Chickens, as a species, became chickens through a long, slow process of evolution. At some point, a chicken-like bird produced an offspring that, due to some mutation in its DNA, crossed the threshold from mere chicken likeness into chicken actuality. That is to say, a proto-chicken gave birth to a real-life official chicken. And since that real-life official chicken came out of its own egg, we can say that the egg came first.
Another way to look at the question would be to ask which came first in evolutionary history. Once again, the egg takes precedence. Many characteristics of the modern avian egg—namely an oblong, asymmetrical shape and a hardened shell—were in place before birds diverged from dinosaurs about 150 million years ago. “A lot of the traits that we see in bird eggs evolved prior to birds in theropod dinosaurs,” says Darla Zelenitsky, of the University of Calgary.
Another key moment in the history of avian eggs occurred at least 150 million years before that, when a subset of four-limbed vertebrates evolved to produce amniotic eggs. The embryos within the eggs were surrounded by three fluid-filled membranes that provide nourishment, protection, and a way to breathe. The earliest amniotic eggs contained large amounts of yolk, says James R. Stewart, a reproductive physiologist at East Tennessee State University. “You still see that in birds, crocodilians, and snakes,” he explains. Like other placental mammals, we humans lost our yolk somewhere along the line, but our eggs still come with a vestigial yolk sac.
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