DNA suggests whales descended from land mammals

If you were asked what kinds of animals a whale most resembles, what would you say? Fishes? Sharks? Manatees? Perhaps even seals and sea lions? Whales live their whole lives in the water, and have a host of anatomical and physiological features that make them feel right at home there. So if DNA encodes these characteristics, you might think that whales should have DNA most similar to other aquatic animals.

Yet evolutionary biologists have long suspected that whales descended from land mammals. Whales are classified as mammals, having hair (albeit not much) and producing milk for their young, and nearly all other mammals are land dwelling. But is there any more evidence of this idea? Indeed, paleontologists have uncovered fossils that seem to document a transition from land dwelling whale-like mammals to fully aquatic whales. The distribution of these fossils further suggests that these mammals were geographically restricted until they made the plunge into water. Developing whale embryos even have tiny hindlimb buds that eventually disappear, implying that these beasts still retain some of the genetic machinery to produce four limbs.

So if fossils and developmental biology seem to be telling the same story, what does DNA say? To the initial surprise of paleontologists, whales have DNA that is extremely similar to even-toed hoofed mammals (artiodactyls), so-named because they always have an even number of toes. This group include animals like cows, deer, giraffes, hippos, pigs and camels.

In fact, whales aren’t just genetically similar to these mammals, they are genetically nested within them. By this I mean they are more genetically similar to some hoofed mammals than these hoofed mammals are to each other.

You can see this in the phylogeny below coming from a study [1] that compared 164 different species of mammals using 35,603 letters of DNA. Whales, indicated in part by paintings of a humpback and sperm whale near the middle of the figure, are most genetically similar to hippos (Hippopotamidae), a perhaps unsurprising finding given the semi-aquatic nature of the latter animals. The next closest group includes a batch of other hoofed mammals such as giraffes (Giraffidae; indicated by the okapi painting), deer (Cervidae; also associated with a painting), as well as cows (Bovidae), pronghorn antelope (Antilocapridae) and others. Outside of this whale + hippo + deer, cow, pronghorn, etc. grouping are the pigs and their relatives (Suidae + Tayassuidae) and camels (Camelidae).

Mammal phylogeny
Mammal phylogeny

It can be rather shocking when you first think about this given how completely different whales look from these animals, but there are distinct clues in the fossil record that point to this same conclusion. For example, some of the proto-whale fossils, such as Pakicetus, share some features that are unique to even-toed hoofed mammals, including a special indentation (trochlea) on the bottom of an ankle bone (astragalus).

Astragalus of Pakicetus compared to other hoofed mammals
Astragalus of Pakicetus compared to other hoofed mammals

When considering the evidence from DNA in conjunction with fossils, biogeography, development and now-defunct genes, a compelling picture is painted in which a group of hoofed mammals, against all odds, transitioned to a life in the oceans. Perhaps next time you go whale watching, think of it as viewing a herd of underwater giraffes or oceanic camels. You probably won’t look at whales the same way again.

Questions for Creationists

Why would the Creator design whales to have DNA so similar to hoofed mammals? With all of their adaptations for living in the water, shouldn’t their DNA be more similar to fish, sharks or other aquatic mammals such as manatees or seals? Is it possible that whales and hoofed mammals belong to the same kind, but whales evolved from these hoofed mammals over just a few thousand years? Wouldn’t this involve major evolutionary change at extraordinary speeds? Is it just a coincidence that whale DNA seems to tell the same story as the fossil record, geography and development?

References

1. Meredith, R. W., Janečka, J. E., Gatesy, J., Ryder, O. A., Fisher, C. A., Teeling, E. C., … & Murphy, W. J. (2011). Impacts of the Cretaceous Terrestrial Revolution and KPg extinction on mammal diversification. Science, 334(6055), 521-524.

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6 thoughts on “DNA suggests whales descended from land mammals

  1. Pingback: Pseudogenes: Whales have remnants of tooth genes | Evolution For Skeptics

  2. Pingback: Transitional fossils: From land mammals to the ancestor of modern whales | Evolution For Skeptics

  3. Pingback: Transitional fossils: Manatees that walked on land – Evolution For Skeptics

  4. Pingback: Ontogeny recapitulates phylogeny: Fetal whales have hindlimb buds – Evolution For Skeptics

  5. Pingback: From the ancestor of modern whales to baleen whales – Evolution For Skeptics

  6. Pingback: Whales walked in India and Pakistan before swimming around the world – Evolution For Skeptics

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