Chickens have been the object of scientific inquiry for at least the past 2300 years, and scientists see chickens a little differently than most of us. I have always appreciated chickens as a homesteader and animal lover; after all, they are fascinating, beautiful, and useful creatures. But when I started looking at how scientists have studied them throughout history, I was fairly impressed with what chickens have shown us.

Since Aristotle's time in ancient Greece, the chicken has been a model organism for studying embryonic development. Aristotle, himself, was the first to observe the daily development of embryonic chickens, detailing their changes in a logbook. Prior to his study it was believed that upon fertilization a very tiny embryo that looked exactly like a young chick was formed, which then simply increased in size until hatching. Likewise, the same was believed to happen in other animals and humans - however, Aristotle observed that development is very much more complex and piecemeal. Aristotle's method was vastly refined in 1951 when scientists outlined chicken development not just to the day but down to the hour and the microscopic level. These studies of chickens were a first step in understanding the basic knowledge of the prenatal development of animals that we now have and led directly to our current understanding of how the nervous system forms in humans.

Chicks sit atop a picture of a genetic map of a chicken. The chicken genome has 39 pairs of chromosomes, whereas the human genome contains 23 pairs. Photo by Peggy Greb, United States Department of Agriculture.

Today chicken eggs are used in everything from introductory biology classes to cutting edge genetics labs studying embryonic development. Eggs are prized for embryonic research largely because it is a lot easier to look at chicken embryos, than it is to look at human (or other mammal) embryos. The theory behind this research is that geneticists can manipulate genes, and then see how the various altered genes affect the growth and development of chicken embryos, something not possible, or legal, in humans. The information gained from these genetics experiments is particularly useful since humans and chickens have many identical and similar basic "developmental" genes that are only active during embryonic development of the body and in tumor growth.

Many of these developmental genes remain inactive most of the time, and are not well understood. One of these inactive genes in chickens has the information needed for the formation of teeth. This discovery was made by a scientist studying developing chicken embryos that had two copies of the recessive trait, "talpid2" a lethal form of an important developmental gene. When active it prevents hatching and causes major abnormalities, including the development of teeth in the embryonic chicken's beak. Taking this knowledge, scientists inserted some of the molecular signals produced by this gene into "normal" chicken embryos (those without the lethal talpid2 gene) and, amazingly the embryos then formed teeth, this indicates that all chickens have the gene to grow teeth but that it normally remains inactive. I have always liked the old French exclamation, "When hens have teeth!" their equivalent to saying, "When pigs fly!," but it seems that hens having teeth isn't so far fetched after all. These chicken teeth, when they develop, aren't like human teeth, but more like a Tyrannosaurus rex's conical dinosaur teeth. No chicks have been allowed to hatch which have these induced teeth, and scientists aren't sure if fully developed chicks would even retain the teeth. Interestingly, all parrot embryos also develop "teeth" briefly during their development but they are resorbed into the beak before hatching.

Scientifically studying chickens has also been a way to learn about diseases in humans—especially tumor formation. In 1911, Peyton Rous's veterinary research on chickens lead to the groundbreaking discovery of cancer-causing viruses. This discovery was not only important, but so unexpected that he wasn't awarded a Nobel Prize for it until 1966 (an amazing 55 years later). Today this cancer-causing virus is known as the Rous sarcoma virus, and while having a cancer-causing virus named after you might not really be an honor, the Nobel Prize certainly is. In 1976 a team of scientists was awarded another Nobel Prize for finding "c-src", a cancer-causing gene in chickens that is very similar to the viral cancer gene found in the Rous sarcoma virus. The "c-src" gene that they discovered was the first time scientists identified a cancer-causing gene in an organism's own genome. Since these historic studies a number of other cancer causing viruses and genes have been identified in humans, chickens, and other animals.

The field of genetics holds the keys not only to understanding embryonic development and tumors, but also to why all living things look and act the way they do. So when scientists announced in 2004 that they had sequenced the chicken genome, it was very important. The chicken was (and remains) the only bird and the only agricultural species to have a map of all its genes made through the process known as genome sequencing.

The human genome had first been sequenced in 2000, so one of the first things done with the chicken genome was to compare it with humans. The chicken genome is 1 billion base pairs long and is only about 1/3 the size of the human genome. Despite its comparative shortness, the chicken genome was found to be quite similar to the human genome in "genetic length." Both humans and chickens have approximately 20,000 genes. This strange accounting is due to the fact that we humans have a lot more "junk DNA" which doesn't seem to code for anything and many duplicate copies of our genes. Chickens, on the other hand, have much more streamlined DNA.

Of the 20,000 genes in both chickens and humans, about 60% (or approximately 12,000) are nearly identical. If it is hard to believe you and a chicken are close to 60% genetically identical, remember that there are a lot of basic similarities between a chicken and a human. We both have heads, necks, bodies, mouths, backbones, two eyes, two nostrils, two legs, two upper limbs, two lungs, skin, a heart, a liver, and a stomach. The list could go on all day.

One surprising area humans and chickens seem to be similar in is their sense of smell. It has long been believed that chickens have virtually no sense of smell. However, recent genetic tests found that chickens have a similar number of olfactory (smell) receptors as humans, indicating that their sense of smell is similar to ours, so keep that coop clean! These same genetic tests also indicated that chickens probably have almost no sense of taste because they have almost no genes that produce taste receptors. So it is safe to say that humans don't taste like chicken.

Chickens have been domesticated for somewhere between 8,000 and 5,500 years, that's a long time for an animal to be associated with humans. Only today, as research continues, are we able to fully understand and appreciate this feathered human partner. Next time you look at a chicken, remember, they are amazing animals, not only do they provide us with food and enjoyment, but they have also provided us with a tremendous amount of information about ourselves. We can learn a lot about ourselves by studying chickens and what we learn may help us to live better and healthier lives. 

Learn more about the poultry genome project at http://poultry.mph.msu.edu.

Chris Packard, M.Ed. lives with his family and 37 chickens on a small homestead in Hampden, Maine. He is a professional science educator and biologist.