Where the whales came from .. maybe

It doesn't really matter how many transition fossils paleontologists uncover. Creationists are forever claiming fasely that they've produced "nothing which would qualify as intermediate" between today's whales and their terrestrial ancestors. So this week's fascinating news that we might have figured out what such a forebearer looked like is unlikely to change anyone's mind. Still, it would be nice if everyone who rejected evolution watched the video that the journal Nature has made freely available to help explain the paper it just published on what may be "the missing Eocene piece of the jigsaw."

The first thing to know about the discovery is it's far too early to add "Indohyus" to the list of extinct critters (Pakicetus, Ambocetus, Rhodocetus, etc.) that share a common ancestor with today's whales, dolphins and porpoises. Other researchers have yet to be convinced that this little raoellid ;;;;; related to today's African mousedeer ;;;;; belongs in that category.

"While this new hypothesis for the origin of whales is compelling, it will require further testing, especially since other recent studies have suggested both hippos and raoellids were involved in whale ancestry," San Diego State University biology professor Annalisa Berta said in an e-mail. Raoellids are the larger grouping of species that include the indohyus. (AP, Dec. 19)

Berta's the kind of expert that will need convincing. Along with James Sumich, she's co-author of one of the most common textbooks for budding marine biologists (Marine Mammals: Evolutionary Biology). But the Hans Thewissen, an anatomy professor at Northeastern Ohio Universities College of Medicine and his co-authors are probably on the right track as their new data address the very issues that form the foundation of Berta and Sumich's perspective on whale evolution.

The raoellid hypothesis is based largely on the fact that the fossils they've found show very heavy bones -- which are necessary if a terrestrial mammal is going to try to walk about in the water, for the same reason divers use weight belts to keep them below the surface. That and the fact that isotopic analysis of those bones shows the oxygen in those bones originated in the water.

That may not sound like a lot, but it is strong evidence in favor of the notion that the species involved, an even-toed ungulate, spent a lot of time in the water. And that is exactly what's been missing in the search to fill in the gap between whatever entirely terrestrial whale ancestor and the first aquatic whales of 50 million years ago.

And it led to Thewissen's working hypothesis, which is:

... that raoellid ancestors, although herbivores or omnivores on land, took to fresh water in times of danger. Aquatic habits were increased in Indohyus (as suggested by osteosclerosis and oxygen isotopes), although it did not necessarily have an aquatic diet (as suggested by carbon isotopes). Cetaceans originated from an Indohyus-like ancestor and switched to a diet of aquatic prey. Significant changes in the morphology of the teeth, the oral skeleton and the sense organs made cetaceans different from their ancestors and unique among mammals.

Not that anything a scientist has done in the past should affect critical analysis, but Thewissen has a good track record. Roger Highfield of The Telegraph's sums up his accomplishments nicely:

The professor has revolutionised our understanding of cetaceans. The discovery of the first, and at that point only, amphibious whale, Ambulocetus natans, was published in Science by Prof Thewissen's team in 1994. In 2001, his team discovered the skeleton of Pakicetus attocki, the oldest known whale. Pakicetus and Ambulocetus represent the two earliest stages of whales, and Indohyus complements this by showing it what the ancestors of whales looked like.

In 2002, his study of the organ of balance in whales revealed when their ancestors first took to the water. The semicircular canals that sense head movements as part of the organ are curiously small, far smaller for their body size than in any other mammal.

The semicircular canals of the huge blue whale are just smaller than in humans, and those of a dolphin are not much larger than in a mouse. By taking 3D X-ray images of the inner ears preserved in ancient fossils the researchers concluded that early whales acquired tiny semicircular canals early in their evolution, taking to the water between 50 and 40 million years ago.

Still, we must keep an open mind. Among others for whom the evidence is less than compelling is Sciblogger Brian "Laelaps" Switek:

Overall, the position of Raoellids as a sister group to cetaceans based upon Indohyus seems pretty flimsy, especially when presented in a few scant pages in Nature. Much more research is required to help resolve this issue,... so at the moment all I can really say about the hypothesis is "It's an interesting idea." ... If nothing else, this is an area that merits some more research and the hypothesis could be tested by going back to Asia to try and find some older fossils closer to when Raoellids and Cetaceans would have split from each other (especially on the cetacean line), but I guess we'll just have to wait.

Brian is also disappointed with the way the AP science reporter, Seth Borenstein, described the species in question, as "an odd raccoon-sized animal that looks like a long-tailed deer without antlers. Or an overgrown long-legged rat." But considering the language that Thewissen et al use in their paper and the illustration by Carl Buell (above) that accompanies the story, I would call it pretty fair. Until someone supplies a better description, it works for me. Yes, the creationists are going to spin it to their ends, but they'll do that regardless of the language. I think we should just be happy that this discovery is getting loads of media attention.

Whale evolution has long been a tricky thing ;;;;; we still can't agree on how sperm whales fit into the the mysticete-ondotocete split, for example. But it makes for fascinating reading, and at least the coverage assumes the reality of biology evolution instead of being sidetracked by ignorant commentators who don't know the difference between a whale and whale shark.