How to Build a Dinosaur (2 page)

The first part of any reconstruction is to understand just what it is you are trying to make. If you’re going to indulge in biological reverse engineering, you have to take your target creature apart to see how it works. That’s the bread and butter of dinosaur scientists. We find fossils, dig them up, date them, put them in a context with other organisms that lived during their time. We use the fossilized bones to establish the shape of the dinosaur. We make educated guesses, some more solid than others, about movement, behavior, social life, parental involvement with the young.

With modern imaging technologies and computing power, we look deeper than ever before into the fossils we find. We can see inside the bones. We use CT scans to make 3-D images of the inside of skulls. We smash up bits of fossils to search for preserved remnants of muscle tissue, blood vessels, red blood cells. We use the tools of chemistry and physics to go deeper yet.

The recent technological changes in how bones are studied are profound. For most of the last century the study of dinosaurs was primarily a collector’s game. It certainly was not an experimental science. But that is changing. We can now retrieve ancient biomolecules, like proteins, from fossils tens of millions of years old. And we can mine the genomes of living creatures to trace evolutionary history. We can bring the history of life into the laboratory to test our ideas with experiments. And right at the top of the list of the experiments we can try is the attempt to bring back the characteristics of extinct creatures that have long been lost to us in deep time. That is how we can build a dinosaur.

We may someday recover bits of dinosaur DNA, but that is not the route to making a dinosaur. That has already been tried, in the movies. But it won’t happen in real life. I’m not putting down the movies. I loved
Jurassic Park,
not least because I worked on it and the sequels as a technical consultant to help get the dinosaurs right. And the idea of cloning a dinosaur from DNA recovered from a mosquito preserved in amber that once fed on dinosaurs was a brilliant fiction. It was, however, a fiction that reflected the science of its time, the fascination with DNA and the idea that we would have a complete blueprint of a dinosaur to make one. Now we are actually much closer to being able to create a dinosaur, without needing to recover ancient DNA.

We can do it because of the nature of evolution, and the way it builds on itself, adapting old plans to new circumstances, not inventing new life-forms from scratch. Much of the writing about dinosaurs in recent times has concentrated on the way we have been correcting our old mistakes. But mistakes are to be expected when you are trying to reach back tens of millions of years. What is amazing, if you stop to think about it, is that we got the main points right about their shape and structure right off the bat. How could we do that so easily when you might think that such ancient animals could have taken any shape imaginable, or unimaginable?

The answer is that evolution does not allow innovation without limit. It did not allow the dinosaurs to pop up in any old shape. They have the same body plan that all other animals with backbones do. Anyone can see that immediately, with or without science. Anyone who has seen a deer skeleton, or a lizard skeleton, or a human skeleton, would recognize the basics in a bunch of
T. rex
bones dug from the ground.
T. rex
has a spine, a skull, ribs, just as an eel or a salmon or a mouse does. It has hind limbs and forelimbs, just as crocodiles and frogs, hawks and people do.

Why? Why do animals of such different external shapes and lives share characteristics so similar that we can immediately recognize the basics of their bone structure? The reason is that shape is not an unlimited smorgasbord from which evolution can pick and choose. All living things are part of a continuum. The shapes of animals evolve over time from earlier ancestral shapes. Different groups of animals have basic body plans that themselves evolved from earlier plans.

All vertebrates have backbones. But before that innovation, they evolved from creatures that all had a front-to-back orientation for eating and elimination. Before that came self-propulsion. Before that energy metabolism. And so on, back to DNA itself, which we share with all living things, unless RNA viruses count as living things. A body plan is an abstract idea—four limbs, spine, skull, mouth at the front, elimination at the other end—but it has remained constant and resilient. Mountains have risen and fallen, seas have appeared and dried up, and continents themselves have shifted, while the standard four-limb plan, the tetrapod blueprint, has persisted with minor modifications.

