The evolutionary dynamics of sex and ageing are fascinating.
The fundamental choice of life is: approach or avoid? Search is therefore built into the structure of life at its most basic level.*
Natural selection is about successfully replicating so as to stay in the game of life. Selection pressure comes from what keeps organisms in the game. Searching for successful survival and replication is how natural selection happens. Genetic mutation is a path to the genetic variation on which genetic selection can operate to biologically entrench and develop successful replication strategies after they have been discovered.
Genetic evolution is driven by subsistence and reproductive strategies, not by mutation. Genetic selection responds to the interactions of subsistence and reproductive strategies with the wider environment. Natural selection is not a random process, it is a highly structured one, and is most certainly not driven by random mutations; most of which are harmful to, or useless for, successful replication anyway.
A good example within human evolution of this process of life strategies driving genetic selection is that pastoralist populations come to develop lactase persistence mutations (the ability to digest milk as adults). But different pastoralist populations developed different such mutations. The mutations were selected due to the shift in life strategies to pastoralism but the selection pressure to be able to consume dairy did not require exactly the same genetic mutation as a mechanism for it to operate through.
We can thus see that mutation is not an important selection pressure, though it can be a significant mechanism for selection to operate through. Background variation in, and the capacity for, mutation enables selection pressures to operate through selected-for genetic pay-offs. Thus, across evolutionary time, genetic selection tends to follow shifts in life-strategies and so shifts in selection pressures.
Random mutation is not an ordering structure within the biosphere. Search for niche possibilities, and selection pressures therefrom, are ordering structures within the biosphere that use mutation.
There is far too much order in the biosphere for random mutation to be the driving force of natural selection. What we have are very non-random selection processes responding to the approach-avoid dynamics of living organisms searching for what works for surviving and reproducing.
Sex causes ageing
If a species reproduces via all its cells, then it does not experience ageing. If every cell-line is needed in the process of replication, then the selection pressure is for all such cell-lines to remain fully functional. Even a multi-cellular being, if it reproduces by division, experiences no ageing of its cell-lines, as they all continue to be needed for the continuation of the species.
Cancer represents cells opting out of the multi-cellular cooperative game and concentrating on their own replication, without contributing to its functioning and regardless of the cost to the larger organism. They essentially abandon the cooperative role they are supposed to be performing as part of a multi-cellular organism. Instead, they revert to the more primitive, every-cell-for-itself pattern.
Once cells become divided into somatic (body) cells and gametes (reproducing cells), then the selection pressure is for somatic cells not to age until the organism starts reproducing. Up until that point, the selection pressure is for optimal adaptation to maximise the chance of surviving long enough to reproduce (and so replicate).
The further into reproduction cycle, the less need there is for the somatic cells to continue to function optimally, reducing the selective pressure. Ageing will thus occur (both in individual cell lines and in their inter-connections) as a result of this declining pressure to be adaptive. This process of loss of adaptive function eventually “bottoms out” in a plateau at the end of the reproductive lifespan.
Sexual reproduction as discovery mechanism
Shifting to sexual reproduction increases the ability to search for and exploit, even construct, new niche possibilities by increasing genetic variation. Sexual reproduction also weakens the ability of pathogens to continue to exploit a particular vulnerability in an organism. This a specific instance of the wider pattern of increased capacity to search for, and occupy, new niche possibilities by increasing genetic variation through genetic re-combinations: including one less vulnerable to whatever pathogen is creating the selection pressure. Occupation of such new niche possibilities are the replication-payoff for the increased capacity to capitalise on search that sexual reproduction provides. Ageing is the pay-off for the greatly increased capacity to find and exploit new niches.
The only evolutionary stable pattern in the increasing-the-value-of-search sexual-reproduction-via-gametes strategy is to have two types of gametes: small, motile (self-moving) gametes and large, sessile (not-self-moving) gametes. Having only two types of gametes — one mobile-and-sent the other retained-and-receiving — maximises the chance of successful gamete match-ups. Yet, there need to be at least two types so as to be able to “throw” the genetic dice, thereby generating genetic variation other than by mutation, thus providing more fodder-for-selection from which better search-and-exploiting-discovery sexual reproduction gains its replication advantage. Incest (apart from any abuse of trust or violation of boundaries that might be involved) reduces genetic variation in offspring, so works against the search-and-discovery advantage sexual reproduction provides.
