similarities are attributed to the 60 percent of protein-coding genes that humans and chickens inherited from a common ancestor. Kowalevsky suggested that sea squirts called tunicates should be classified as chordates instead of mollusks because tunicate larvae have notochords and form neural tubes, making them more like chordates and vertebrate embryos. DNA analysis of the tunicate genome has since proven Kowalevsky correct.

The notion of recapitulation drew plenty of critics, notably Karl von Baer , who also took a disliking to Darwin's ideas. Modern evo-devo experts like Michael Richardson agree there are similarities in the embryonic development of related species, but mainly at the molecular level. For instance, human embryos have a tail that becomes the tail bone.