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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