Whales are a tremendous pool of evidence for Evolutionary theory …
"The smoking gun for tooth loss in Mysticetes turned out to be exactly what was predicted: a fourth gene, necessary for enamel production, and mutated with the same inactivating mutation in all modern toothless whales. The gene in question, named enamelysin, was destroyed when a mobile genetic element called a SINE transposon inserted into it, breaking it into two halves and removing its function: . . .
“The fact that the same SINE insertion mutation at an identical location is found in all modern Mysticete species indicates that this mutation happened once in a common ancestor and then was inherited by the entire group. Since this must have occurred early in the evolution of toothless whales in order to happen in the common ancestor of the entire group, the picture from the genetics and the fossil record match.”
" Once again, findings in one discipline (in this case, paleontology) can be used to make very detailed predictions about what another, unrelated discipline (comparative genomics) should reveal. These results are also entirely consistent with the observation, made in the 1920s, that toothless whales form tooth buds during embryogenesis that are later reabsorbed prior to the point when the deposition of enamel would begin. As with the hind limb story in whale evolution, lines of evidence from genetics, paleontology and embryology converge to support the hypothesis that modern toothless whales descend, through modification, from toothed ancestors."