Reverse Evolution


Way over 90 percent of all the species that have ever lived on this planet—ever lived—are gone. They're extinct. We didn't kill them all. They just disappeared. That's what nature does. They disappear these days at the rate of twenty-five a day. And I mean regardless of our behavior. Irrespective of how we act on this planet, twenty-five species that are here today will be gone tomorrow. Let them go gracefully. Leave nature alone.

—George Carlin hope that by now I have convinced you that we can start with a chicken embryo and hatch out something that looks like a nonavian dinosaur. Will it really be an extinct animal brought back to life? Would a tail, claws, and teeth be enough to say that we have brought such an animal back from extinction?

No. We would have brought back some of the characteristics of the dinosaurs. We would have used the signatures left by evolution in the chicken's DNA to rewind evolution. But we would have re-created ancestral traits, not the ancestor itself. We could never truly re-create a species or genus that was lost, unless we had a complete genome. If we reached that point, then there would be philosophical disputes about whether we could really bring back an animal from deep time. There would always be gaps in our knowledge and, not ever having seen the animal, we could not be absolutely sure. But we would have brought back an animal that, by all our tests, was identical to the extinct animal.

This seems unlikely for animals from deep time, for practical reasons in retrieving a full picture of an ancient animal's genome, as well as other influences on its development. But the more we pursue creating experimental atavisms, the closer we will come to this achievement, and the more we will learn about evolution, which is the deeper goal. The fundamental reason for attempting to rewind evolution is to learn how evolution occurs. Like a teenager with an old car, you take it apart to learn how it works.

On a shorter time scale we could certainly come close enough to re-creating an actual extinct species, but the achievement could well be an empty one. Let's use the ivory-billed woodpecker as an example. This was the largest woodpecker in the United States and Canada and it is generally thought to have gone extinct perhaps half a century ago, although there have been many claims of sightings, including one that was published in Science on June 3, 2005.

Earlier that spring, when the announcement of the sighting was made and the paper on the woodpecker released to the press, I was planning this book with my coauthor, Jim Gorman, and he was called away in the middle of our conversations to fly to Alabama and write about the woodpecker. Since then, the sighting reported in the Science paper has been roundly criticized as a sighting of a pileated woodpecker, and no hard evidence in the form of a clear photo or video, or DNA, has emerged to prove the ivory bill's continued existence.

There are woodpeckers that are in the same genus as the ivory bill, Campephilus, such as the Magellanic woodpecker from South America, Campephilus magellanicus. The Magellanic woodpecker is not descended from the ivory bill, but they share a recent common ancestor, recent in evolutionary terms. It seems possible that if we carefully documented the embryonic development of Campephilus, we might discover a way to make a magellanicus embryo develop to be indistinguishable from an ivory bill.

Whether we would want to is another question. I don't know what there is to be learned in this case, since the differences are subtle, and the re-created ivory bill would not breed true. We could also sequence the genome of the ivory bill from skins saved in museums and compare it to magellanicus and find differences in the genome. If we found obvious differences, we could perhaps change the genome.

But even if we were able to create a bird that was indistinguishable from the old ivory bill, it would always seem ersatz, particularly since the dream of finding the ivory bill still living is about proving to ourselves that we were able to stop our-

The ivory-billed woodpecker is thought to have become extinct, although there are reports of sightings. Similar woodpeckers might provide a genetic basis for reconstructing the species.

selves from driving a beautiful species into extinction. And for science and conservation and our own sense of the planet, the issue is not so much the bird itself but the bottomland hardwood forests it lived in. Without them, re-creating the bird would be like re-creating tigers without a jungle.

Other experiments might also be undertaken with a good chance of success. We could probably make the embryo of a domestic chicken grow into Gallus gallus, the wild chicken that is its ancestor. And if wolves disappeared we would have a great reservoir of genes in the domestic dog. Dealing with mammalian embryos is, however, quite difficult. Still, we have the dog genome, we can get the wolf genome now, while they are still living.

These would be very, very small triumphs of reverse engineering. Even to an untrained eye a Siberian husky and a wolf are not so far apart in appearance and behavior. We would be reversing microevolutionary changes. An untrained eye might not see the difference in the first place between one woodpecker and another. And behavior would be hard to re-create since it would depend, no doubt, on environment as well as ge netic heritage.

In contrast to those experiments, however, the one that I am proposing—or campaigning for, I suppose you would say— promises significant benefits both in terms of basic research and applications. Turning the clock back from chickens to dinosaurs would open up to us a method to tackle the major changes of macroevolution and help us tie them to changes in the control of genes. And what we find out about intervening in embryonic development, particularly involving the growth of the spinal cord, could prove of great practical, medical use.

In the attempt to re-create a dinosaur, we can't pick a species. That's too fine a target. In another way that distance and time are connected, the farther away a target is at a shooting range, the larger the area you need to aim for. The farther back in time you go, the larger the target you might be able to hit. Research would have to aim at something phylogene-tically larger, perhaps at the level of genus, or family. The farther we go back in time the less information we have about the extinct animals. Once we are in deep time, we are dealing with animals that we have imagined based on limited information. In some cases species have been named on the basis of a tooth, or not much more. Even with a relatively complete skeleton, there are so many areas where we would have to guess. Wait, let me rephrase that. We would have to hypothesize, based on the evolutionary context of the animal and other information. The color of the skin? The way the animal moved? Evidence based hypotheses, otherwise known as educated guesses.

What we can aim at with some certainty are the characteristics that we know from fossils—size and skeletal structure, teeth, musculature, and in some cases skin. We can make reasonable conclusions about movement and diet, and good guesses on certain aspects of behavior. For some behaviors, however, we would need herds of dinosaurs, complete with the appropriate predators and environment, to observe them. In other words—Jurassic Park. That is not something I will see in my lifetime. And probably not something worth pursuing.

How much we will eventually be able to achieve is impossible to say. We can keep pushing the boundaries of knowledge and ability, which will continue to grow. But the scientific capability must be balanced against mundane questions ofmoney and usefulness and profound issues of ethics and social responsibility.

I have no doubt that we can and will do what I've proposed, to bring back teeth, tail, and forearms with claws. It won't be easy and the money may not be forthcoming, but it will happen, and I'm convinced that it will be worth doing. I also think we could change the kind of feathers a chicken grows to make them more primitive. I think we could achieve a suite of changes in one embryo so that the resulting animal could hatch and live out a normal life span, eating, moving, and functioning without difficulty.

Beyond that, we'll have to wait and see. As embryologists work out the details of the program for tetrapod development, I would expect many barriers to reengineering extinct life-forms to fall. Right now, we could change limb growth with many small interventions—adding and suppressing growth factors in different locations at different times. We may, however, find higher-level signals that lead to a cascade of developmental changes so that instructions for forelimb growth, for example, do not have to be adjusted piecemeal. There may be fewer changes needed than we now imagine to prompt development of a hand rather than a wing.

We may never know the physiology of long extinct animals. We can intuit certain internal arrangements from living birds, but I doubt that we will ever know exactly what the inside of T. rex looked like. Still, the more we try to rewind and replay the tape of evolution, the more we will learn about how animals are put together, how they grow. And we have clues in the digestive systems, to take one example, of modern birds with different diets. Combining the variations found in living birds and what we can learn of the diets of extinct animals, we may refine our ideas about the digestion of an extinct animal. If T. rex was a scavenger, for instance, we might look to see how modern avian scavengers cope with their diet.

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