DNA suggests that whales are descended from hoofed mammals, and the fossil record also appears to document a transition of four-limbed whales to modern species that only have forelimbs. Looking at the ontogeny, or development, of whales also provides evidence of a past when the ancestors of these aquatic creatures walked on land.
Below is a picture of an Atlantic spotted dolphin (Stenella frontalis), which very clearly has forelimbs (flippers) but no hindlimbs, typical of whales.
But when this and related species are still embryos, you can see that tiny hindlimb buds form .
If you look at a series of embryos through their development, the hindlimb buds form but then they disappear.
Interestingly, dolphins seem to retain some of the genetic developmental machinery to make hindlimbs. Occasionally, people have found individual dolphins that have hind flippers, such as the one in the picture below.
This is called an atavism, and is thought to arise from one or more mutations that somehow turn the development of the hindlimbs back on.
Together, this is a remarkable example where DNA, fossils and ontogeny all tell the same story, specifically that whales descended from ancestors that walked on four legs. These animals appear to have gradually lost their hindlimbs, presumably as an adaptation to streamline their bodies for swimming, but their ontogeny seems to retain a record of their fully-limbed past.
Questions for Creationists
Is it just a coincidence that DNA, fossils and ontogeny all suggest that whales descended from four-limbed ancestors? Why would God create dolphins with hindlimb buds that simply disappear? Is it just a coincidence that we see the same pattern of limb buds forming and then disappearing in legless lizards?
1. Thewissen, J. G. M., Cohn, M. J., Stevens, L. S., Bajpai, S., Heyning, J., & Horton, W. E. (2006). Developmental basis for hind-limb loss in dolphins and origin of the cetacean bodyplan. Proceedings of the National Academy of Sciences, 103(22), 8414-8418.