Getting in the middle of a scientific controversy is more dangerous than you might think. I have fond memories of a conference at which, under the influence of the product of yeast anaerobic sugar metabolim, I watched two well known professors very nearly come to blows over a question regarding the role of chromatin in gene expression. When you throw in a group of pseudo-scientific know nothings, that debate becomes both fiercer and more absurd. Such is the “controversy” around macroevolution – evolution at the species level – it regrettably melds science, religion and politics in a sort of perfect storm of dispute.
Catherine Boisvert knows a little bit about this particular scientific controversy. The Canadian graduate student (studying at Uppsala in Sweden) found herself in the midst of an ridiculous debate when some creationists took, completely out of context, a couple of quotes that she gave in an interview with The Scientist and loudly and ignorantly claimed that they supported their position.
In reality, Boisvert’s recent Nature paper offers irrefutable evidence supporting macroevolution. Boisvert and her colleagues at Uppsala used fossilized remains of Panderichthys, a prehistoric fish, to shed light on an age old evolutionary question: whether digits (fingers and toes) are an evolutionary novelty to tetrapods (four limbed critters like us) or were present in some form in the last common ancestor of tetrapods and our fishy kin.
Recently, Boisvert was kind enough to take a seat on A Free Man’s virtual couch for a chat about evolution, fish and creationism. In a nod to my audience’s diversity, I’ve tried to start the interview out with more general topics and to move into the hard science as we move on.
AFM: My readers run the gamut from working scientists to lay persons. Can you clearly and concisely explain to the latter class why they should pay attention to your research?
CB: Anybody interested in their origins and how they evolved would be interested in knowing where their fingers come from. The origin of finger precursors in fish is fascinating given the fact that it allowed fish to transform into land animals and later exploit all ecological niches, from land, trees, air and even returning to the seas! The success of land vertebrates is closely tied to the evolution of limbs and fingers and I certainly would not be able to type this, or play violin for that matter, without them!
AFM: I’m interested in Panderichthys as a species. Where would it have fit in the food chain of the late middle Devonian? What kind of species roamed the sea with Panderichthys? What did it eat? What ate it? Paint a picture of Panderichthys’ neighbourhood.
CB: Panderichthys was a medium to large predator (1 – 1.5 m in length) living in shallow waters in deltaic systems (so probably a mix of salt and fresh water). It ate other fish, so you could say it was at the top of the food chain and I doubt that it had many, if any predators. At the time, the seas (in terms of vertebrate life) were dominated by lobe-finned fish such as Eusthenopteron and Panderichthys, placoderms (armoured jawed fish) as well as acanthodians. Sharks were beginning their radiation and so were ray-finned fish, which were still relatively rare and very small in the Devonian. There were of course an abundance of invertebrates at the time and insects were beginning their foray onto land, only preceded by plants.
AFM: It’s been proposed that the driving pressure to diversify limbs came from demands of feeding and locomotion in the Ordovician and Silurian seas (Shubin, et al. 1997). Do you agree with this assessment? What were the challenges faced by paleozoic fish?
CB: It is obvious that feeding and locomotion are main drivers of evolution since they are so closely related to survival. What Shubin and co-authors refer to in their 1997 article pertains to the diversity of fin forms observed in vertebrates in the Ordovician and Silurian seas and how much “experimentation” there was. As agnathans (jawless vertebrates) are being outcompeted by gnathostomes (jawed vertebrates) in the Silurian, the body plan of vertebrates stabilizes at two sets of paired fins (pectoral and pelvic) (the body plans of gnathostomes). The challenges of moving to feed and of moving to avoid predators are no different in gnathostomes than they were in agnathans and are therefore some of the challenges faced by Paleozoic fish.
AFM: Your paper has pretty much put the nail in the coffin of the hypothesis that gained some credence in the 1990’s that fingers and toes are an evolutionary novelty. What was that hypothesis based on? And for those who haven’t read your paper, can you briefly outline how your finding rejects that hypothesis.
CB: The hypothesis was based on the comparison of developmental data from Zebrafish and mice. Zebrafish lack the second phase of expression of the gene Hoxd13 which is responsible for the formation of digits in mice. This led Denis Duboule’s team to hypothesize that digits were novelty in tetrapods (four footed vertebrates like amphibians, reptiles, birds and mammals). This was supported at the time by the fact that the pectoral fins of the fossil fish Panderichthys, then the closest fish to tetrapods, had been described as being composed of large plates at the end of the fin. This pointed to the fact that this fossil did not have any elements that could be identified as precursors of fingers. Our new analysis of Panderichthys performed by CT-scan showed that this was an artefact of preparation. Panderichthys has small elements at the end of its pectoral fins that we interpret as fingers precursors.
