What is your favorite transitional form?

Aetiocetus

Carl Buell’s restoration of Aetiocetus weltoni. From Demere et al., 2008.

By now many of you have no doubt seen the abysmally bad story on evolution and creationism in yesterday’s Telegraph. After referring to the reactions of fundamentalist Christians to the forthcoming Charles Darwin biopic Creation (based upon the book Annie’s Box), the anonymous author of the piece presents the “Top 5” arguments for both evolution and creationism. The choices were baffling; it appeared that rather than do any actual research the writer extracted the selections from a bodily orifice that I will refrain from naming specifically.

The first point selected in favor of creationism immediately set my teeth on edge;

No evidence for evolution
There is no evidence that evolution has occurred because no transitional forms exist in fossils i.e. scientists cannot prove with fossils that fish evolved into amphibians or that amphibians evolved into reptiles, or that reptiles evolved into birds and mammals. Perhaps becuase of this a surprising number of contemporary scientists support the Creation theory.

I do not care if the author was trying to play “devil’s advocate”; it is grossly irresponsible to perpetuate this myth. Creationists deny the existence of transitional forms because for them evolution is not a possibility. It is a circular sort of reasoning. Anything that questions their fundamentalist interpretation of religion is automatically wrong and therefore any species, living or fossil, exhibiting transitional features is automatically barred from providing evidence for evolution.

Now I could point out the flaws in the piece point by point, but I do not think it would be of much benefit. (And PZ already did it.) If you are reading this blog you probably already agree with me about how important and exciting evolutionary science is and I have no desire to waste your time. Instead I have decided to write up a little something about one of my favorite transitional forms, and I encourage you to do the same in the comments or on your own blog. Contrary to the ignorant statements of the Telegraph piece there are so many transitional forms in the fossil record that I could spend months writing about them and still not cover them all. Rather than try to cover many examples in brief I thought it would be more profitable to examine one particular case in detail.


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The origin of whales has long been a controversial subject. I am not referring to the lame protestations of creationists, but to debates between scientists over how, when, why, and from what cetaceans evolved. Our present understanding (built upon an convergence of fossil, genetic, and developmental evidence) is a relatively new thing. During the latter half of the 19th century and much of the 20th century the details of whale evolution were frustratingly difficult to draw out.

One of the now-forgotten debates hinged upon whether living whales shared a common ancestor or not. All living cetaceans fall into one of two groups, the odontocetes (“toothed whales” like dolphins) or the mysticetes (“baleen whales” like the humpback whale), but the two groups seemed so dissimilar that some scientists doubted that they could have shared a common ancestor. The characteristics shared between the two groups of whales would hence be examples of evolutionary convergence in the extreme, with baleen whales having one ancestor and toothed whales having another.

Enter Aetiocetus. In 1966 D. Emlong of the University of Oregon described this 25-million-year old whale which exhibited a strange mix of features. Its skull was long, broad, and flat like that of a baleen whale yet it also had teeth. While initially identified as an “archaic whale” on the basis of its toothy grin, in 1968 the paleontologist Leigh Van Valen proposed an alternative view. Even if mysticetes had different early ancestors than odontocetes those ancestors still would have had teeth. Thus Van Valen deemed Aetiocetus to be a “baleen whale”, and this was the correct assessment.

Even though we can lump living whales into the “toothed” and “baleen” categories this distinction ceases to be useful as we look back into the fossil record. As Van Valen pointed out baleen whales clearly evolved from ancestors with teeth, so by what features can we more reliably tell the difference between an odontocete and mysticete? One way is to look closely at the bones of the skull.

Those who know little about evolution often assume that new anatomical features, like the blowhole of a whale, invariably appear out of nowhere. This just isn’t so. The blowhole of a whale is its nasal opening which was pushed back to the top of the skull by the elongation of other skull bones, primarily the maxilla (or the bone that makes up the upper jaw in mammals), during evolution. This is important in that the elongation of the maxilla did not happen in exactly the same way in toothed and baleen whales. In mysticetes the maxilla was elongated to the extent where it scoops downward and backward beneath the eye socket. This osteological quirk is not seen in odontocetes, and when we look at the maxilla in Aetiocetus it is immediately clear that it was a “baleen whale” with teeth.

Baleen

The skull of Aetiocetus weltoni, and a close-up of holes that once contained blood vessels that would have nourished its baleen. From Demere et al., 2008.

Things only got stranger from there. When paleontologists looked at the underside of the upper jaws of a particular species of Aetiocetus, A. weltoni, they discovered that it possessed nutrient foramina just like modern baleen whales. Nutrient foramina are little holes in bones that once housed blood vessels, and in living mysticetes these vessels supply blood to the plates of hair-like baleen that hang down from the roofs of their mouths. Whales without baleen do not exhibit the pattern of nutrient foramina seen in living baleen whales and Aetiocetus. This led scientists to a startling hypothesis; Aetiocetus had both teeth and baleen!

Eventually, though, baleen whales lost their teeth entirely, a fact confirmed by the presence of “fossil genes” that still exist in living mysticetes. In a 2008 paper a team of scientists not only laid out the evidence that Aetiocetus weltoni had baleen, but they also showed that baleen whales possess two genes involved in tooth formation, AMBN and ENAM, which are slowly being mutated away. Sometime during mysticete evolution a mutation caused a stop codon to form in these genes, which (as the name suggests) acts a kind of genetic “STOP” sign that prevents the genes from being fully expressed. (A recent PLoS Genetics paper also discusses this.) Some baleen whales develop tooth buds that are resorbed during fetal development, as well, clearly showing that they still possess some vestiges of their toothed ancestry. Whenever the loss of teeth in mysticetes occurred, though, Aetiocetus shows that it happened after the evolution of baleen.

It is difficult for me to conceive how anyone truly interested in science can look at this kind of evidence and deny that evolution is a reality. The existence of prehistoric whales with both teeth and baleen and the fact that modern mysticetes still carry the (albeit degraded) genes for tooth formation only make sense when considered in an evolutionary context. Despite the cries of creationists that “Darwinism” is ready to crumble the truth is that right now evolutionary science is an extremely vigorous area of research. It is extremely exciting to see more interdisciplinary work that incorporates evidence from various biological fields to help us better understand how life has (and continues to) evolve. To deny this is to be willfully blind.

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