Planet Earth

Missing The Wrist

The LoomBy Carl ZimmerJul 14, 2008 9:07 PM


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We're all for open and objective discussions of scientific theories, right? Who wouldn't be? If your kids are taking physics in high school, you want them to read critiques of gravity, right? After all, shouldn't they know that there are some serious weaknesses in the theory of gravity? Right? For instance, the theory of gravity says that gravity makes things fall down. But planets don't fall into the sun. They go around it. So which is it--down or around? Clearly the theory of gravity is deficient. Right? Wrong, of course. You don't teach critical thinking with patent nonsense. A couple weeks ago Louisiana passed a new science education act that promotes "critical thinking skills, logical analysis, and open and objective discussion of scientific theories being studied including, but not limited to, evolution, the origins of life, global warming, and human cloning." Along with the regular textbook, the law states, teachers "may use supplemental textbooks and other instructional materials to help students understand, analyze, critique, and review scientific theories in an objective manner." The law "shall not be construed to promote any religious doctrine, promote discrimination for or against a particular set of religious beliefs, or promote discrimination for or against religion or nonreligion." What the law does not make clear, however, is how schools will determine whether the extra instructional material is good science or nonsense. There is nothing in the law that would keep a teacher from introducing a bogus non-argument about gravity and the revolution of the planets. I was reminded of this sad fact when I read a post published today by Casey Luskin, a staffer at the Discovery Institute, an outfit that promotes intelligent design. Luskin has been one of the leaders of the Discovery Institute's efforts to get so-called "academic freedom" bills passed in states around the country. He personally testified in Louisiana in favor of their new education bill. When he's not busy with politics, Luskin writes posts at the Discovery Institute's "Evolution News and Views" site, where he "critiques" research on evolutionary biology, claiming to find major flaws. But his critique make as much sense as the falling-or-revolving challenge to gravity. The subject of the post is a 375-million-year-old fossil that helps reveal the transition of our ancestors from the water to land, known as Tiktaalik. I've written about Tiktaalik here, and you can get more details from the book Your Inner Fish, written by Neil Shubin, one of Tiktaalik's discoverers. (Here's a review I wrote in Nature.) Luskin claims that Neil Shubin calls Tiktaalik a fish with a wrist, but "from what I can tell, Tiktaalik doesn't have one." The bulk of the post is taken up by Luskin's fruitless search for a diagram or some other helpful information, either in Shubin's book or the original papers. He is frustrated not to find a picture showing a wrist on Tiktaalik compared to the wrist of a tetrapod (a land vertebrate). This sort of "evidence" leads Luskin to conclude that Shubin has something to hide. "In the end, it’s no wonder Shubin chose not to provide a diagram comparing Tiktaalik's fin-bones to the bones of a real tetrapod limb," he writes. Instead, Luskin is forced to read a scientific paper. He writes:

So we are left to decipher his jargon-filled written comparison in the following sentence by sentence analysis: 1. Shubin et al.: "The intermedium and ulnare of Tiktaalik have homologues to eponymous wrist bones of tetrapods with which they share similar positions and articular relations." (Note: I have labeled the intermedium and ulnare of Tiktaalik in the diagram below.) Translation: OK, then exactly which "wrist bones of tetrapods" are Tiktaalik’s bones homologous to? Shubin doesn’t say. This is a technical scientific paper, so a few corresponding "wrist bone"-names from tetrapods would seem appropriate. But Shubin never gives any.

Um...Shubin did give them. They are called the intermedium and ulnare. (I just double-checked, for example, in Vertebrates by Ken Kardong, on p.332.) Shubin and his colleagues found two bones in the limb of Tiktaalik that bear a number of similarities to the intermedium and ulnare in the tetrapod wrist--in terms of their arrangement with other limb bones, for example. That's why Shubin and company refer to the bones in Tiktaalik's limb by the same two names. They are homologous--in other words, their similarities are due to a common ancestry. So Luskin wants to know what bones in the tetrapod limb are homologous to Tiktaalik's intermedium and ulnare. The answer is...the intermedium and ulnare. He has unwittingly answered his own question. Now, perhaps Luskin got tripped up in Shubin's "jargon-filled" writing. But that doesn't change the facts--merely Luskin's understanding of them. Luskin's entire post is based on a mistaken notion of homology--the similarity of traits due to common ancestry. The bones of a bird's wing do not look just like a human arm. Many of the wrist bones in our arm are not present in a bird's wing, for example, and instead of five fingers the bird has a rod-like bone at the end. But they still bear an overall resemblance in their arrangement. And when evolutionary biologists arrange birds and mammals in an evolutionary tree, they can see some of the steps by which an ancestral tetrapod limb evolved into our arm in one lineage, and into the bird wing in another. Shubin and his colleagues offer a detailed analysis in their paper of how the intermedium and ulnare in Tiktaalik are homologous to the bones of the same name in tetrapod wrists. Not only do the bones have similar arrangements, but they also allow the limb to bend in a similar way to how the tetrapod wrist bends the hand. They also present evidence for the homology of other bones in Tiktaalik's limb to the tetrapod limb. Some bones in the tetrapod limb don't exist in Tiktaalik's, and some of the bones that are there are different in some respects--size, and shape, and so on. But the relationship of the bones to each other makes sense if they're the result of a shared ancestry with tetrapods. To say that Tiktaalik lacks a wrist because it doesn't have all of the bones in a tetrapod limb is to misunderstand how evolution works. Luskin suggests instead it would be easier to make Tiktaalik a forerunner of lungfish. (Lungfish are among the closest living relatives of tetrapods, but our last common ancestor with them lived over 400 million years ago.) "Without trying to force-fit the fin of Tiktaalik into a pre-conceived evolutionary story," he declares, "the living species that Tiktaalik's fin seems to bear a much closer relationship to is the lungfish." Note the word seems. If Luskin were offering a real scientific hypothesis, he could do an anlysis of lungfish, Tiktaalik, tetrapods, and other vertebrates--comparing not just their limbs but their heads, spines, and so on to figure out their evolutionary relationships. That's exactly what Shubin and his colleagues did in their original paper on Tiktaalik. They compared 114 traits on species from nine different lineages of tetrapods and their aquatic relatives, including the lineage that produced today's lungfish. And that analysis shows that Tiktaalik is more closely related to us than to lungfish. Luskin apparently doesn't need to do this sort of science. He can just announce what seems right to him personally. If this is the sort of stuff that's used to promote "critical thinking" in Louisiana classrooms, don't be surprised to hear about the great gravity hoax. Update: PZ Myers has more.

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