People probably make too much fuss about defining biological sex in terms of its organic components. The term “chromosomes” gets thrown about, maybe because it is commonly used in basic biology education and is consequently a bit more accessible than “gametes,” although gametes are in fact the heart of the matter. Several different chromosomal combinations exist in humans (as abnormalities) besides XX and XY, but gametes come in only two forms – sperm and ova, the component factors of sexual reproduction.
But why does sexual reproduction itself exist, and by extension, why do the two sexes themselves? It is not a given across all species. Quite a few species of plants and some unicellular organisms practise autogamous fertilisation, effectively a slightly modified form of cloning in which the variants of sex are applied to an otherwise identical genetic template. Others, like the New Mexico whiptail, are parthenogenic, meaning that females can produce more females (clones) with no fertilisation at all. Most often this manifests as a “fail safe,” in species such as the Komodo monitor, for environments with a shortage of males. Obligate parthenogens are rare. When it happens, it tends to be the result of an unusually torpid environment combined with some kind of recent fuck-up. In the case of the obligately parthenogenic New Mexico whiptail: it lives primarily in the desert and owes its existence to cross-breeding between two parent lizard species which cannot produce viable males. If its environment changes too much, it is fucked: cloning and autogamy place a hard limit on gene recombination, and therefore adaptation, which is why the latter really only exists in plants and invertebrates, and the dominant presentation of the former is as a “failure mode” in otherwise sexually reproducing species. It is only “practical” in species with extraordinarily high reproductive potential, short gestation periods, sedate or undemanding environments, low metabolic needs, or high mutation rates.
Given this, it is not hard to see where males and females came from. Think of The Sexes™ as a strategy of gene propagation, and then secondary sex differences, in morphology and psychology, as strategies which reflect the different selective pressures the sexes were subjected to and/or subjected each other to (dimorphism). Viewed through this lens, females represent the “default” strategy which began with the oldest organisms (e.g. asexual bacteria): the “incubators,” reproducing through cloning and self-fertilisation, whereas males, the “fertilisers,” are a comparatively recent innovation. The degree of “sex-differentiation load” that falls upon males varies by species according to the aforestated variables in selection. Since females are, as is often noted, the gatekeepers of reproduction, the selection pressures that act primarily on females tend to be similar across species and relate, directly or obliquely, to their ability to bear offspring. For males, the story revolves around the conditions of access to females, which is why the male sex “morph” (form) differentiates itself from the female in completely different ways across species.
Sometimes male and female are barely distinguishable from one another. This is the case for many monogamous avians, whose environments, for whatever reason, do not lend themselves to significant sexual differentiation, which reduces female choosiness, which limits dimorphism: it is a negative feedback system. Other birds, like the crested auklet, engage in a kind of mutually eliminative sexual selection, whereby each sex vets the other for organically expensive sexual ornaments for reasons that are not well understood. In elephant seals, the degree of sex differentiation, just in size, borders on the absurd, although their (relative to humans) feeble brains mean that the possible scope of behavioural differentiation is not all that striking most of the time. Exactly where humans “fit” on these continua of male sex differentiation is something of a relative judgement call, but we are obviously not auklets or crows.
Sexual dimorphism and monomorphism have special behavioural correlates, most of which are obvious. Monomorphic species tend to be monogamous with fairly equal parental investment in offspring and low variance in male reproductive success. Dimorphics tend towards, well, the opposite of those traits. Humans also have a lengthier post-reproductive schedule than most animals, largely because of how long it takes the human brain to develop, which probably limits sex differentiation in e.g. aggression compared with some species that practise effective polygyny, and different normative mating systems between human societies will also affect it notwithstanding other forces such as judicially enforced genetic pacification. There is also considerable variation in these “life history traits” through time: from a time when “childhood” was seldom acknowledged as its own entity and children were expected to be responsible, to the point of execution, for criminal wrongdoing from an extremely young age, to … whatever you would call the situation we have now. Certain kinds of change may be inevitable, in this respect. Other things are remarkably changeless even in the face of new environments.
Human sexual dimorphism is an example of this changelessness. If aliens were to observe the human sexes 100 years ago and now, they would note stability in a range of male and female responses to exogenous stimuli, and note the differences in underlying strategy. Males are the strategy of high risk, aggression, dominance, status-seeking, agency and systems orientation; females are the strategy of low risk, passive aggression, emotional dominance, comfort-seeking, agency by proxy, and social orientation. (A great example of the agency/agency by proxy distinction can be seen in sex-specific antisocial behaviours such as psychopathy in males and Briquet’s syndrome in females.) They would note that human females are the limiting factor in reproduction, but human males are the limiting factor in just about everything else (obligatory Paglia quote about living in grass huts, etc). Intelligence is probably not a sexually selected trait in humans, or at least, there is little good evidence for it, and sex differences in intelligence per se are trivial. The sex difference is in application. Human brain complexity and its antecedents mean that the domain of activities germane to preserving one’s genetic line are rather more elaborate than normal, and since females are the “selector” sex, those tasks, and selection for assiduous task-doing, are upon the males.
There is no real sense in which human beings can “escape” natural selection, because natural selection is the reason behind everything that we are, including the desire (of some) to “overcome” natural selection, whatever that means. However, natural selection has also given us moral instincts and reasoning abilities which, combined with the technologies born mostly of male ingenuity, could allow us to divert evolutionary selection pressures in a way that could never happen without our technology. The crapshoot of genetic recombination, by the lights of human morality, is just that: a crapshoot. At some point, artificial gametogenesis could allow humans to become effective hermaphrodites, even if we still have the old equipment. CRISPR, and eventually full genome synthesis, could render natural recombination processes obsolete, and therefore sexual reproduction itself obsolete. Childhood will increasingly resemble adulthood as we produce children of extremely superior intelligence, and thus, reduce the need for high investment. Male breadwinning social roles will run into a brick wall of automation, or perhaps cloning of the 99.999th percentile most workaholic and intelligent workers. Female homemaking roles will (or have?) run into a brick wall of washing machines. As technology outpaces our obsolescent biological hardware, one seriously has to wonder: how much of the human intersexual dynamic, i.e. behavioural sexual dimorphism, is worth preserving? Maybe we could do with being more like the monomorphic crows.
Alternatively, perhaps one imagines a world of nearly infinite morphological freedom where individuals can modify their own physiology and psychology with ease, unconstrained by sex, like character profiles in an RPG, and where sex and gender, insomuch as they exist, amount to little more than fashion. One may dream.