The bones in the hands and arms used to type these words are almost the same as the bones in Buffalo chicken wings. If you follow the development of a chicken embryo closely, you will see five buds at the end of the developing wing, buds that also appear in the embryos of mice and people. The buds become fingers in a human embryo and claws in a mouse. In a chicken the five buds on a forelimb will lengthen, shorten, disappear, and be fused to fit into the familiar structure that cries out to us for hot sauce. The record of such astonishing and persistent continuity of form has been described as the main gift the fossil record has given to evolutionary biology.

Evolutionary change is added to existing plans. Genetic blueprints are not thrown out. We don’t have to start from scratch to grow a dinosaur. We don’t have to retrieve ancient DNA for cloning. Birds are descended from dinosaurs. Actually, they
are
dinosaurs, and most of the genetic program for the dinosaur characteristics we want to bring back should still be available in birds—in fact, in the chicken.

You can see evidence of this continuity in the way an embryo develops. And chicken embryos, in those perfectly functional containers, hard-shelled eggs, have been endlessly studied. Aristotle was the first to record the stages of growth of a chicken embryo. Other scientists have followed his example, partly because chickens and chicken eggs are so readily available.

You can see easily and clearly with a low-power dissecting microscope that a tail like a dinosaur’s is well on its way in the growing chicken embryo before something stops it. The result is a plump tail stump called the pope’s or parson’s nose (if your specialty is eating chickens), or the pygostyle (if you are more given to studying them). The pygostyle is a hodgepodge of different bones, the growth and purpose of which have been redirected, just the kind of jumble that evolution specializes in. It is a perfect demonstration that evolution does not suggest intelligence, planning, or purpose, but rather accident and opportunism. Evolution is by definition not revolution. It works within the system, using what it finds.

 

One of the hottest fields in science now is evo-devo, for “evolutionary developmental biology.” It has also been called devo-evo, or DE, for “developmental evolution.” Whatever the name, it is the investigation of how evolution proceeds through changes in the growth of embryos. The limb-to-wing transition does not require a complete new set of genes, but rather changes in the control of a few genes that promote or stop growth. These genes produce chemicals called growth and signaling factors that give directions to the cells in a growing embryo. When they are turned on and off at different times, that can drastically change the shape of an animal.

It’s a bit like remixing an old recording. Let’s take a band with a banjo, guitar, and a mandolin playing “She’ll Be Coming ’Round the Mountain.” The original recording features the banjo, but you want just the guitar and mandolin, so you turn the banjo tracks way down. If you look at it this way, a bird is just a new arrangement of an old tune. The dinosaur melody and the old genetic information are in there, but the sound is more contemporary. Of course, the better tune right here might be the old standard, the chicken song, “C-H-I-C-K-E-N, That Is the Way to Spell Chicken.”

I have a chicken skeleton on my desk at the Museum of the Rockies. I have for decades kept a chicken skeleton at hand wherever I have worked, because it looks like a dinosaur, and I like being around dinosaur skeletons. Sometimes I look at it and turn it this way and that and think, If I could just grow these bones a little different, tilt this one way, that another, I’d have a dinosaur skeleton. Over the past few years I have been looking at that chicken skeleton more often and more intensely. As I’ve looked at the bones I have started thinking less about the bones and more about the underlying molecular processes that caused the bones to grow. And the more I’ve learned about evo-devo and looked at that chicken skeleton, the more reasonable an idea it has seemed. That skeleton started out as an embryo, a single cell, dividing and growing, the cells differentiating into different types. Chemical signals directed by DNA turned it away from the path of growth that would lead to a nonavian dinosaur, but it seemed highly likely that all the raw material, all the genetic information needed to grow a dinosaur, was in that embryo. How much, I wondered, would it take to redirect its growth so that it ended up looking like a dinosaur?

Experimentalists have already caused a chicken to grow teeth. Other researchers have chemically nudged chicken embryos to develop the different sorts of beaks that the famous Darwin’s finches display.

Once I got the idea in my head that it could be done, I started talking to researchers who were truly grounded and fluent in the language, ideas, and techniques of both paleontology and molecular biology, like Hans Larsson at McGill. He was already working on what he called experimental atavisms as a way of understanding evolution. That is, he wanted to prompt a living creature to develop an ancestral trait.

