FEATHERED DINOSAURS
In the mid-1990s one of the best-preserved dinosaur skeletons ever was found in China, in early Cretaceous sediments that provided an extraordinary record of all sorts of life. Three scientists, Pei-Ji Chen, Shi-ming Dong, and Shuo-nan Zhen reported in
Nature
in 1997 finding two skeletons of a chicken-sized dinosaur with the longest tail of any theropod dinosaur and a very large and strong first digit, perhaps a killing claw. The preservation was so striking that internal organs and a last meal of a lizard, as well as two eggs about to be laid were found in one specimen. Most remarkable, however, was the preservation of skin and filaments that the scientists identified as feathers. The dinosaur was named
Sinosauropteryx prima.
(At first it was thought to be a bird.) It was very similar to
Compsognathus,
a dinosaur that early on was thought to be an ancestor of birds. Sinosauropteryx was a coelurosaur, a kind of dinosaur close to birds, in fact the group that includes birds in current thinking.
Shortly thereafter, two theropod dinosaurs with clearly defined feathers were found in the same geological formation in northeastern China that yielded
Sinosauropteryx,
in Liaoning Province. These two dinosaurs were found by two Chinese paleontologists, Ji Qiang and Ji Shu-An; one Canadian, Philip J. Currie of the Royal Tyrrell Museum in Alberta; and one American, Mark Norell of the American Museum of Natural History in New York. These dinosaurs, named
Protarchaeopteryx
and
Caudipteryx,
had both downy feathers and longer branching feathers similar to those in modern birds.
These finds removed feathers as one of the defining characteristics of birds. The two dinosaurs were both classified as maniraptorans, the kind of dinosaur thought to have given rise to birds. More discoveries followed, including feathered dromaeosaurids, another kind of theropod dinosaur. One of the most surprising of these was one called
Microraptor,
a dinosaur about three feet long, with feathers on all four limbs and hind feet that seemed adapted to perching. It certainly looks like it was a tree-living glider and offers considerable support for the idea that flight evolved from dinosaurs that lived in the trees. Xu Xing reported the find in 2003.
Richard Prum, an ornithologist and evolutionary biologist at Yale, who has studied the evolution of feathers, wrote in
Nature
in 2003, in the same issue as the report of
Microraptor,
that the origin of dinosaurs was a settled question. “Birds are a lineage of dinosaurs, and are most closely related to dromaeosaurs and troodontids.” With
Microraptor
apparently being a gliding dinosaur, Prum wrote, “there remain no major traits that are unique to birds—with the possible exception of powered flight.”
This brings us back to our original and overriding purpose, to build a dinosaur. Since, as Prum writes, powered flight is the only trait unique to birds, we can see quite clearly that causing a bird to grow up as a nonavian dinosaur crosses a thin boundary that grows less clear the more we know. Only small skeletal traits would distinguish a nonavian theropod dinosaur with feathers from an avian dinosaur with feathers. In real terms, however, what I want to see is quite clear—a feathered, running theropod with a tail, teeth, and forelimbs with usable claws. I could put it another way. I want to have a chicken grow up so that we can’t tell whether it is an avian or nonavian dinosaur. That would certainly constitute rewinding the tape of evolution.
One objection to this version of the evolution of dinosaurs is that the fossils of theropod dinosaurs with feathers are not older than
Archaeopteryx,
the first known bird, so it doesn’t make sense to pick them as ancestors. Clearly those particular dinosaurs were not ancestors of a creature the same age as themselves, but that is not the point. Some of the feathered theropods show primitive characteristics that indicate that their group evolved before birds did.
The much loved duck-billed platypus might help make this clearer. The platypus is a favorite of children, evolutionary biologists, and the sort of person who likes to throw the word
monotreme
into the conversation. A monotreme is a very ancient kind of mammal that lays eggs. There are only two of them: the platypus and the echidna (spiny anteater), both of them native to Australia. They are oddities among the odd, since Australia is set apart from the rest of the world by having no native placental mammals except human beings. Kangaroos, koalas, and the rest are all marsupials, with protective pouches for their tiny young to continue their development until they are ready to face the outer world. Placental mammals like us give birth to fairly well developed young that survive outside the mother’s body.
But the platypus is something else again. As Ogden Nash, who might be said to have his own evolutionary branch among poets, wrote, “I like the duck-billed platypus, Because it is anomalous.” It has a duck’s bill, more or less, and lays eggs, but it has mammary glands, although no nipples. The young, once they hatch, must suck the mother’s milk through thin skin over the glands. To top it all off, the platypus has venom, delivered by spurs on its legs. It seems like a cross between a mammal and a reptile and, unsurprisingly, its genome has what we think of as reptilian and mammalian characteristics.
Most of us like the platypus for the same reasons as Ogden Nash, but it has evolutionary importance because we think the first mammals probably had some of these reptilian characteristics such as egg laying. So the platypus has primitive characteristics that were lost in other mammals as time and evolution proceeded. But evolution is not restricted to one line. At the same time that mammals with what are called derived characteristics, such as nipples, were evolving, and other animals like the platypus were disappearing, the platypus survived. In the same way, single-celled life-forms did not disappear or stop evolving as multicellular animals appeared and diversified.
So when we look at fossils we try to identify primitive characteristics, and derived, or novel, characteristics. Naturally, the novel characteristics appear later in time. And our knowledge is always changing and developing. At one time we thought that feathers were a derived characteristic that identified birds. No longer, since we know of nonavian dinosaurs that had feathers. A number of such fossils have been found in China.
