Zebrafish Limbs

For Monday Jeff Tomkins writes “Did Scientists Make Fish Grow Hands?” The subject is a recent Developmental Cell paper called Hoxd13 Contribution to the Evolution of Vertebrate Appendages, which describes and experiment on zebrafish that induced them to grow what look more like limbs than fins, by promoting the expression of the Hoxd gene. This plugs a gap in our knowledge of how land animals evolved from fish – though, obviously, Tomkins disagrees.

A recent news story featured a variety of science writers repeating the meme “Fish grow ‘hands’ in genetic experiment.” These sensationalized stories attempted to describe a new genetics research study published in the journal Developmental Cell. The primary results of the study actually produced data that refuted the accompanying evolutionary hype.

Reference to memetics from a creationist cracks me up, but I suppose it’s now part of the general lexicon. Tomkins is quick to point out that no hands were actually produced:

For starters, the genetically modified zebrafish embryos under investigation had no hands at all. When New Scientist magazine questioned one of the researchers, Fernando Casares, about the popularized claim, he responded, “Of course, we haven’t been able to grow hands.”

The New Scientist article instead referred to the developed structure as an “autopod”:

One full day [after “extra copies of the gene” were injected “into the zebrafish embryos”], all of those fish whose cells had taken up the gene began to develop autopods instead of fins. They carried on growing for four days but then died.

“Of course, we haven’t been able to grow hands,” says Casares. He speculates that hundreds of millions of years ago, the ancestors of tetrapods began expressing more hoxd13 for some reason and that this could have allowed them to evolve autopods.

Hands are autopods, as are feet – autopods are apparently just the end of limbs. This distinction is the reason for the scare quotes around “hands” in most headlines, and why the New Scientist title itself called them “pre-hands.” They’re not all the way in one magical step, but it’s a start.

With that settled, where does Tomkins disagree?

Nevertheless, Casares and his colleagues did claim that their data had profound implications for the hypothetical evolutionary change required for fish fins to magically morph into legs, arms, hands, and feet. This mythical process would have been necessary for fish to transition to land animals. And contrary to common evolutionary claims, this dogma is not supported by any actual transitional forms in the fossil record.

The claim that there are no applicable transitional forms – a reference to the opening line of the abstract, which states that “[f]ossil data suggest that limbs evolved from fish fins by sequential elaboration of their distal endoskeleton, giving rise to the autopod close to the tetrapod origin” – is cited to The Fossil Record: Unearthing Nature’s History of Life, by “Morris, J. D. and F. J. Sherwin” (sic). Presumably this book tries to deny the existence and/or significance of Tiktaalik and the like.

Tomkins’ counter is that, instead of producing limbs, the mutant zebrafish were, well, mutansts – that and they died:

So, what were the effects of over-expressing the hoxd13 gene? Not only did the fish not develop hands or any other novel evolutionary [sic?] favorable appendage, but normal fish fin development was completely and grotesquely perturbed. The resulting embryos died within four days on average—hardly a hallmark of evolutionary progress.

To a certain extent this is a straw man. As we already saw, the scientists are not claiming that hands were produced – though Tomkins is likely just reiterating this point. More important is the “favourable” slipped in there.

Nobody is saying that the mutants should have been viable. The researchers made one change (out of many that would have taken place in parallel), and all at once – jumping straight from the expression level of a normal zebrafish to that of a mouse, rather than slowly. The fish died, yes, but they also showed that the mere modification of the expression of a gene common to both fish and tetrapods has the affect of modifying a number of different features in the way expected in the evolutionary development of hands. Tomkins may say that “normal fish fin development was completely and grotesquely perturbed,” but these are by no means mutually exclusive descriptions. From a certain point of view the development of an arm complete with opposable thumbs would also count as the perturbation of normal fin development, but that is a perspective that ignores the more important aspects of the change.

Nevertheless, the perturbations and the deaths lead Tomkins to the diametrically opposite conclusion to that of the study – he concludes:

Evolutionary media were quick to propagate a story where fish supposedly grew hands in the lab. But the actual experimental results told just the opposite story. Altering only a single gene’s expression level disrupted the finely tuned system of hundreds of interacting genes within the irreducibly complex developmental genetic network, resulting in death of the organism.

Rather than showing how limbs could be produced from fins, this research showed how wonderfully fine-tuned and built the genome is.

Ah, irreducible complexity, fine tuning – Tomkins’ signature is trotting out these kind of claims on a whim, without backing them up.