The short version of the story is that I recruited Hans to drive the chicken/dinosaur express, at least part of the way. Hans is not growing a dinosaur, not yet. And none of the embryos in his experiments will hatch. But his research and my waking dream of having a chicken-sized dinosaur with teeth, a tail, and forelimbs instead of wings fit well together. So I supported research on getting that chicken embryo to express its inner tail. He has already discovered aspects of tail growth that tie this process to the very basic and early directions for the growth of any vertebrate, including humans. So the work may have unexpected value for some of the most common and devastating birth defects, those affecting the early growth of the spinal cord.

Researchers have shown how beaks can be modified by a change in one gene. If successful, Hans will show how the dinosaur’s tail turned into the chicken’s pygostyle and how to grow a chicken with a tail instead of a pygostyle. He will also have laid the groundwork for future research and, perhaps, for finally hatching that living example of how evolution works.

The scientific rewards of the process of learning how to rewind evolution would be enormous. It would be a remarkable demonstration of a direct link between molecular changes in the developmental process and large evolutionary changes in the shapes of animal bodies. We know quite a lot about the evolution of different forms from the fossil record. And we also know about specific changes in DNA that cause identifiable changes in body shape in the laboratory, among fruit flies in particular. But we are just now beginning to link molecular changes to large changes in the history of life, like the loss of a tail.

A nonavian dinosaur has not yet hatched from a chicken egg. This book is about how that became a goal I want to pursue, and how that pursuit is continuing, about how I and other scientists have tried in every way to travel into the past and bring it to life, how we began with pick and shovel, moved to CT scans and mass spectrometers, and have now arrived at the embryology lab.

The story starts with old-fashioned fossil hunting, with the hunt, the discovery, and the digging, always digging. There are two excavations here, both of the same
Tyrannosaurus
skeleton, a treasure of a find nicknamed B. rex, taken from the badlands of eastern Montana. The story continues with a second, microscopic excavation of the fossil bone itself by Mary Schweitzer, and the discovery of what seem to be fossilized remnants of sixty-eight-million-year-old blood vessels, still flexible; what may be the fossil remains of red blood cells and bone cells; and protein molecules, or parts of them, with unchanged chemical structure.

There comes a point, however, in studying dinosaurs and in this story, when fossil bones can yield no more. The search for more information about how dinosaurs evolved has to shift to the genes of living creatures. That is where the story takes a sharp turn, like the one the chicken’s growing tail makes as it stops growing and turns into something else. And there it chronicles how and why we know that the history of evolution is written in the genes of modern dinosaurs, the birds, and how and why we can take a chicken egg that might have become part of an omelette or an Egg McMuffin, and convince it to turn into the kind of dinosaur we all recognize.

When we succeed, and I have no doubt that we will, and sooner rather than later, it will be another step in a long chain of attempts to re-create the past. The first public exhibition of dinosaurs was in England in 1854, at the Crystal Palace in Sydenham, five years before Darwin published his
Origin of Species
. It was groundbreaking, although the stances of the animals were wrong, putting dinosaurs that stood on their hind legs on all fours instead. Over time, our reconstructions of dinosaurs have become more sophisticated and more correct. We have changed the stances of dinosaurs in museums, learned that many of them were warm-blooded, that dinosaurs, not birds, were the first to have feathers, that some dinosaurs lived in colonies and cared for their young in nests, that many of them were much smarter and more agile than we’d ever imagined.

We have made accurate robots and 3-D reconstructions of the internal cavities of skulls. We have theorized about the colors and sounds and behavior of dinosaurs. And now, we can try to make a living dinosaur. This is a project that will outrage some people as a sacrilegious attempt to interfere with life, and be scoffed at by others as impossible, and by others as more showmanship than science. I don’t have answers to these challenges, really, because the answers are not mine to provide. I have my ideas, my concerns, my own questions about the value and difficulties of such a project, but the story I have to tell is, like science itself, more about questions than answers, and the book is not a recipe or lecture. When we get to the point of hatching a dinosaur, it will be a decision that involves society as a whole, not just a few scientists in a laboratory. Most of all, this book is an invitation to an adventure. I can say how it begins, but all of us will have a say in how it ends.