Gametes are usually produced in equal-number sexes in a species, as an excess of one sex will tend to improve the replication possibilities of genetic packages being expressed in the other sex, until their numbers equalise.
Having non-replicating individuals within a species can evade this equal-numbers-of-each-sex dynamic. Neuter female ants and bees are a manifestation of the replication strategy of their queen, a feature of ants and bees being eusocial species.
The sex that produces (and so retains) the large sessile gametes presumptively takes more risks in reproduction than does the sex that produces (but deposits elsewhere) the small motile gametes.
There is therefore a lot of courtship behaviour, including courtship displays, across species, as members of the sex facing lower reproduction risks seek to convince members of the sex facing higher reproduction risks to let them make a reproductive deposit in their body.
An alternative path, common in harem species, is for males to be selected to generate the capacity to win dominance conflicts, often including evolving reliable signals of success in such contests. In harem species, males are likely to be strikingly larger than females (e.g. male gorillas can be twice the size of female gorillas) or develop physical instruments and markers of dominance (e.g. antler horns). In harem species, females typically have reasons to congregate, due to access to resources or mutual protection, enabling a male to dominate a group of females. Male-to-male contest, raising the risks males face to reproduce, largely substitutes for courtship. (Though kleptogyny, aka the “sneaky fucker” phenomenon, may have an element of courtship.)
Humans are not a harem species. Our cooperative subsistence strategy (sharing food from hunting and gathering and congregating together for mutual protection) precluded it.
There is also a strong tendency towards ornamentalism in the sex that faces lower risks in reproduction. Ornamentalism being the investment in physical features structured to attract the opposite sex, such as the peacock’s tail. For it is the sex that seeks-to-be-chosen (because it is facing lower risks) that has reason to invest in features to make them attractive to the other sex. The sex doing-the-choosing (because it is facing higher risks) does not have to bother, as they are doing the choosing.
Hence, peahens (who choose) are drab, but peacocks (who are chosen) are highly ornamental. Male ornamentalism is a very common pattern in birds. (Though, if it is the males that look after the eggs, then the females tend to be ornamental.) Ornamentalism is clearly not likely to be a feature of harem species.
Yet this is one of the many ways that humans are weird. Human childbirth is unusually risky. Trying to get big infant heads (due to big brains) through pelvises designed for bipedal motion is a bit fraught. Moreover, human infants are unusually helpless and human childrearing typically requires prolonged parental investment. In contemporary forager societies, children do not produce as many calories at they consume until they are almost 20 years old. Yet, in humans, it is the females who are the ornamental sex. It is human females who have evolved features specifically to appeal to human males.
Human males have not evolved any features clearly and specifically designed to attract human females. They have, however, evolved features to signal dominance in contests between males; features that women often find attractive. This suggests that inter-male contests have been important in human evolution.
Cultural display may have developed, at least in part, as a way of making human males more sexually attractive; but that was likely a development relatively late in our evolutionary history. Larger penises also may have developed as sexual attraction features. But increased penis size could also have evolved as a means of retaining female sexual interest (so as to reduce the risk of infidelity), given that human females evolved both to remain sexually active when not-fertile and to obscure signs of their fertility, likely to maximise material benefits from human males. As evidence suggests that women shift their sexual interest when they are fertile, in a pattern than suggests “gene shopping”, and that men tend to become more possessive when their partner is fertile, selection for larger penises likely fits in with the evolution of the Dad strategy rather than against it.
The pattern of human females being the ornamental sex means, as online dating data indicates quite clearly, that human males typically find a broad range of women physically attractive, but human females typically find a much narrower proportion of men physically attractive. The ornamental sex (human females) has evolved to be broadly physically attractive to human males, due to seeking to be chosen. Human males have not evolved to be broadly physically attractive to human females, because, sufficiently intensely across evolutionary time, they did the choosing.
