Male and Female

We are all half-human. Think about it.

Imagine an alien zoologist who abducts one of us to conduct a biological study of the human species. However judiciously chosen, the sample will lack one major organ system and some attendant secondary characteristics. Furthermore, our inquisitive alien will be at a loss trying to figure out how the species propagates.

Much ado about little has been made in human society over the “inexorable differences between male and female” – from different dressing conventions to different expectations about career and lifestyle.
In fact, some philosophers have gone so far as to explain the entire Universe as the interaction of “male and female principles” – Shiva and Shakti, Yin and Yang.This is, of course, one of the many examples where over-extrapolating from the human condition leads to complete nonsense. The “principles” governing the Universe are impersonal mathematical laws which know nothing of male, female or transgender.

All right then, what about the biosphere?
Aren’t male-female distinctions fundamental to the living world? Hardly.

Let’s start at the very beginning – with microorganisms.
In case that’s too humble a beginning for you, consider the fact that at least 50% of the Earth’s biomass consists of bacteria alone, and for 85% of its history, life on Earth was nothing but microorganisms.
There are no genders in the unicellular world.
Virtually all microorganisms reproduce asexually – splitting into clones which mutate slowly over the ages. Among bacteria, a pair of creatures will sometimes come together to exchange genetic material, but this is not connected to reproduction.

Still too microscopic for you? Then let’s consider the green world of autotrophs.
Plants are the basis of the biosphere, converting the energy of sunlight into useful forms which all other creatures can process.
An alien botanist would face none of the “incomplete sample” problems plaguing our hapless zoologist. The vast majority of plant species are hermaphrodites – a single sample gives you the entire picture. The situation is worth a deeper look.

Each plant has many reproductive organs. We call them flowers.
In the vast majority of cases, each flower has both male and female parts – an androecium that produces pollen, and a gynaecium that contains ova. In botanist’s jargon, these are “complete flowers”.
These include roses, lilies and the hibiscus, much beloved of authors of sixth-grade biology texts. In fact, virtually all the bright and colourful flowers that gardeners cherish are complete flowers, and the corresponding plants are called “hermaphrodite”.

Now some plants produce flowers that only make pollen or only have ova – male and female flowers so to speak. But each plant has both male and female flowers.
Only a paltry 6% or so of plant species have exclusively male or female plants, with flowers of only one type. These tend to be rather small and unattractive, usually pollinated by wind.
Clearly, plants think gender is a bad idea.

So, how about things that move? If you take a walk in your garden, the earthworms and snails you come across are all hermaphrodites. During mating time, two individuals will line up and simultaneously fertilize each other!
Looking for something less slimy? Take a cruise down to the Great Barrier Reef. Among the colourful corals and sea anemones, you will see schools of bright orange and white fish, which may remind you of the movie Finding Nemo.
These are clownfish, and their gender behaviour is most interesting. Each school consists of many males and one dominant female. But if the female dies or goes missing, one of the males changes sex to become female. How extremely convenient!
In fact, clownfish will sometimes pair up and inhabit a sea anemone. If the pair consists of two males, no problem at all. One of them just “changes over” and all is well.

Such sex change behaviour, where males become females or vice versa in response to the gender ratio in the population is in fact quite common among fish in the coral reefs and several species of amphibians as well (some of you may recall that this was a plot element in Jurassic Park). Imagine the wonders such a system would work in your average engineering college.

So then, what happened to us?
Why are we stuck with the most boring of alternatives, where each individual is doomed from birth to see only “one half of life”, as it were, with much inane speculation about what the other half “really wants”? Why aren’t we hermaphroditic “complete individuals” like the plants, or conveniently mutable like the clownfish?
Turns out, this is quite a puzzle.

The only major classes of animal where all individuals are exclusively male or female are reptiles, birds and mammals (and dinosaurs too, from fossil evidence, but then birds are just living dinosaurs.) But they are produced rather differently in each case.
If you think male versus female is a matter of different versus similar sex chromosomes (XY vs XX) think again. That’s only for mammals.
Among reptiles, the sex of the individual depends on various external factors like the temperature at which the egg was incubated. For birds, it’s the males that have the same sex chromosomes (ZZ, as opposed to ZW for females).
And even for humans, chromosomes aren’t destiny. It turns out that in addition to XY chromosomes, you need functioning testosterone receptors in the foetus, otherwise it just develops into a female.
Nature has to work quite hard to produce separate-sex individuals, apparently.

But why? What’s wrong with good old hermaphrodites?
The possible diversity in the gene pool is reduced to half in every generation, thanks to the exclusive single-sex system, so what are the compensating features for evolution?
Is some kind of fitness parameter being optimized by having separate males and females?
Or is it just a “frozen accident” of biological history – the common ancestor of mammals, birds and reptiles had males and females, so that’s how it remained?

The answers lie hidden somewhere in the genetic code. But for now, we half-humans can only wonder and our alien zoologist needs a sample of at least two....