Now reading The Evolution of Chickens

The Evolution of Chickens

The story of Darwin, evo-devo, and the ouroboros of avian genealogy.


As anyone who learned their dino-science from Dr. Alan Grant of Jurassic Park knows, birds descended from dinosaurs. And no bird is more closely related on a genetic level to the attractions at Jurassic Park than Gallus gallus domesticus, or chickens (and fine, turkeys, too).  In fact, today’s most populous birds serve not only as our most direct reminder of velociraptors—they also had a part to play in establishing the idea of evolution itself.

While it was the finch that famously spurred Charles Darwin to formulate the theory of evolution, chickens helped him think about selection (natural, artificial, sexual, or otherwise), survival, and fitness, and about how these changes accumulate in organisms over time and generations. Upon returning to England in 1836 with trunkloads of samples from a five-year stint as the onboard gentleman-naturalist of the HMS Beagle during its voyages in and around South America, twenty-seven-year-old Darwin mapped where he’d collected every one of the numerous finches he brought back with him. He noted that while each species had similar bodies, they also had huge differences in beak shape that were unique to their own semi-isolated islands. Pondering this geographical-morphological relationship—that the species were related to one another, but each adapted to its own specific locality—Darwin began working out ideas on transmutation of one species into another, descent, and adaptation. Because natural history could take him only so far, he looked for inspiration in works on animal breeding and husbandry. In the writings of Sir John Sebright—a politician, agricultural innovator, and breeder of chickens (one of his breeds, the Sebright Bantam, is still around today)—Darwin found some familiar concepts shaping up. In a monograph entitled The Art of Improving the Breeds of Domestic Animals, Sebright mused on topics like sexual selection (“The greatest number of females will, of course, fall to the share of the most vigorous males”) and the survival of the fittest. He wrote that natural conditions could “select” the traits that get passed onto future generations of animals as much as a farmer deliberately breeding specific animals could.

With visions of chickens selectively coupling in his mind, Darwin developed the foundation of the theory of evolution, though it would take him another twenty years working out the finer points before finally publishing On the Origin of Species in 1859. In it, he asked and answered the important question, “If species have descended from other species by insensibly fine gradations, do we not everywhere see innumerable transitional forms?” In other words, why aren’t there live specimens or fossils of half-chicken/half-dinosaurs all over the place? And how did we get from giant reptiles to the animals we have today?

He attributed the lack of obvious living transitional forms to the probability that evolved species replaced their original lineages, and hedged that transitional fossils weren’t being found because fossils are really hard to make and, in the grand scheme of things, only happen very rarely. “The crust of the earth is a vast museum,” he wrote, “but the natural collections have been made only at intervals of time immensely remote.” In other words: chill out with the transitional fossil demands.

Then, two years later, a new find gave naturalists a frisson of excitement and vindication.

In a limestone quarry in Bavaria, German scientists excavated a dinosaur-ish fossil that they called the Urvogel (“first bird”), which later became known as the Archaeopteryx (“ancient wing”). It was roughly the size of a raven and had, like most dinosaurs, a long tail, a jaw full of teeth, and a flat sternum. It also had a wishbone, some hollow bones, downy feathers all over its body, and wings covered in asymmetrical flight feathers. It was a clear transitional link between dinosaurs and birds. Darwin’s buddy Hugh Falconer, who’d seen the fossil when it was presented at the British Museum in 1863 (Darwin was a recluse, so he stayed home), wrote to Darwin: “Had the Solenhofen quarries been commissioned—by august command—to turn out a strange being à la Darwin—it could not have executed the behest more handsomely—than with the Archaeopteryx.”

It took more than a century for the scientific community to come to agreement, but eventually the idea that birds evolved from dinosaurs made its way into high school science classes everywhere. Except Tennessee.

The Ur-Chicken, or Thanks Again, Darwin

It’s now widely accepted that the chicken is (mostly) descended directly from a bird called the red jungle fowl, which looks like a bug-eyed, wily cousin of the birds we see on farms, and that has a natural range in South and Southeast Asia, from northern India to Indonesia.

