Which Came First, the Chicken or the Egg?

So, did the chicken or the egg come first? It’s one of those questions that can send you around in logical circles until you have a headache. The chicken must have come first because it laid the egg, but the chicken came out of an egg so maybe that came first, but it was laid by another chicken, and pretty soon you’ve got a headache but are no closer to an answer. However, if we look at the puzzle from an evolutionary standpoint, the answer is very clear for a couple different reasons. Undoubtedly, the egg came first.

Image of chicken and eggs with arrows drawn to make a circle going back and forth between them
It's a logical circle that never ends! Unless, Science!

In absolute evolutionary terms, the egg came about millions of years before chickens. The oldest fossilized eggs ever found are about 600 million years old, and probably belonged to a tiny species similar to modern coral. Many invertebrate and all vertebrate females produce eggs in order to reproduce. Most mammals (excluding the platypus and echidnae, which lay eggs) and even a few other species, including some sharks, skinks, and snakes, have live births after their eggs are fertilized internally and their embryos develop in the womb. The eggs of fish and amphibians are soft and jelly-like and are not surrounded by a shell, whereas reptilian eggs have a leathery shell and bird eggs have hard shells in different textures. Eggshells first evolved in reptiles and allowed them to lay eggs away from the water, a major step in the evolution of land animals. Dinosaurs laid eggs, and since birds descended from dinosaurs, so do they. So from that standpoint, eggs clearly win the debate.

But what about if we just look at the evolution of the chicken? Speciation is really cool, but it can be a little confusing. There’s never a strict dividing line where you can say with certainty that the mother is a different species from her offspring. Species diverge via minute genetic changes that add up over time until there are two or more groups of descendants from a single ancestor that either will not mate if given the opportunity or will produce sterile offspring such as zedonks (half zebra, half donkey). It can take countless generations for a new species to emerge, with each generation being slightly less like their ancestral species and more like the new one that will eventually be recognized. That’s why it’s so hard to find “missing links” in evolution and can sometimes be difficult to determine the exact species of transitional fossils. Thousands of years ago there was a long-gone bird species that slowly evolved into the chickens we recognize today. On average, each egg contained a chick that was slightly more chicken-like than its parents. Once again, the egg came first!

Any other fun science or math questions you’d like to see answered in a future post? Leave ’em in the comments below. And no, so far as I know there is no scientific reason that I know of that caused the chicken to cross the road, so don’t even try that one, wiseasses!


Eggs and Their Evolution by Paul R. Ehrlich, David S. Dobkin, and Darryl Wheye.

Wikipedia articles on egg biology and chickens.

Image via Wikimedia Commons, unattributed.

By [E] Hillary

Hillary is a giant nerd and former Mathlete. She once read large swaths of "Why Evolution is True" and a geology book aloud to her infant daughter, in the hopes of a) instilling a love of science in her from a very young age and b) boring her to sleep. After escaping the wilds of Waco, Texas and spending the next decade in NYC, she currently lives in upstate New York, where she misses being able to get decent pizza and Chinese takeout delivered to her house. She lost on Jeopardy.

19 replies on “Which Came First, the Chicken or the Egg?”

As one who has cared for chickens (chickensitting – I’m not THAT portlandish) I can tell you why a chicken will cross the road.

Because that’s where there’s a vacant lot and the neighbor dumps their trash over there in the brambles and as any chicken caregiver knows (or will soon find out), chickens peck at or eat everything, including food scraps. They run like mad to old potatoes and squash.

(Also, that sentence up there may have just been a great example of the “they’re, their, there” grammer issue.)

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