The evolutionary path to birds is now seen as follows. The first dinosaurs emerged in the Triassic, about 225 million years ago, from reptiles called thecodonts, and split into two sorts, ornithischians and saurischians. Here the terminology is a bit misleading, because although the ornithischians are named for birdlike hips and the saurischians for lizardlike hips, the birds arose within the saurischian lineage. The saurischians split into sauropods, like the big, long-necked herbivorous brontosaurs, and the theropods, carnivorous dinosaurs. Birds are theropods, and although we don’t know which theropod gave rise to them, it was small, fast, smart, and carnivorous. The best guess is that birds arose from primitive coelurosaurs, which are first known from the early Jurassic, between 175 and 200 million years ago.
Archaeopteryx
is, however, the first known bird. It emerged around 150 million years ago. After it we can trace bird evolution through several steps. Modern birds appeared about 55 million years ago, and within those, the galliform birds appeared about 45 million years ago. The domestication of
Gallus gallus,
the red jungle fowl that became our domestic chicken, apparently began around 5,000 years ago.
Remarkably, it is in this genome, 50 million years removed from its nonavian theropod ancestors, that the information resides to grow a dinosaur. I mentioned earlier one of the most recent calls for changes in paleontology, by three scientists, including Kevin Peterson at Dartmouth. It summed up progress to that point in merging paleontology and molecular biology and pointed the way for much more mixing of the two disciplines, in the new, hybrid field of molecular paleontology. He and his colleagues pointed out that there is a vast repository of molecular fossils within the genomes of living animals, and that “we are now in a position, both technically and methodologically, not only to explore this molecular fossil record but also to integrate it with the geological fossil record.”
What is particularly interesting to me, perhaps because I agree with them, is their argument that there must be a marriage or merger of the skills and knowledge of molecular biology and paleontology. Neither is sufficient without the other. The skills of the molecular biologist and the understanding of the mechanisms of genetics are necessary, as is an understanding of the fossil record and the grand sweep of evolution and the classification of life-forms.
This is certainly true. And although it may be my bias, it seems to me that it is often paleontology that sets the table and makes possible the questions that molecular biology has the knowledge and skills to answer. That is certainly the case when it comes to dinosaurs and birds. It is in birds that we will find the molecular fossils that lead us to learn more about dinosaurs and their evolution.
5
WHERE BABIES COME FROM
ANCESTORS IN THE EGG
The problem of development is how a single cell, the fertilized egg, gives rise to all animals, including humans. So it really is about life itself.
—Lewis Wolpert,
The Triumph of the Embryo
A
map of the chicken genome, actually the genome of the ancestral chicken, the red jungle fowl
(Gallus gallus),
was published in 2004. The achievement followed on the mapping of a number of other genomes, including, of course, our own. So it did not receive any great fanfare. But this was the first avian genome and it was immediately compared to human and other genomes in a search for insights about the separate paths evolution has taken. The last common ancestor of mammals and birds dates to about 310 million years ago, which is a long time for separate evolution.
And there were a number of intriguing differences.
One major difference is that the chicken genome is one third the size of the human genome, which contains twenty to twenty-five thousand genes. Chickens also have many fewer repeating sections of DNA. Humans, for reasons that are still not understood, have much DNA that has been called junk because it was thought to be leftover and nonfunctional. The thinking now is that much of it is useful, in ways that we hope to figure out as we map the details left out in the original studies of the genome. But birds are more economical in their DNA. Not surprisingly, the chicken also has a specialized set of genes for the keratin that goes into beaks and feathers. There are long sections of DNA that are the same in chickens and humans, but some of these are of unknown function.
Comparison has always been one of the key techniques of science, and using other creatures that are more easily observed has also been a key to understanding human biology. Genome mapping is equally easy in chickens and humans, but studies of genes are only part of the way molecular biologists mine the treasures of modern animals to understand the path of evolution.
Another technique of great importance has been the study of development, of embryology, to penetrate the great mystery of how a fertilized egg—one cell—grows to an adult organism. Today we study how this process is directed by genes, and how it relates to evolution, but development has been studied since antiquity.
Aristotle is considered the father of embryology, if not biology. In 345 BC he observed and recorded the development of the chicken embryo in the egg. As far as we know, he is the first experimental embryologist. In
Great Scientific Experiments
(1981) Rom Harré—a philosopher of science, prolific popular writer, longtime professor at the University of Oxford, and now teacher at Georgetown—examines one of the philosopher’s Hippocratic writings, in which Aristotle follows up on an experiment proposed by an unknown author:.
“In the work
On the Nature of the Infant
,” Harré writes, “an exploratory study is suggested in the clearest terms. ‘Take twenty eggs or more, and set them for brooding under two or more hens. Then on each day of incubation from the second to the last, that of hatching, remove one egg and open it for examination.’ ”
Aristotle apparently followed this suggestion to the letter. (The unknown author who suggested the experiment never seems to have actually done it.) In the
Historia Animalium
Aristotle recounted the results, providing a source that was relied on for more than a millennium. He described the first hint of an embryo after three days, the development of the yolk, the first hint of the heart, which, he wrote, “appears, like a speck of blood, in the white of the egg. This point beats and moves as though endowed with life.”
Even for the modern reader Aristotle’s eye for detail and the clarity of his writing are remarkable. “When the egg is now ten days old the chick and all its parts are distinctly visible. The head is still larger than the rest of its body, and the eyes larger than the head, but still devoid of vision.” He continued to observe after hatching, and noted that “ten days after hatching, if you cut open the chick, a small remnant of the yolk is still left in connection with the gut.”
Aristotle was not pursuing an idle interest, or a particular attachment to chickens. The original proposer of the experiment was writing about human development, and the chicken egg was a means to watch an embryo grow, the presumption being that human infants had to share some aspects of this development. Also, to the Greeks and still to us, the growth of an organism is one of the most profound biological mysteries. It is the child’s inquiry writ large as a scientific question that still demands our full attention: Where do babies come from?