What Does Embryonic Development Say About Evolutionary Theory?
Most anatomical homologies, whether active or vestigial, are present in the adult members of a species. However, a few appear only briefly during the embryonic stage of an animal's development. These short-lived anatomical homologies are referred to as embryonic homologies.
The word homology is used to describe similarities. In biology, it is used to compare similar features in various species. The arm of a human is often compared to the wing of a bat, for instance.
Embryonic homologies are those similarities that are seen prior to adulthood. They too serve as evidence that the species in question is related to another species, even if those similar organs or anatomical structures are only found in embryos.
As the embryo develops it goes through a variety of stages, many of which show homologies between different species. Bird limbs are an important example of this: birds are tetrapods, all of which have five-digit limbs, but adult birds have a three-digit limb in its wings. This might appear to be a problem until you examine the embryos of birds. It is then that you will find that this limb develops from a five-digit precursor.
Another example is teeth in toothless whales. Some toothless whales do develop teeth as embryos and these are later absorbed in embryonic development.
Charles Darwin also noted that some snakes have immature pelvic bones. Remnants can be found in certain species, while these bones are reabsorbed in other species.
Even before Darwin, J.V. Thompson observed that the larvae of barnacles and crabs were strangely similar. This explains why barnacles are classified in the phylum Arthropoda rather than Mollusca. The barnacle may be more visually similar to mollusks like clams, but biologically -- specifically in embryonic terms -- they are crustaceans.
Embryology provides a strong source of homologies that need to be explained. Why should a toothless whale develop teeth that are later absorbed? Why should organisms which are so different as adults have so many similarities as embryos? Why should a bird's three-digit limb develop from a five-digit limb?
If life forms developed independently, one would think that their embryonic development would be distinct. In theory, the embryo should reflect what the organism will look like when it is fully developed.
The evolutionary answer is that evolution is conservative: evolution makes use of what has gone before. From the point of view of a natural process with limited resources, developing something new is much more difficult than modifying what already exists.
The embryological similarities are explainable by common ancestry. Whales develop teeth embryonically because they evolved from ancestors which had teeth. Birds develop their three-digit limbs as embryos from five-digit limbs because they evolved from five-digited ancestors.
Such development makes sense in light of evolution. Creationism has no explanation aside from "it's a mystery" and "God did it." Scientifically, these are obviously not legitimate arguments.