Explore Evolution argues that if common descent predicts both similarity and dissimilarity of embryos, it is impossible to challenge the theory. The similarity of embryos is best explained by common descent. The dissimilarity of embryos can be explained by environmental adaptations under natural selection, and while it can cause scientists to re-evaluate particular claims of common ancestry, no embryological evidence now available would cause scientists to reject universal common ancestry.
Explore Evolution insists
By arguing that common descent predicts both embryonic similarity and dissimilarity, Darwinists have effectively made it impossible to challenge the theory with counterevidence. When the case is stated this way, common descent would be consistent with whatever we observe in embryos.Explore Evolution, p. 70
In this claim, Explore Evolution fails to distinguish two main threads of modern evolutionary biology; common descent and the role of natural selection in adaptation. As noted earlier, Darwin realized that embryos would show differences if they had adaptations to different environmental circumstances. Perhaps the most important adaptation is the amount of yolk, which affects the egg size and the amount of stored nutrition available for the developing embryo. One of the most striking differences is between animals that undergo metamorphosis from a feeding larval stage and animals that undergo direct development, bypassing the larval stage. The comparative analyses of sea urchin embryos and larvae has shed light upon the adaptive strategies of these different modes of development.
Observations of a [sic] sea urchin larvae show that most species adopt one of two life history strategies. One strategy is to make numerous small eggs, which develop into a larva with a required feeding period in the water column before metamorphosis. In contrast, the second strategy is to make fewer large eggs with a larva that does not feed, which reduces the time to metamorphosis and thus the time spent in the water column. The larvae associated with each strategy have distinct morphologies and developmental processes that reflect their feeding requirements, so that those that feed exhibit indirect development with a complex larva, and those that do not feed form a morphologically simplified larva and exhibit direct development.Smith, Zigler and Raff, 2007. "Evolution of direct-developing larvae: selection vs loss." Bioessays, 29:6, p. 566
Significantly, these different modes of development are found in closely related species who diverged relatively recently, about 4 million years ago, and have occurred in multiple instances in other echinoderm lineages. The comparison of indirect and direct development has demonstrated that the earliest stages of development are relatively plastic (Raff ref).
Shared features of organisms are normally most parsimoniously explained by inferring common descent. The support for common descent, which is disputed by Explore Evolution but accepted by the vast majority of biologists, arises from the independent convergence of evidence from a wide variety of fields including biogeography, biochemistry, molecular biology, and embryology. One such shared feature of primate embryos, including human, is a tail. Primates which lack a tail as adults, such as humans and chimps, resorb the tail during later embryogenesis. Primates which have a tail as adults, do not resorb the embryonic tail. Phylogenetic analysis have unambiguously demonstrated that the ancestors to primates were tailed. The vast majority of biologists would consider human embryonic tails to be best explained by common descent. How would Explore Evolution explain it to students?