Carolus Linnaeus, the father of modern taxonomy, was perhaps the first Western naturalist to classify the chicken as a relative of the red jungle fowl. Technically, the domestic chicken is classed as a subspecies (Gallus gallus domesticus) of the red jungle fowl (Gallus gallus) and a relative of other Gallus species, all of which are different kinds of jungle fowl. But Linnaeus wasn’t advancing an argument about ancestry. (“God created, Linnaeus organized” is one of his famous humblebrags.) To Linnaeus, God had created both the chicken and the jungle fowl, and made them able to breed; thus, they were both Gallus gallus.

The scientific basis for the connection, it turns out, is another of Darwin’s great gifts to chickenology.

After publishing On the Origin of Species and before writing The Descent of Man, Darwin made a deeper exploration into artificial selection in a book called The Variation of Animals and Plants Under Domestication, in which he fleshed out the argument for the red jungle fowl as the progenitor of the modern chicken.

At the time, colonialism and the Industrial Revolution were combining to create faster and cheaper transport (via the clipper ship) between Britain and its empire. In 1842, a gift of huge Chinese Cochin chickens arrived for the young Queen Victoria, setting off a Tulip Mania–esque frenzy of chicken fancying that burst in the mid-1850s, bringing a lot of exotic chickens and hybrids to England. This gave Darwin access to specimens and data that would not have been available to a similarly educated naturalist of an earlier generation. In addition to extensive comparisons of appearance and analysis of reported data from chicken and jungle fowl observers in Asia, he also approached the question empirically, through breeding experiments.

He was happy to report that, of the wild jungle fowl species in Asia, the evidence showed that only red jungle fowl could produce non-sterile offspring with chickens. (A phenomenon that’s currently edging the red jungle fowl toward extinction as a distinct species; it interbreeds so much with domestic chickens that it’s become tricky to find a red jungle fowl with only wild genetics.) Darwin also bred a purebred black Spanish cock with a harem of hens, most of which were white or black and white, and none of which had shown in their breed lines the characteristic red/orange-shouldered, black-breasted, diagonally plumed tail looks of the red jungle fowl. Despite their achromatic parentage, many of the resulting chicks grew up to have orange and red hackles and shoulders and dark bodies—reverting to ancestral characteristics, according to Darwin.

Rounding out his breeding experiments and ornithological analysis, Darwin drew creatively on scholarship from the humanities as a record of evidence that was no longer tangible. The Greeks called chickens “the Persian birds,” but since no chicken-like wild birds had a natural range that included Persia, they probably came to Greece via Persia from somewhere even farther east. In Asian languages, the etymology of words for “chicken” suggested that the word may have originated in Malaysia or Java; and translated ancient Chinese texts suggested chickens were being imported into China in the fifteenth century BCE. Darwin also cited the Bible as evidence, since chickens are mentioned in the New Testament but not in the Old.

After Darwin, the debate over the primary origin of the chicken stalled until the 1990s, with the rapid development of molecular biology and molecular genetics, fields in which precise chemical analysis of DNA and proteins allowed scientists to tell new stories about the relationships between organisms over long stretches of time and across distant places on the globe. Intensive studies of the genetic sequences of modern chickens, ancient chicken bones, and various wild jungle fowl, considered in tandem with archaeological, linguistic, and historical evidence, have enriched, complicated, and sometimes muddied the picture of where and when the jungle fowl became the chicken.

Genetic studies suggest that a key feature of most modern chickens—their yellowish skin—could not have come from red jungle fowl, which have whitish skin, but may have come from gray jungle fowl, a chicken parentage that Darwin had ruled out. Bone analysis from a site in northern China suggests that domesticated chickens were found much farther north (in a climate where no jungle fowl now live) and much earlier (as early as 8,000–5,000 BCE) than anyone expected. Was this the product of an earlier original domestication event, outside the current range of the red jungle fowl that subsequently spread southward? Or was the chicken domesticated multiple times in parallel? (Or, as has been suggested in critical follow-ups, mitochondrial DNA confirmation aside, could the bones have been misidentified? It’s happened, more than once, before.) As of now, the jury is still out.

