Who came first, the chicken or the egg?

Eggs come from chickens and chickens come from eggs: that’s the basis of this ancient riddle. But eggs – which are just female sex cells – evolved more than a billion years ago, whereas chickens have been around for just 10,000 years. So the riddle is easily solved…or is it?

Taken at face value, there is no doubt that the egg came before the chicken. We tend to think of eggs as the shelled orbs laid by birds from which their chicks hatch – unless we eat them first. But all sexually reproducing species make eggs (the specialised female sex cells). That’s 99.99 per cent of all eukaryotic life – meaning organisms that have cells with a nucleus, so all animals and plants, and everything but the simplest life forms.

We don’t know for sure when sex evolved but it could have been as much as 2 billion years ago, and certainly more than 1 billion. Even the specialised sort of eggs laid by birds, with their tough outer membrane, evolved more than 300 million years ago.

As for chickens, they came into being much later. They are domesticated animals, so evolved as the result of humans purposefully selecting the least aggressive wild birds and letting them breed. This seems to have happened in several places independently, starting around 10,000 years ago.

The wild ancestor of chickens is generally agreed to be a tropical bird still living in the forests of Southeast Asia called the red junglefowl –  with other junglefowl species possibly adding to the genetic mix. From these origins, humans have carried chickens around the world over the past two millennia or more.

So, eggs dramatically predate chickens. But to be fair to the spirit of the riddle, we should also consider whether a chicken’s egg predates a chicken. As humans consistently chose the tamest red junglefowls and bred them together, the genetic makeup of the resulting birds will have shifted. At some stage during this domestication process the red junglefowl (Gallus gallus) evolved into a new subspecies, Gallus gallus domesticus, AKA the chicken.

In practice, it is impossible to pinpoint the moment when this happened. But in theory, at some point two junglefowl bred and their offspring was genetically different enough from the species of its parents to be classified as a chicken. This chicken would have developed within a junglefowl egg and only produced the very first chicken’s egg on reaching maturity. Looked at this way, the chicken came first.

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