Aug 22 2011

Crocochicken

You have probably heard of the crocoduck – the impossible chimera that exists only in the pseudoscientific imagination of Kirk Cameron and Ray Comfort. There is also the duckcroc, which is a real creature – an extinct crocodilian with a duck-like snout.

Well now, meet crocochicken. This creature does not yet exist, but only because the scientists who are tinkering with its genetic program are not allowed to let it hatch. Arkhat Abzhanov is an evolutionary biologist at Harvard University. He is engaged in a research program, the goal of which is to reverse some of the evolutionary changes that resulted in birds evolving form theropod dinosaurs. In essence, he want to reverse the evolutionary clock and produce a chicken with more reptilian features.

The particular feature he is working on is the duck’s bill. He is coaxing it down a different developmental path so that it ends up looking more like a crocodiles snout – hence the name “crocochicken.”

There is another researcher also doing similar research. Hans Laarson of McGill University and colleagues are also trying to reverse chicken DNA to develop along more primitive path – he is working at the other end, trying to get chickens to grow a dinosaur-like tail. His creation has been dubbed “dinochicken,” which is a much better term as chickens actually evolved from dinosaurs and not crocodilians.

The two teams, working independently as far as I can tell, are not tweaking the DNA directly. Rather they are altering proteins that control how the DNA is expressed, and specifically how DNA controls the development process. The research is teaching us a lot about this process. You can imagine the development of an embryo into a mature creature as all the different body parts following a winding and twisting developmental pathway. Proteins, themselves coded by genes, and other biochemical signal turn on and off genes in different cell populations and are thereby steering development down these various paths.

This means that the introduction of a key regulatory protein at a key time in development could take the embryo down a different path, and result in a different morphology. At some point, for example, the chicken’s tail is given a signal to stop growing. If you block or change that signal, then the tail will continue to grow and end up more like the dinosaur tail of its ancestors.

There are a couple points worth emphasizing here. The first is that the researchers are not changing the actual DNA of the chicken embryos. They are simply introducing altered protein signals that influence the expression of DNA and therefore the developmental process.

The second, and more profound, point I wish to emphasize is that the chicken’s DNA is capable of creating dinosaur-like structures, with simple prodding here and there. The chicken DNA, and indeed the developmental process that leads to a chicken, is not a clean and direct process. It does not appear as if the chicken were created out of nothing as a chicken. Rather, the developmental process is messy and meandering, and reflects the long and chaotic evolutionary history of the chicken. These researchers are taking advantage of that fact, and exploiting the dino-DNA inside every chicken to alter those developmental meanderings into a more dinosaur direction (at least for some parts of the chicken).

I know this by itself is not “proof” of evolution – but it is one more line of evidence that supports evolutionary theory. This kind of thing should be true if evolution is a fact. It is similar to prior evidence that chickens can grow teeth. Researchers can famously coax chicken embryos into growing teeth by methods similar to the dinochicken and crocochicken experiments. They just introduce the right signals at the right time in development.

The implication of this (pointed out by Stephen Gould) is unavoidable – chickens have the genes for teeth, they are just dormant and turned off. We can turn those genes back on and voila – a chicken with teeth (smilochicken?). We are getting pretty darn close to independent proof of evolution. I won’t say “undeniable” because (you guessed it) those loons at the Discotute do in fact deny this line of evidence.

Casey Luskin “explains” that the chicken teeth experiments must be mistaken because neutral mutations (without selective pressure against them in a vestigial gene) would render the genes for teeth useless within 6 million years. Yet, he argues, birds and dinosaurs diverged 60+ million years ago. Luskin does not have a coherent explanation for the experiments, he just wants to cast doubt on the evolutionary implications. He glosses over the assumptions of his point, however. He assumes that the genes that contribute to teeth in birds have no other purpose, and are therefore under no selective pressure. He also assumes that any loss of function dates to around the split of birds and reptiles, but relevant changes in gene expression could have occurred more recently. Further, the chicken’s teeth that result from these experiments are not perfectly formed – they are highly degraded, as one would expect.

We are now also moving far beyond just chicken teeth. The more recent experiments seek to reverse many developmental features of chickens to their more reptilian form. It is true that, because of mutations, the evolutionary clock cannot be perfectly turned back using these methods. We will not get a velociraptor from a chicken by tweaking development. But we will get a dinochicken. We would have to actually reverse a lot of the mutations that have accumulated over the last 80 million years to get an actual dinosaur from these experiments. And for that we would probably need some reference dino DNA, which is highly unlikely to have survived.

I do not rule out that future researchers will hit upon some workaround. We may have a Jurassic park yet. Not from these experiments, as cool and informative as they are.

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