AFM: Macroevolution isn’t my strong suit and as a geneticist I’m often swayed by developmental genetic data. Is there any molecular work that has been done that supports your hypothesis?
CB: Indeed, there is. It is in combination with new fossil data (from Tiktaalik and Gogonasus for example) as well as developmental genetic data from skarks, the basal actinopterygian (ray finned fish such as zebrafish and salmon), Polyodon (paddlefish, a close relative of the sturgeon) and the sarcopterygian (lobe finned fish who gave rise to all amphibians, reptiles, birds and mammals) Neoceratodus (the Australian lungfish) that we are able to confidently homologize the distal radials to fingers. Several studies have been conducted in the past few years about the expression of Hoxd13 in those species and, in all of them, a second, late phase of expression is present. Zebrafish, being a derived ray finned fish, has a very reduced fin. It lacks the metapterygium, from which the entire fin of lobe finned fish is derived. It is therefore understandable that by loosing this part of the fin, genetic expression associated with it would be lost as well. Sharks and paddlefish have a more primitive fin retaining all parts and therefore show Hoxd13 expression in their fins where the metapterygium develops. In the Australian lungfish as well as all tetrapods, only the metapterygium is retained and the late phase of Hoxd13 is expressed where distal radials, or in the case of tetrapods, fingers, develop.
AFM: As you’re no doubt aware, creationism in the mantle of “Intelligent Design” is creeping back into schools in the U.S., Britain and many other parts of the world. What could a secondary school teacher take from your research into their classroom to serve as a counterpoint to the pseudo-science of ID?
CB: Our work shows how similar the fins of Panderichthys are to that of Tiktaalik, Eusthenopteron and the arms of Acanthostega while being slightly different. These similarities and how they differ can only be explained by shared ancestry and evolution. Paleontologists have hypothesised a long time ago that Panderichthys was more closely related to tetrapods than Eusthenopteron was and by examining the fossil and finding that its fins are intermediate in morphology between Eusthenopteron and Acanthostega for example, it proves the predictive nature of evolution.
AFM: The creationist Discovery Institute has pounced on some of the statements in your paper regarding sample quality as evidence that scientists are trying to backpedal on previous hypotheses regarding digit development and evolution. Can you clarify your statements regarding sample quality of Tiktaalik and Panderichthys?
CB: As you know, the “Discovery” Institute tactic is not to go to the primary literature in order to understand it but rather to use quotations from secondary, even tertiary sources, reorganise or use them out of context opportunistically to their own convenience. In this case, they used an article where the journalists unfortunately misunderstood me. Tiktaalik’s material is in fact exquisite, it is very well preserved, basically uncrushed and can be prepared out to be examined in three dimensions. I never said the quality was poor. I have simply explained that the morphology of the fin of Panderichthys is more tetrapod-like than that of Tiktaalik, which has nothing to do with the quality of the material.
AFM: Specifically regarding the sample quality of Panderichthys, how does CT scanning permit the type of analysis that you presented in the Nature paper?
CB: The material of Panderichthys is of high quality but it is material that is extremely difficult to prepare and manipulate because it is so fragile and preserved in clay. Previous analyses of the fin was based on prepared material but when I visited the institution in Moscow where the specimens were housed, I noticed that it was incompletely prepared, producing the results published in the 1990’s. It is almost impossible to prepare this material without destroying the underlying fin endoskeleton and I do not think many palaeontologists would have dared preparing this precious specimen. Our analysis was based on the CT-scanning of another specimen, housed in Estonia. This technique permitted us to visualise the endoskeleton, the scale covering as well as the shoulder girdle without destroying anything. We then produced three-dimensional models that can be rotated and manipulated to understand the morphology.
AFM: What’s next for you? I see that you’ve joined us Down Under. What have you got going on in Australia?
CB: I was indeed in Australia when you joined me. I was continuing developmental studies on the Australian lungfish, provided by Jean Joss’ laboratory at Macquarie University. I am interested in understanding how the pelvic girdle transformed during the fish tetrapod transition and, in addition to my fossil work, I am studying the development of that structure in the Australian lungfish and salamanders. I will be finishing my PhD soon, here in Sweden and will return to Australia for a post-doc in developmental genetics.
Man, they don’t mess around in Uppsala. Catherine has two first author Nature papers to her name and is still working toward her Ph.D.! Thanks to Catherine for taking a time out from a clearly strenuous Ph.D. project to talk about her work today.
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