Nothing in Biology Makes Sense Except in the Light of Evolution was the title of an essay written in 1973 by evolutionary biologist Theodosius Dobzhansky. It was written to counter the notion of Creationism or Intelligent Design which is an alternative ‘explanation’ of the things observed in biology held by some religious people.
It’s a phrase that has stuck - because it’s true.
The structures, processes and behaviours we see in the world of biology have all arisen as adaptations to evolutionary (selection) pressures.
Without the overarching ‘theme’ of evolution we can’t understand why life in its incredible variety is the way that it is. “It woz God wot dun it” is no kind of explanation at all, really.
It seems to me that with the notion of sex we’re in a kind of parallel pickle today. Largely inspired by political and social selection pressures some biologists today appear to argue that the notion of sex in humans is more complicated than the traditional ‘binary’ version we have all known about since Ugg first got slapped for making a lewd suggestion to Ugga.
Dobzhansky’s sage advice stands as a kind of warning here. We could re-phrase the ‘sex’ question as follows.
What are the selection pressures that have given rise to a third sex in humans?
If, as the spectrumists argue, there are more than two sexes in humans, then there must, at the very least, be a third one for us all to examine. This human with a third sex must exist somewhere on this planet. Where is it?
And what were the selection pressures in operation that led to its creation?
I’m going to anthropomorphise a bit here and use a shorthand that ascribes things like ‘intent’ to evolution. It’s a convenient rhetorical device that avoids lengthy and tedious (but correct) descriptions when talking about evolution.
Biologists when trying to support the notion of some kind of spectrum of sex in humans point to things like intersex conditions which are disorders of sex development (DSD’s). They’re sometimes called differences in sex development, but that terminology hides what’s really going on.
In blunt terms, then, there are men, women, and a very small group of people for whom something has gone wrong. Biology does that - it messes up from time to time - but without that ‘messing up’ we wouldn’t have had evolution at all.
What do I mean by “wrong” here? There are two ways things can go “wrong”. The ‘blueprint’, the body plan that is intended by evolution, goes awry and some mutation occurs as a result of errors in that blueprint itself. Or it could be that the blueprint is OK, but that something goes awry in the implementation of this blueprint.
It’s the former case that evolution acts upon. If these mutations are heritable then they might lead to a slight (reproductive) advantage in a given environment. Over time this beneficial mutation gets ‘fixed’ in the population. Most mutations are not beneficial (they do not confer any subsequent reproductive advantage). Most are deleterious (they make things worse as far as subsequent reproductive success goes and so get ‘phased out’ by evolution), or neutral (evolution gives a big fat ‘meh’).
Life first arose on this planet some 3.5 billion years ago1. At first, this life reproduced asexually, but at some point a couple of organisms got a bit frisky with one another and sexual reproduction was, erm, born. This first fumbling attempt at (probably) bacterial passion almost certainly looked nothing like the sexual reproduction we see today, but it was enough to confer a reproductive advantage and so evolution got a hold of it and ran with it.
It is thought that the reproductive technique of sexual reproduction arose around 2 billion years ago (give or take a day or two).
There have even been speculations that the development of sexual reproduction and the development of the first eukaryotic2 cells are linked.
A lot of this is a kind of ‘guesswork’ - we can’t go back 2 billion years and examine it for ourselves - but there is a lot of evidence (not least in examination of DNA lineages) for this overall picture. It’s scientific guesswork - which is a very different beast to normal guesswork.
So, if we want to understand what sexes are, and what sexes exist, we really need to understand sexual reproduction - because, to adapt Dobzhansky, nothing about the sexes makes sense except in the light of the evolution of sexual reproduction.
The point, for humans, as it is for almost all sexually reproducing organisms, is that the ‘plan’ provided by evolution for ensuring the success of sexual reproduction as a reproductive strategy comes in two flavours; plan M and plan F.
There is no room for plan X in the mating strategy of humans. There is no ‘third’ sex because this confers no reproductive advantage (and confers no reproduction at all, in fact).
The postulated existence of a third sex in humans does not represent an evolutionary adaptation as a result of selection pressure.
And that’s all there is to it really. We can point to the physical manifestations of how these plans become implemented and we find that a very good distinguishing characteristic for plans M and F is in the size of the gametes, but this is really a secondary, and convenient, placeholder for the more fundamental description in terms of a specific evolutionary adaptation that resulted in two ‘plans’.
The notion of there being some kind of ‘spectrum’ of sex in humans is not even remotely understandable in terms of sexual reproduction and the evolutionary selection pressures that gave rise to the two sexes. It’s political pandering - an adaptation of meaning as a result of societal pressures, but not scientific ones.
On the other hand, when I see a picture like this
I do wonder whether I’m wrong about there being only two sexes. What is this creature?
If you want a really good introduction to how we ‘know’ this then I would recommend my all-time favourite popular science book which is Nick Lane’s Oxygen. This book examines, not surprisingly given its title, the somewhat necessary gas we call Oxygen; why and how it arose, what it does for life, and what the problems for life with using it are.
Cells with a nucleus
I am less enthusiastic about evolution as the big explainer (I put more weight on the axiom that there is an objective reality - which provides some common ground for evolutionary biologists and religious traditionalists for the fight against the postmodern relativists), but I very much appreciate your writing, and respect your point of view.
Well, I am enjoying the new relationships being formed between creationists and evolutionists. (Similarly, “TERFs” are finding friends in the conservative right they would have once railed against, and I could have written that sentence backwards.)
I do think the trans activists made an error when they decided to try to rewrite the biology textbooks. Maybe some people could have accepted “gender affirming care” as something along the lines of orthodontia or cochlear implants, an appendectomy or chemotherapy— a corrective for nature’s errors— but claiming that this particularly ideology applies to all human beings who have ever lived and we just never noticed before is certainly overplaying their hand. (And that hand— we now want to inform you— might have five fingers, but it could also have six or seven— the number of human fingers exists on a spectrum, and if you’ve never felt trans -fingered, well, then, you’re cisfingered, but I hope you’ll be an ally.)