The first chicken was born in Southeast Asia in 7500 BCE, a gene study recently confirmed. An article published by the American Association for the Advancement of Science traced the common bird back to an area near southern China. However, eggs came long before then.
According to an article published in Science, a gene study established that chickens did not originally come into existence in northern China, as was previously thought. “The first extensive study of the bird’s full genome concludes that people in northern Southeast Asia or southern China domesticated a colorful pheasant sometime after about 7500 BCE,” the article said. “Migrants and traders then carried the bird across Asia and on to every continent except Antarctica.”
Now we know where the first chicken came from, but which came first—the chicken or the egg? Eggs predate chickens, but thanks to studies in evolution, we have a thorough answer for why.
The first egg laying comes from the evolutionary transition from amphibians to reptiles. Scientists know this because of a site in Nova Scotia, Canada, that’s home to some very valuable reptile remains.
“Joggins is a world heritage site, which is at least partially because of its fossils, and among those fossils are the oldest known reptiles,” said Dr. Anthony Martin, Professor of Practice in the Department of Environmental Studies at Emory University. “One site in particular at Joggins named Joggins Fossil Cliffs became quite famous for its fossils, consisting of carboniferous age plants, invertebrates—including insects—and vertebrates.
“The fossils of nearly 200 species have been identified from this site, and that has provided paleontologists with this extraordinary glimpse of the paleoecosystems there from about 310 million years ago.”
The paleoecosystem in Joggins showed that ancient trees, which were once part of a forest, were buried by flooded riverbanks, leaving only their trunks and roots behind. As the wood decayed, it attracted small invertebrates like insects and snails who ate it—or each other. These insects attracted hungry amphibians and reptiles, including the oldest known reptile in existence, whose fossil was discovered in 1859.
This important evolutionary transition, whose earliest records are in the Joggins fossil site, led to the first land-based laid egg. Here’s how.
Sunny Side Up
Dr. Martin said that amphibians utilize external fertilization, where the females lay hundreds or even thousands of eggs outside their bodies that require fertilization by males. Once the tadpole emerges from the egg, it soon grows legs and becomes a tadpole frog, then an adult frog, though it stays dependent on bodies of water its whole life—most importantly, during reproduction. Reptiles exhibit different behaviors.
“They have internal fertilization with the female receiving sperm placed directly inside her by males,” he said. “There’s growth of a membrane around each fertilized egg and development of other organs enclosed by this membrane. Laying relatively smaller amounts of enclosed eggs in a wide range of environments was all done out of water, too.”
Finally, a longer period of embryonic growth happens in reptile eggs, so they hatch looking like tiny adult reptiles. This ecological difference separates amphibians from reptiles.
Without a dependence on water for all its stages of life, the reptile evolved eggs that came 310 million years before chickens.
Dr. Anthony Martin contributed to this article. Dr. Martin is Professor of Practice in the Department of Environmental Studies at Emory University, where he has taught courses in geology, paleontology, environmental science, and evolutionary biology since 1990. He earned his BS in Geobiology from St. Joseph’s College (Indiana), MS in Geology from Miami University (Ohio), and PhD in Geology from the University of Georgia.