Transplanting completely failed to keep the chick embryo’s tail growing, but the retinoic acid, Larsson said, “pushed tail growth to the upper range of normal development. It had some effect, but it didn’t break it out of the cycle.” The tail was a far more complex system than he or anyone else had imagined.
The initial hint of growth from the retinoic acid was encouraging, but Larsson needed to know much more. In particular he needed to avoid creating something that looked like a longer tail but was simply an embryologist’s trick, a growth that occurred without utilizing an ancestral pathway. Unless he knew the normal pathway of development in detail, all he would have achieved with a tailed chicken would be a circus attraction.
Unfortunately, there was no foundation of basic research describing how a tail grows. Studies had been done on how embryos initiated tail growth, but not on how they maintained that growth and what genes might be involved. Larsson had to start from scratch and do fundamental developmental research.
Larsson is now working on a model of how the tail grows. This will involve labeling cells in the growing embryo tail and using microinjections of dye to follow the pathways that these cells take as the tail develops. His goal is to see how zones of growth and organization move as the embryo grows and to probe what is going on biochemically. He has been able to piggyback on previous work on the development of the chick embryo: When researchers interested in some process use a stain to show gene activity, the stain affects the whole embryo, providing clues about which genes are active in tail growth.
Although the growth of the tail is very complicated once begun, the action that turns that growth off may be quite simple. Larsson compares the situation to a mechanical one. “It’s kind of like the key to a car. You could turn the key on and the motor will run and produce all these patterns and rhythms. Once you turn the key off, it stops. The key is relatively simple, compared with the rest of the car. I think that’s the kind of system we’re dealing with. Or I’m hoping.”
But building a dinosaur requires more than just a tail. “The experiment I’m envisioning is that you have a single embryo developing in the egg with multiple injection sites and multiple kinds of molecules to be really fine-tuning the regulation of genes,” Larsson says. “We’ll be able to inject different parts of the embryo at different times of development with different things. If we do that, if the timing and position are correct, we should be able to manipulate lots of different kinds of morphologies—feathers, wings, teeth, tails.
We don’t have to give an embryo new genes, just adjust the growth factors and other chemicals that direct development. It would be the first step in growing a dinosaur.
“It would take just a little bit of time to work out each one of those systems in very great detail, which we’re now doing for the tail. Other people are doing it for the limbs for clinical work. And teeth are being worked out by other people for mammals and such, and then we can just sit down and play with all these in concert, which has never been done before.”
The goal, in the end, would be to steer the embryo down the path it would have gone if it were something like a very early coelurosaur, a dinosaur grouping that counts tyrannosaurs and velociraptors among its members. If the genes in the chick embryo are very close to those of an ancestral, nonavian dinosaur—and if the changes, over more than 150 million years, have been almost all in regulation of the genes—then we could reactivate the old pattern of regulation.
We don’t have to give the embryo new genes, just adjust the growth factors and other chemicals that direct development. And by doing that we can see what must have changed during evolution, and what the old pattern of regulation was. If we learn enough, this will give us enormous insight into the fundamentals of biology, development, and evolution. It will also be the first step in growing a dinosaur.
In the end, there is an image that keeps popping into my mind. I give an awful lot of lectures. I don’t read from notes; I prefer to use slides, each of which fits with a topic that I want to talk about. I don’t need to memorize a speech or make it formal. I can stay conversational, which is what I find most comfortable.
So the image I have is that I walk onstage with a dinosaur on a leash. It’s small, but bigger than a chicken. Let’s say it’s the size of a turkey, one day maybe even the size of an emu. The dinosaur, or chickenosaur, or dinochicken, the emu-size version of a dinosaur (that one might have a muzzle or a couple of handlers) is the ultimate slide. Instead of a lecture, this would be a public science class with questions and challenges about how it was done, what its skin feels like, does it have teeth, what does it eat, how close is it really to a dinosaur? What would inevitably follow would be a discussion about the nature of dinosaurs, of birds, of evolution and development, of the relationship of molecular biology to big changes in evolution, of how we know what we know, and whether we were justified in doing what we did.
That would be the most satisfying lecture I could possibly give. I don’t like providing answers. I never have. I like questions. I like asking them, trying to figure out answers, trying to figure out what we are really asking, and seeing what new questions come up. For this event I won’t have to prepare any speech at all. My entire prepared text will consist of one simple question from which everything else will follow.
I’ll walk to the edge of the stage, point to the creature on the leash, look at the audience, and say, “Can anyone here tell me what this is?”
From How to Build a Dinosaur, by Jack Horner and James Gorman. Published by arrangement with Dutton, a member of Penguin Group (USA), Inc. Copyright © Jack Horner and James Gorman, 2009.