Which Came First – The Chicken or The Egg?

Photo of Steve Lysne
Author: Steve Lysne
Published: May 11, 2017

This age-old question seemingly proposes a conundrum that is not easily answered. That is: Chickens emerge from eggs so eggs must come first… but eggs are laid by chickens so the chicken must come first. At first glance this does seem to be an unanswerable question. However students of Dr. Lysne’s and Dr. Strickler’s Biology 2 classes know the answer and soon you will too!

The answer lies with an understanding of biological evolution. That process you probably don’t know a ton about but do know that it involves Charles Darwin and something to do with “survival of the fittest.” Biological evolution tells the story of where we came from; indeed where all of life in its present form came from. Importantly though, biological evolution does not tell us where the original spark of life arose; that is an entirely different question!

With regard to the problem of which came first, the answer is clearly demonstrable and evident in the fossil record as well as in our own DNA. From fossils we know that the first amniotes – those animals that produce “eggs” as we understand them – arose around 350 million years ago (MYA) and probably looked something like small lizards with pointy teeth (indicating that they were carnivores). Birds on the other hand do not appear in the fossil record until much later; some 200 million years after the first amniotes or about 140 MYA. And since the chicken is a bird, the egg preceded the chicken by nearly 200 million years! Not even close to a conundrum as it turns out.

Interestingly we can also see evidence of this amazing story in our own DNA. While the egg may have evolved in early reptiles, we are mammals and all mammals still retain an egg in some form and the genes for the development of that early reptilian egg still exist within you and me! However, instead of laying eggs outside of the animal body as is the case with reptiles and birds, most mammal embryos complete development partially or entirely within the mother. The yolk sac – that yellow part of the fried egg you may have had for breakfast – is still retained in humans but the genes for the yolk itself are no longer functional as human embryos derive nutrition directly from the mother via the placenta. These non-functional genes are called “pseudogenes” and represent a large proportion of the human genome as well as a connection to our reptilian ancestors. Thus the age-old question has been resolved – like the relationship between reptiles, birds, and mammals – and you now know, incontrovertibly, which came first. The egg!

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