Why Did the Chicken Cross the World? (24 page)

Mutant chickens might also provide insight into bird-dinosaur evolution. In 2004, a biologist working with a chicken embryo found tiny bumps inside the developing mouth. These were not the flat-topped enamel that dominates human jaws, but sharp and conical structures resembling miniature alligator teeth. The researcher went on to create a virus that could copy the signals sent by the genetic mutation and set off similar teeth growth in normal chicken embryos. The teeth didn't last and were absorbed into the beak, but the experiment offered a glimpse into that long-lost era when hen's teeth weren't rare.

Another recent experiment produced crocodile-like snouts on chicken embryos. Arkhat Abzhanov, an evolutionary biologist with glasses and a trim black goatee, conducts the research at his Harvard lab. “I'm not sure that the goal of re-creating a ‘dinosaur' is necessarily a good science project,” he says when I stop by his office. Abzhanov runs what may be the world's only chicken boot camp, in which young researchers spend six grueling weeks learning how to make the best use of eggs for the greater glory of biology. “They are truly a fantastic system.”

Chicken embryos are tough and large and very predictable. They can be stored in a cooler for two weeks without developing. In an incubator, their changes from hour to hour have been well mapped. What begins as a little disk that is a mere two cells thick turns into an intricately structured and complex organism. Cut a hole in the shell, cover it with clear duct tape, and you can watch it happen. Eggs are also cheap and easy to store and manipulate, unlike primates, mice, and even zebra fish. So long as he doesn't hatch them, Abzhanov is free to do what he wants with chick embryos since they are outside lab-animal regulations. His entire lab is a small, windowless room
with a long counter, a couple of microscopes, empty egg cartons, and a clock featuring a rooster. A grad student is at work injecting a virus expressing a certain protein into embryonic tissue; its purplish-­brown stain will spread as the embryo cells grow and multiply, providing a trail for her to follow in the coming days. This delicate operation, she says, is the toughest part of a boot camp.

Abzhanov grew up in a big city in Kazakhstan in what was then the former Soviet Union, and his earliest memory is chasing chickens at a cousin's farm. As a biologist, he grew fascinated by the evolution of bird heads. He extensively studied Darwin's finches from the Galà­pagos Islands to understand what genes were central to the development of their remarkably varied beaks, which make it possible for them to rule a particularly ecological niche. That led him to wonder how the beak developed from dinosaur snouts in the first place, so he turned to chickens and alligators. In the reptile, there are two bones that structure the snout, but in chickens the two are fused. Abzhanov set out to find and turn off the gene that orders a beak instead of a snout to form on the fifth day of a chick's gestation. In 2011 he succeeded—though they were not allowed to hatch because of ethical guidelines.

Short of an ancient DNA sample, you can't “go back” to dinosaurs using chickens. Dinosaurs had many skull features that would have developed later in their embryonic cycle, and that information has been erased in chicken eggs, which develop much faster than, say, those of a
T. rex
. Abzhanov is curious about how dinosaur genes differ from those of a chicken, but his goal is both grander and more practical. “I want to find a more mechanistic view of evolution and disease,” he says. Studying the way bird heads develop, for example, might lead to a way to switch off the gene that creates a cleft palate in a human embryo.

Increasingly, dinosaurs are starting to look birdlike without any genetic manipulation at all. In 2007, paleontologists spotted quill knobs on a velociraptor. Four years later, chunks of 75-million-year-old amber revealed actual preserved dinosaur feathers with hints of pigment. These may have been more for display than flight. Triceratops, that hulking, horned, four-legged dinosaur, had feathers on
its tail. Even
T. rex
may have been feathered. Feathers, in fact, may have come before flight. Why did these massive animals go extinct 66 million years ago, while the chicken's ancestor survived and thrived?