To put it even more bluntly, (long-term) female mate selection has not been a significant factor across human evolution. We can tell this because human females are the ornamental sex, so the sex seeking to be chosen, not the sex choosing. (Short-term mating is a more complex issue, but human societies have generally sought to suppress short-term mating.)
Nor was such mate selection by human females particularly important anthropologically. Across 190 foraging societies, in only 8 was courtship the primary mode of marriage (and those instances may be a more recent development). Far more common was parents selecting spouses, followed by close kin selecting spouses. If either of the prospective couple had a say, it was much more likely to be the groom than the bride (who might have a veto).
Moreover, the paleo-archaeological record suggests strongly that human technology and culture was homogeneous for very long periods of time, for thousands of generations, with signs of significant technological and cultural diversity only showing up relatively late in the Pleistocene. So, human sexual-selection patterns were likely also stable across that long period of time.
This evidence against the evolutionary and anthropological significance of mate selection by human females appears to be contrary to human child-birth being unusually risky, for both mother and child. As noted above, trying to get infants with large heads (from big brains) through pelvises designed for bipedal motion sitting on top of relatively narrow thoraxes (due to our economising on gut tissue) is somewhat fraught. This has imposed major selection friction against the expansion of human brains across evolutionary time. Which shows that the selection pressures for larger brains must have been very strong for human child-birth to become as risky as it has. Nevertheless, it seems very odd that in a species where childbirth is unusually risky, it is the females who have the ornamental features.
The fundamental selection pressure is to replicate so as to stay in the game of life. Either via replication of cell line(s) or via replication of cell lines long enough to reproduce and so replicate one’s genes. To say that the selection pressure for big brains was very strong, is to say big brains were being selected for so as to stay in the game of life.
The solution to the problem of the big-infant-heads-through-bipedal-pelvises, yet with clearly very strong selection for larger brains, has been to have much of the brain development happen after the infant is born. That creates remarkably helpless infants. It also creates long childhoods, with children who have to be taught, either directly or by example. Children, in other words, who require a lot of investment of time, attention and feeding. Those large-brain infants growing in relatively narrow thoraxes also tends to make human females very vulnerable during pregnancy.
That human females are vulnerable during pregnancy, have remarkably vulnerable infants and their children need years of care and support, has created archetypal human female responses to risk: to avoid risk or to transfer risk. That only works if there is someone to transfer the risks to.
At which point we get to another human weirdness. Humans are highly unusual among non-eusocial terrestrial species in having a strongly sex-differentiated subsistence strategy. (Eusocial species such as ants, termites, etc have sex-differentiated subsistence strategies, but they are obviously special cases.)
Across foraging societies, human males usually do the high-risk, high-variance foraging (hunting larger animals, gathering honey) and females usually do the low-risk, low-variance foraging (gathering plants, hunting small animals such as lizards). If women do hunt larger animals, it is typically because they do not have to engage in childcare, whether due to being childless, their children are adults, or are being looked after by others. (Exactly the patterns that female writers replicated for millennia, until very recently.)
A study of contemporary foragers found that men provided two-thirds of the calories and almost nine-tenths of the protein consumed by foraging bands, including almost all of the fat and protein consumed by children. These sex-differentiated subsistence patterns represent the transfer of risks away from child-bearing and child-rearing. A pattern in the allocation of tasks that we find across human societies.
It is a mistake to see greater male strength as the underlying driver of allocation of tasks in human societies and across human evolution. Greater male strength is an evolutionary response to the risks males face, the risks inherent in their replication strategies.
Which is why human females became the ornamental sex. If, despite the risks of child-birth, over the time it takes to raise a human child, a human male pursuing the Dad strategy (helping to protect and provide for the child) faces more risks than the human female, then males becomes the sex facing higher risks across the full reproductive strategy, females the sex facing lower risks. Hence, males became the choosing sex, females the sex-that-is-chosen. Thus human females became the sex that invests in ornamentalism to attract a human male willing to invest years of effort and risk in being a Dad to her children.
Including the (genetic replication) risk of investing in a child that is not biologically his child. (About a 9% risk worldwide, though with considerable local and cultural variation, with human populations typically having rates less than 5%.)
