From Our Inbox: May 2013
Question: “According to evolution, which came first: the chicken or the egg? And how could a reproductive system change to or from egg-laying in a single generation?”
Dennis Venema: Nothing changes dramatically in one generation. Changes are slow - think of a child’s flip book, where you flip pages rapidly to see the image change. Evolution is like that - each generation (page) is pretty much identical to the one before and after it. The changes add up over successive generations.
As for eggs, egg-laying was the reproductive mode for all tetrapods (and fish, the group tetrapods arose within) until the advent of marsupial and placental mammals. So, egg laying goes back to at least the origin of fishes... and of course much further back than that. The point being that chickens, as highly derived tetrapods, are relative newcomers compared to eggs.
You can read more in Dr. Venema’s ongoing “Evolution Basics” series.