All birds now are classed among the theropod dinosaurs, which appeared more than 200 million years ago as the planet's first large meat eaters. Even in that early period, some had feathers, hollow bones, and a wishbone still retained by modern birds. Avians are a particular sort of theropod called maniraptorans, a group of long-armed, three-fingered creatures that include velociraptor and
Microraptor gui
, which lived in trees. Another maniraptoran is Oviraptorosauria, which had a beak-like toothless jaw and feathers and produced only one egg from an oviduct at a time—like birds and unlike reptiles—and then sat on its nest to incubate its progeny. Some researchers even class oviraptors, which weighed less than a hundred pounds, as birds.

Abzhanov believes that birds survived and thrived better than their larger cousins because of their small size, which made them more adaptable and less vulnerable. Today's chicken lacks teeth and the huge talons of its massive ancestors, but deep within its DNA, it retains a reptilian ferociousness that—like our fascination with ­dinosaurs—both appeals and frightens.

For a thousand years in Europe, people believed that the chicken could on rare occasions turn deviant and deadly. A cock could lay an egg that hatched a fearsome monster. Called a basilisk, the rooster­headed creature with the body of a snake or dragon could kill with a glance. Recently, biologists discovered that a cock laying an egg is more than a superstitious myth.

Basilisk means “little king” in Greek. In Roman times, Pliny the Elder called it “a snake with a light crown upon its head,” which may describe a hooded cobra imported from India in the first century AD. As the rooster gained ascendancy in Christian symbolism, the basilisk morphed into a more satanic beast combining aspects of the cock and snake long associated with healing in the classical world.
The basilisk haunted the Middle Ages. The twelfth-century German mystic and naturalist Hildegard von Bingen warned that “nothing living is able to endure it” since the basilisk is ruled by the Antichrist. Panic swept thirteenth-century Vienna when word passed that a basilisk was loose in the city's twisting streets. A hysterical mob of sixteenth-century Dutch villagers strangled a rooster and crushed the eggs it was incubating. Warsaw's senate met in an emergency session when several people died gruesomely in a cellar during a basilisk attack. It was destroyed when a condemned prisoner donned a suit of mirrors to defeat the creature. The only thing said to frighten the monster, besides its own reflection and a weasel, was the crowing of the cock.

Fear of the basilisk led to one of history's most peculiar trials. On an August afternoon in 1474 in the Swiss city of Basel, a judge declared an eleven-year-old rooster guilty of laying an egg and ordered it beheaded and burned at the stake. After the executioner chopped off the cock's head and opened its innards, the officials were horrified to find three additional eggs waiting to be laid. All four, along with the corpse, were laid on the fire. Ironically, the basilisk was the city's emblem. The monster, even today, is everywhere in this ancient city. Basilisks spurt water in fountains, open their wings at the entrance to the Wettstein Bridge, and perch on the helmet of a gilded Renaissance statue. The popular local beer is even called basilisk.

As late as 1651, members of the royal Danish court in Copenhagen panicked when a castle servant reported a rooster laying an egg. King Frederick III, an amateur naturalist, kept a cool head and had the egg closely watched. When it did not hatch, he had it placed in his collection of curiosities rather than destroyed. A century later, the basilisk was nothing more than a silly fairy tale. “Know, Sir, that there is no such Animal in Nature as a Basilisk,” the sensible character in Voltaire's 1747 book
Zadig
tells the Queen of Babylon. Her retinue seeks the beast to cure the ailing king's health. The basilisk has resurfaced more recently without its rooster looks, in
Harry Potter and the Sorcerer's Stone
,
as pure reptile.

At the Roslin Institute in Scotland, which has been at the forefront
of finding ways to manufacture protein-based drugs in chickens, the biologist Mike Clinton is the resident chicken expert. A beefy man with a heavy Scottish brogue, he grew up on a remote Hebrides island, where he gathered eggs from his grandmother's farm and cut peat for fuel. Clinton wanted to be a veterinarian, but preferred studying animals to euthanizing them. Fascinated with what determines sex, he used embryonic chicks to see what turns them into roosters or hens.