The children of a man’s sister often have special significance within cultures, as they are children the man is guaranteed to be biologically related to. Maternity being biologically certain when paternity is not. The lack of biological certainty in paternity have led to all those attempts to arrange social interactions so as to make biological paternity as certain as possible. Something that matters because of the crucial importance of paternal investment in successfully raising children. Including the importance for the prospects of children of paternal connections that have been constructed from the base of male provision and protection.
Given this deep evolutionary history, it is not surprising that father absence continues to have a range of negative effects on children’s well-being and prospects. Including ill-effects specific to sons and to daughters.
It is also useful to remember how human courtship has usually worked in societies where it is a thing. The male targets the female, who chooses whether to accept the courtship. Courtship that is usually structured around the male proving commitment to the female.
Human females have, across evolutionary time, somatically invested in features that signal long-term fertility (prominent breasts, curvy hips, etc). Physical signals that they are good long-term investments in fertility. Signals to attract a male wiling to accept the risks and burdens of the human Dad strategy. Signals that may also work for parents or kin engaged in selecting marriage partners for their son, nephew, etc.
Across human cultures, the most common way human females express aggression towards another female is to attack their reputation, particularly their value as a romantic partner (for a male). Including attacking their commitment to fidelity norms. In other words, to send the (damaging) message “she is not worthy of being chosen, of investing your effort”.
As part of the pattern of avoiding or transferring risk, women and girls typically tend to engage in more indirect aggression. What is known as relational aggression, as it attacks reputation; the ability to sustain relations with others.
As part of the risk avoidance strategy, human females often cloak their aggressive behaviour (even from themselves). Such as casting their aggression as normatively positive, even as being concerned for the well-being of the target of their aggression.
As the more physically capable and resilient sex, human males are more likely to express aggression openly and physically.
Unsurprisingly, women also tend to be higher in agreeability and neuroticism than men. They tend to build intimate friendships based on emotional investment and protection that eschew hierarchy in favour of fitting in. (What we can reasonably call cliques.) “You can’t say, that’s offensive!” is a typically female-pattern response, insisting on the protection of feelings.
As the more physically capable sex that risks have been transferred to, men and boys typically form teams based on roles and risk-management that are often somewhat hierarchical. Male teams tend to be built on a lot of bantering behaviour, as such banter are ways of testing trust worthiness: can I say outrageous things and you still support me? Will you fold under pressure?
“Sticks and stones may break my bones, but names will never hurt me” is a typically male-pattern response (within a dignity culture), showing that you will not fold under pressure. (Conversely, men in honour or bravado cultures can be very touchy at any possible insult, due to the pressure to defend their social space.)
Boys forming teams and girls forming cliques can be observed in any school yard. Girls low in agreeability and neuroticism (“tomboys”) will tend to join the teams. Boys high in agreeability and neuroticism (“sissies”) will tend to join the cliques. But the overall pattern still holds and has a deep evolutionary history. (As a profoundly cultural species, we are not slaves to our evolutionary heritages, but we are systematically subject to influence by them.)
As male roles have typically been higher risk roles than female roles, males seeking to perform female roles have tended to be dis-respected, as they are avoiding risks that men are expected to accept. This is also why there are no matriarchal human societies. If men are performing higher risk roles, they will be undertaking public (i.e. external to the domestic sphere) roles that accrue prestige, so cannot be presumptively excluded from authority, as matriarchy requires. (Patriarchy being the presumption that authority is male, matriarchy that authority is female.)
(Part 2 covers why humans evolved to pair bond.)
* Psychologist Jonathan Haidt makes the point that approach-avoid is the fundamental question of all animal life in The Righteous Mind, p.55. I have heard him express it online as the fundamental question of all life. Any organism with any element of mobility (including patterns of growth or sending of seeds) faces the question. Evolutionary biologist Bret Weinstein has discussed online the notion of search heuristics being built into evolutionary processes, that explorer modes permitting the systematic searching of evolutionary possibilities are embedded in evolutionary dynamics. (Also, this is a bit of a work in progress, so has been updated to reflect further information and thought.)