Better Living Through Artificial Selection and Chemistry

In the nineteenth and early twentieth centuries, interest arose in “improving” breeds to make them grow larger, faster, more efficiently, or lay more eggs. Rearing flocks specifically for egg laying became common (with the male chicks in these flocks used as “broilers,” or meat chickens), especially after World War I.

Over successive generations of selective breeding—really accelerating after World War II—chickens have been transformed from a variety of breeds that gave both eggs and meat into an industrial bioreactor for turning corn into protein as efficiently as possible. With these chickens, you can choose eggs or meat, not both. And you certainly don’t get to choose flavor.

The infamous Chicken of Tomorrow contest of 1948 marked a watershed moment in the creation of the industrial breeds that make up most of the chickens alive today. It started innocently enough—with beef being rationed during the war, Americans’ chicken consumption almost doubled, and grocers were interested in keeping sales up. A&P launched a promotion for chicken breeders of all stripes to submit for consideration the largest, most breasty, picture-perfect bird that could be raised in twelve weeks on the least possible food. The winning chickens were a hybrid, the first generation of a cross between a New Hampshire Red hen and a California Cornish rooster. At twelve weeks, they weighed 3.75 pounds each; an improvement on the contemporary average of three pounds.

Selective breeding continued over subsequent years, expanding from grocery chains to universities’ animal-science departments and commercial chicken breeders. By 1973, chickens were reaching maturity at eight and a half weeks. It’s now possible to rear chicks that reach maturity even faster, in five weeks, and put on four pounds and change in that time.

A few other developments helped make this possible. The 1950s and ’60s were the first decades of the heyday of cheap corn, thanks to intensive farming methods, commercial seed lines, and selective breeding. Previously, “free-range” chickens weren’t a thing, because all chickens were free to range outside and eat grasses, insects, and grubs, even if they were fed largely grain-based chicken feed. Chicken farmers actually had to let their animals forage—if they didn’t, they got metabolic diseases (similar to beriberi and anemia in humans) from missing the micronutrients they would usually get from their grub supplements.

This brings us to the third element of the commercial-chicken trifecta: the “vital essences,” or vitamins and minerals, discovered in the early part of the twentieth century. With these isolated micronutrients in hand, chicken farmers could fortify inexpensive corn and create a feed that fattened chickens quickly without killing them. This allowed producers to go from flocks of fifty to two hundred chickens to facilities with thousands upon thousands of chickens, since there was no longer any need to let chickens out onto pasture to keep them alive.
And just like that, you’ve got a recipe for billions of giant, inexpensive, tasteless chickens.

Chickens of Future Past

Back to chicken’s dino roots. We know that toward the end of the Jurassic period, a branch of ancestrally carnivorous theropods emerged as ancient birds. Recent fossil finds, like the sinosauropteryx (discovered in 1996 in Liaoning Province in China, and covered in primitive downy feathers), quill knobs found on velociraptors, and wing and tail segments encased in amber, show that non-avian theropod dinosaurs (so cousins to ancient birds, though not actually birds) commonly had feathers, suggesting that feathers evolved before birds did.

And while we haven’t been able to isolate dinosaur DNA yet (sorry, Jurassic Park fans), we’ve learned a lot about which genetic changes are necessary to make changes in an animal’s shape. The basic map of a vertebrate isn’t revised wholesale when something like a bird emerges from the lineage of dinosaurs. Dinosaurs and chickens (and humans) have skulls with two eyes, a rib cage, a pelvis, a spine, and forelimbs and hind limbs. Dinosaurs had tails where chickens have a stumpy structure called a pygostyle, jaws and teeth where chickens have beaks, and their five-fingered front limbs differ from chickens’ three-appendaged wings (though those wings develop in the chicken embryo from hand-like structures).

