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Which came first: the chicken or the egg?

It’s that old riddle that’s sparked many arguments through the ages: was it the chicken or the egg that came first? It’s such a tricky question because you need a chicken to lay an egg, but chickens come from eggs, leaving us with an intractable circle of clucky, feathery life that apparently has no clear starting point.

Thankfully, there’s no need to keep brooding over this forever. This is a riddle we can unscramble with the tools of science—more specifically, the principles of evolutionary biology.

Let’s get cracking.

The first eggs

Eggs are found throughout the animal kingdom. Technically speaking, an egg is simply the membrane-bound vessel inside which an embryo can grow and develop until it can survive on its own.

Scientific illustration of a range of different eggsEggs of all shapes and sizes exist throughout the animal kingdom. Image adapted from: Adolphe Millot; CC0

But let’s focus on the type of bird's egg we recognise today. These first came on the scene with the evolution of the first amniotes many millions of years ago. Prior to their arrival, most animals relied on water for reproduction, laying their eggs in ponds and other moist environments so that the eggs didn’t dry out.

At some point, a different kind of egg began to evolve, which had three extra membranes inside: the chorion, amnion and allantois. Each membrane has a slightly different function but the addition of all these extra layers provided a conveniently enclosed, all-in-one life support system: an embryo can take in stored nutrients, store excess waste products and respire (breathe) without the need of an external aquatic environment. The extra fluids encased in the amnion, plus the tough outer shell, provide extra protection too.

Diagram of a chicken egg in its 9th dayDiagram of a chicken egg in its 9th day—an example of an amniotic egg. Image adapted from: K.D. Schroeder; CC-BY-SA 3.0

Amniotic eggs were a big deal. They opened up a whole new world of opportunities for land-based egg-laying locations, and the extra membranes paved the way for bigger (and mostly better) eggs.

We’re still not sure of exactly when this happened, largely because eggy membranes don’t make very good fossils, leaving scientists with no clear record of when, or how, amniotic eggs developed. Our best guess is that the last common ancestor of both tetrapods (four-limbed animals with a backbone) and the amniotes (four-limbed animals with a backbone that lay eggs with all those extra layers) lived around 370-340 million years ago, though some sources put the first amniote species as living closer to 312 million years ago. Today’s mammals, reptiles and birds are all descendants of the first amniotes.

(This leaves us with another eggsellent question: which came first, the amniote or the amniotic egg? But let’s just stick with chickens for now.)