Comparative embryology supports the theory of evolution because scientists have found that the embryos of many different species show similarities, which implies they share a common origin. For example, in humans the embryo passes through a stage in which it has a gill structure similar to that of fish. Human embryos also have a tail, much like other primates, though the tail is usually re-absorbed before birth, and this suggests that, even though their adult forms are different, these various species all have a common ancestor.
Furthermore, the embryo serves as a microcosm for evolution. The embryo passes through many stages of evolution until it finally reaches its adult form.
While the appearance of ancestral traits in the embryos of many organisms is well documented and can provide insights into the evolution of a species, an early attempt to draw direct parallels between the development of an embryo (ontogeny) and the development of its species (phylogeny), known as the Recapitulation Theory, is widely believed to have been discredited by later science.