We’ve learned from a new branch of science called evolutionary developmental biology (or evo-devo) that the evolved changes in the anatomy of an adult organism are generally laid out as it develops from a single-celled zygote through its embryonic phase into a baby animal. This development isn’t directly guided by a gene that codes for a tail or a beak, per se, but (because nothing in nature is ever simple) from feedback among the so-called Hox genes, other genes that produce enzymes that start and stop growth, and genes that directly cause different types of cells to grow. The Hox genes are the master guides of the development of an embryo along its head-to-tail axis: change one Hox gene in a fruit fly, and it will develop wings where its antennae should be; change another, and it will grow an extra pair of wings. The Hox genes evolved very early in the evolutionary tree—about 500 million years ago, and are found with extremely similar sequences throughout the animal kingdom (fruit flies, chickens, and humans alike).

But crucially, the Hox genes don’t code for legs or wings—the proteins they code for bind to other sections of DNA, and by binding, turn those genes off and on. The genes that Hox proteins bind to don’t code for a leg or a wing either; they code for proteins that control other genes, the coordination of which eventually leads to bone or eye (or wing or tail) development. Hox genes, and the genes that Hox proteins bind to don’t code for a leg or a wing either; they code for proteins that control other genes, the coordination of which eventually leads to bone or eye (or wing or tail) development. Hox genes, and the genes that Hox proteins bind to, are called transcription factors: they control which sections of DNA getread and transcribed into RNA, which then gets translated by the ribosome into a protein. An organism can easily have dozens of transcription factor genes, each of which can bind to and regulate a hundred or more target genes. And the target genes might code for a specific enzyme or type of tissue, but just as often they are also regulatory genes, so what gets switched on or off is yet another cascade of switching.


But the basic takeaway is that the big physical differences in, say, a bird and a dinosaur don’t actually come from changes in the genes that directly build the tail or wing, but in genetic changes that affect the regulation of those genes. Which means that the changes in dinosaurs that led, eventually, to the chicken, were regulatory: down regulation of tail development, changes in forelimb development that mutated hands into wings, changes in the genes that direct the locations where feather follicles develop, and inactivation (but not necessarily deletion) of genes that cause tooth buds to develop, among others.

Most of you will take this as food for thought or a factoid for your next dinner party, or maybe an opportunity to get philosophical with yourself (All human beauty comes from arbitrary differences in the regulation of cells that make structural materials, and the face that gives me my social currency is just an emergent property of transcription factors making slight differences in bone secretion rates in cells in different locations! My idea of myself is an illusion!n Society is a sham!). A paleontologist named Jack Horner is taking it and running with it—backward in time. He looked at evodevo and decided that if we know how gene regulation changed tooth into beak formation in the chicken embryo, then we can just as easily regulate in the other direction, creating “experimental atavisms,” causing a chicken embryo that has the code for a beak to develop teeth. We can stimulate the embryonic chicken’s pygostyle to develop into a tail; swoop in and down regulate the growth of the long digits that make up the wing, thus creating a hand instead; and alter the regulation pathway that directs where feathers develop.

You’d think that as the science advisor for Jurassic Park (he was, seriously, and also was the inspiration for Dr. Alan Grant), Horner would know better than to mess with nature like this. “Life finds a way,” after all, and rarely are the results clean or predictable, but he’s forging ahead, working with evo-devo labs that study the pertinent regulatory networks in embryos. When and if they create an embryo that can actually grow (remember the scene in Alien: Resurrection when Ripley finds all the horribly mutated earlier attempts at cloning her? That’s basically what an evo-devo lab’s archive looks like, with chicken), it remains to be seen whether their ethics-oversight committee will let them hatch it. But as a common, genetically understood bird, chickens are the standard avian model organism, and thus the natural subject of Horner’s reverse evolution. What came from dinosaurs through jungle fowl, and Darwin’s research into the modern chicken, may loop the evolutionary tree into a circle, taking us to dinosaurs once more.