Which 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.