In 2001, a poultry inspector phoned the institute about a very strangely built fowl that he had just acquired in the south of England. The man had encountered a farmer's son playing with a pet chicken named Sam. Whether that moniker was short for Samantha or Samuel depended on which side of the bird you addressed. Sam's left side had the hulking body of a rooster with white feathers, complete with large comb, wattles, and spur. On the right side, Sam was pure dark-hued hen. Intrigued, Clinton agreed to take the bird, thought to be a one-in-a-million specimen. Two weeks later, the same inspector called again with news that he had found two more.

Sam and the other birds were bilateral gynandromorphs, an animal containing distinctly male and female parts on separate sides of the organism. Lobsters and fruit flies and butterflies occasionally turn up with aspects of both sexes, but this is very rare in vertebrates. Unlike hermaphrodites, which can have sexual organs from both genders but otherwise appear to be either male or female, gynandromorphs are true sexual mosaics.

Scientists have been struggling since Aristotle to understand the mechanisms that determine whether an animal is male or female. The Greek philosopher believed the hotter the sex a man had with a woman, then the greater likelihood that a resulting fetus would be male. This is not as absurd as it sounds, since temperature can play a role in sex differentiation among some animals. The hotter the nest during incubation of alligator eggs, for example, the more likely that males will result.

By the twentieth century, scientists determined that the key to most animals' sex was in the sex chromosomes that take up a chunk of our genetic hard drives. The overwhelming majority of men have
one X and one Y, for example, while females have two Xs. One part of the Y chromosome induces the human embryo to create testes rather than ovaries. Those organs can then manufacture and secrete the chemical signals called hormones—testosterone from the testes and estrogen from the ovaries—that tell other cells to produce male and female traits. Before the gonads decide whether to be testes or ovaries, human cells could become either sex.

Many reptiles and birds—and at least one mammal, the ­platypus—operate slightly differently. Females have a Z and a W chromosome, while males have two of the same kind—Z and Z. Clinton and other researchers had been trying without success for decades to find the equivalent of the human sex-determination gene in birds, the mechanism that makes them male or female. That's why the three gynandromorphs were a surprise and welcome gift.

For two years, Clinton and his colleagues kept the three unusual chickens in a shed isolated from the other Roslin birds. Sam behaved like a male, while the second gynandromorph acted like a female. Two of them appeared female on their right and male on the left, while the third was a mirror image. The third also did not seem to favor either traditional rooster or hen behavior. Sam had testes on his male side, while the second bird had an ovary there instead. The third had a testes-like organ on its female side. After the researchers tried without success to culture living cells from the birds, a veterinarian put them to sleep by injection. In the autopsies that followed, the team took hundreds of tissue and blood samples from both sides of each bird. When Clinton chopped up Sam's testes, he found that the bird made healthy sperm, although his plumbing wasn't up to delivering it into a female and its ovary did not produce eggs. Other gynandromorphs, Clinton says, could look like a rooster and lay eggs.

Thanks to a new dye technique developed as part of Roslin's work in protein-drug creation, the team could color code the tissue according to each Z and W cell. Clinton expected to find that one side of each bird would be normal, while the other side would show some sign of chromosome damage or mutation. To his surprise, each bird
had predominantly male cells on one side—Z and Z—and female on the other—Z and W—while they shared blood mixing both.

He realized that almost every cell in these chickens had their own sexual identity, depending on which side they fell. Since the hormones coursing through the birds' bloodstream were the same, it was clear that the cells on each side were marching to a different drummer unrelated to testosterone or estrogen, long assumed to be the real drivers in sex differentiation. It was a chicken-or-egg question. If hormones drive sex determination, then how could they do so before the organs that make them even exist? To understand what was going on, Clinton and his colleagues conducted hundreds of experiments on chicken embryos. The team implanted female chicken cells in male host cells, and males in females. The stubborn implanted cells refused to change their sex role. That means that the fowl is more set in its sex than humans, who go through a unisex phase.