A few years ago I debated an evolutionist who claimed that the human embryo's gill slits are powerful evidence for evolution. The idea is that as new species evolve, their embryonic development tends to build upon the embryonic development stages of the ancestral species. Imagine a 10-story building is constructed, and then years later a few more stories are added to the top of the building. The first 10 stories would remain unchanged. Similarly, nineteenth century evolutionists expected that the embryonic development of an organism would reflect its evolutionary history.
Darwin contemporary Ernst Haeckel gave this idea the memorable moniker “ontogeny recapitulates phylogeny.” Its more technical title is "the biogenetic law." As with most evolutionary expectations it is now a relic. Indeed, even cousin species are found to have profound differences in their embryonic development.
Nonetheless evolutionists, like the one I debated a few years back, remain fond of the biogenetic law, if in a weaker form. Perhaps ontogeny does not recapitulate phylogeny, but those gill slits in the human embryo are obviously powerful evidence of an evolutionary history. Are they? Let's briefly review the biology.
Fish use their gills for gas exchange. The main problem for fish is in obtaining sufficient oxygen from the water. The oxygen content in water is about 20 times less than that of air. Both air and water breathers rely on the process of diffusion to obtain their oxygen, but that process is about 300,000 times slower in water. Also, because water is denser than air, more energy is required to move the fluid over the gas-exchanging surface. On top of all this, warm water presents even more of a problem for fish, because its oxygen content is reduced while the fish metabolism is increased, thus requiring more oxygen. Fish overcome these obstacles with a sophisticated active gas transport system--the gills--which transport oxygen from the water to the blood.
First, the many gill filaments that float off of the gill arches provide a large surface area over which the oxygen diffuses from the water into the bloodstream. Next, a near continuous flow of water over the gills is created by a combination of positive pressure from the mouth cavity upstream of the gills and negative pressure from the opercular flaps downstream of the gills. The opercular flaps also serve to protect the vulnerable gills. Finally, the diffusion rate is maximized by using unidirectional and opposing fluid flows. That is, the external water flow and the internal blood flow do not reverse direction (as with the air in our lungs, for example) but continues along in the same direction. Also the water and blood move in opposite directions. This counter current flow produces a higher diffusion rate than if they flowed along together in the same direction.
Of course none of these designs are apparent in the early stages of development. In these stages fish and human embryos take on different forms. One exception is the pharyngula stage in which the different vertebrate embryos have a fish-like form, including a series of paired folds or grooves. In fish these lead to gill slits while in other vertebrates they lead to various structures.
So why is this such powerful evidence for evolution? Simple, it refutes design and creation. As evolutionist Tim Berra explained, as though reporting on a scientific finding, "The passage through a fishlike stage by the embryos of the higher vertebrates is not explained by creation, but is readily accounted for as an evolutionary relic."
Similarly philosopher Elliott Sober, in a recent paper, informs the reader that human fetuses have gill slits, and that such structures are evidence for common ancestry precisely because they are useless in humans.
Aside from the fact that human fetuses, in fact, do not have gill slits and that therefore reports of their uselessness are greatly exaggerated, Sober's paper reveals the subtle, yet enduring reason why evolutionists find this evidence so persuasive. The reason does not come from science. Religion drives science, and it matters.