This post has been adapted from a twitter thread.
Some time ago, I lived in a house that had a number of peafowl living in the garden. (Long story). They used to come in the house and steal my cats’ food.
We can tell this is a female peafowl – a peahen – because she is busy feeding the children and not squawking and strutting around outside with show-off feathers. But importantly, she is the one that laid the eggs. The eggs are the important thing.
The cat is a male cat – Dylan – he is male because he is fat and lazy and does not produce eggs. His sister, Lottie, is a female because she developed an anatomy that supports egg production. She remains a female even though she was spayed.
When we say Lottie is female and the peahen is female we mean the same thing – a reference to eggs. And that link and common meaning is rooted in deep time. The cat and the bird (and us) share a common ancestor – a common mother who also produced eggs.
That last common mother of all of us looked a little like this…
She was an Amniote – a lizard-like creature that had evolved on a different path than the amphibians with an evolved trick of laying eggs out of water.
Our last common mother laid a clutch of eggs – perhaps under some fallen logs, and they hatched to form little baby amniotes. But that family line then diverged, with one side resulting in Synapsida – the cats, and the apes that open the tins for them…
… and on the other side, the Sauropsida – the lizards, crocodiles and avian dinosaurs we call peafowl. Some of the Synapsida evolved an additional neat trick of allowing the eggs to develop internally rather than having to go through the bother of building a nest.
And each female here can trace their ancestry back along an unbroken converging chain of mothers and daughters across 300 million years to that last shared mother – the Amniote lizard-like thing. Males have been along for the ride too – sometimes providing the sperm.
Each daughter looked pretty much like her mother. But over that vast expanse of time, those creatures changed and adapted to a changing world to produce all the species of mammals, reptiles and birds you see today.
And we can keep tracing that line of mothers and daughters back about 1,000,000,000 years to a multicellular organism that started producing different sized diploid sex cells. We do not know quite why, but sexually reproducing organisms that shared genetic material started adopting a system where each sex cell involved in sharing had to be different in size. Large sex cells became eggs and small ones became sperm. That first large sex cell producer was the first female. A billion or more years ago.
That asymmetrical requirement for a small and large sex cell to combine has been remarkably conserved across deep time. That binary asymmetry is what leads to the development of two organism types – males and females. No other sex cell type has emerged in our shared lineage.
Evolution has created two development paths for organisms like our cats and peafowl that allow each organism to develop with respect to each asymmetrical gamete type. Our sex characteristics though vary enormously from the absurd tails of peacocks to the milk glands of my cat.
How each sex is determined has become enormously complex, with mammals – XX/XY and SRY gene mechanism – to the ZW/ZZ mechanism in birds – which I understand is still not fully understood. Some of our cousins use environmental switches, like crocodiles.
And of course those male and female development paths are absurdly complex and it is remarkable that they only go wrong very rarely to produce development conditions – what are sometimes called ‘intersex conditions’. These are not new sex classes though as there is no new gamete.
What is strange is that in the past 10 years, an idea has evolved among the cat-food tin opening apes that sex is not real, it is an arbitrary construct, it is ill-defined and difficult to talk objectively about. It is a spectrum of possibilities rather than these two categories.
This is not an idea you will find in the biology peer-reviewed literature where sex is universally described as above – a category based on gamete types. The idea is used to undermine the ability of females to describe their unique position in life & their associated experiences.
It is a political and social idea that impacts females and their rights to define themselves as a material class. Biology rejects this absurd idea of the arbitrariness of sex. We need biologists to speak up and to say that rather than leaving it up to children’s fantasy authors.