The evolution of human mating (2) Why pair bond?

The evolutionary science behind why humans are so weird.

Part 1 of this investigation into the evolution of human mating touched on why ageing happens, the discovery advantages of sexual reproduction and how our genus evolved a cooperative subsistence strategy that selected for big brains. This resulted in human males adopting a Dad strategy that shifted the risks in raising children to reproductive adulthood more to males than females, even given how unusually risky that big-brain evolution made human child-birth.

Yes, but why pair-bond?

That all still raises the question of how humans came to pair-bond, as the Dad strategy requires. None of the other great apes do. Orang-utans are relatively solitary, with an adult male mating with a range of females in his territory. Gorillas are a harem species, with a male having a harem of females, who are individually about half his size. Chimpanzees operate as a coalition of males dominating a group of females. Bonobos have a coalition of (mostly) males, but including an alpha female, who share a group of females. Both chimpanzees and bonobos have evolved relatively large testes, so as to “squirt out” the gametes of the last male to mate with that female and to maximise the chance that his gametes stay long enough to successfully fertilise the female. None of the other great apes follow the human pattern of mildly-polygynous pair-bonding while living in groups.

As chimpanzees are our closest living genetic relative, it is reasonable to assume that our common ancestor was a group-living species operating via male coalitions dominating a group of females. Especially as humans have a lot of similarities with chimpanzees. We are both group-living species. While chimpanzee diets are dominated by fruit, male chimpanzees hunt (female chimpanzees generally don’t), so chimpanzees have a (mildly) sexually differentiated subsistence strategy. Male chimpanzees share hunted food with members of their male coalition (and sometimes females in return for sex).

Humans and chimpanzees are equally the most pro-actively aggressive primates. To the extent that human males (with associated females) placed in the same situation as bands of chimpanzees (independent groups with little or no outside connections armed with simple weaponry) will display the same level and patterns of territorial ambush violence as do chimpanzee bands.

We both use tools. We both act strategically within groups. Indeed, chimpanzees tend to do even better than humans in strategy games in behavioural labs as they conform more to the Homo economicus model that game theory presumes than we do.

As for differences, humans are much less reactively aggressive than chimpanzees. (Indeed, we are less reactively aggressive than bonobos.) We are much more normative and cultural than chimpanzees. We have longer childhoods. Human females have children even if they are still caring for older children. (That is, human females “stack” children, rather than having them sequentially.) We live considerably longer than chimpanzees. In particular, human females have much longer post-menopausal life spans.

We share food much more widely than do chimpanzees. We pair bond when chimpanzees don’t. Human males typically adopt the Dad strategy of protective provisioning of children rather than the Cad strategy of fertilising female(s) while providing little or no food or other care to mate or offspring.

We also spend a lot less of the day eating. Chimpanzees spend almost half of each day chewing, we spend about a twentieth of the day chewing. That is because we are far more adapted to carnivory (especially animal fat), both physiologically and behaviourally, than are chimpanzees. Such adaptations include: high stomach acidity; easy achieving of fat-adaptation; social patterns more like social carnivores than primates; carnivore-pattern fat cells; insulin sensitivity; gut morphology suited for animal fat and meat consumption, rather than internally fermenting plants; earlier weaning ages; body energetics adapted to protein and fat, including carbohydrates not being essential to the human diet; adaptations to throwing and endurance running suitable for hunting.

Hence signs that humans displaced large carnivores and our likely involvement in the extinction of large carnivores and mega-herbivores. Moreover, stone tools for acquiring and processing plant foods begin to appear late in the Pleistocene, during and after the disappearance of the mega-herbivores, suggesting a shift from a high level of carnivory towards a more plant-based diet.

The more our diet resembles a chimpanzee diet of continuous eating, high in carbohydrates, the more we suffer from metabolic disorders, including cancers. (There are no vegan human cultures and, historically, humans have only been vegetarian due to poverty or religious commitment.)

Besides, the chimpanzee eating pattern would not be sufficiently energy-and-nutrient dense to support our very energy-expensive brains and provide sufficient time for complex tool construction, prolonged child care and skill transfer.

So, how do we get from chimpanzee patterns to human patterns?

Via bone-smashing and stone-throwing. A group-living species able to pick up stones and smash open bones and skulls of a kill left after some big carnivore has finished eating has access to high quality energy-and-nutrient-dense food (brains, which are mostly fat, and marrow, also mostly fat). Over time, that means we can invest a bit less in guts (as animal food takes less digestive effort than does plant food) and a bit more in brains that help us coordinate this group-subsistence strategy. We invest less in climbing trees and more in running along the ground. So we become (more?) bipedal.

Throwing rocks as a group enables us to protect ourselves in grasslands. We can even drive bigger carnivores away from their kills. So, we invest a bit more in the ability to throw. If we shape stones — so make tools rather than just using them — we can do all this better, including rippng up meat for easier consumption. We can even think about hunting (as a group) in our own right. While males unencumbered with offspring can do all this better, there is no reason to think that the females would not also participate and feast.

So, there is subsistence-and-reproduction return in both tool-making and in social coordination. This intensifies the selection pressure for bigger brains. We move up the trophic level, becoming more carnivorous. We begin to displace other carnivores. Making us increasingly the biggest danger to us. This intensifies the selection pressure for tool making and social coordination, so bigger brains.

But we are trying to get bigger and bigger infant heads through pelvises designed for bipedal motion on narrowing thoraxes increasingly selected for fat-focused carnivory. So, more and more brain growth is after birth. So childhoods get longer. The ability of nursing mothers to participate in hunts weakens. Unless they can get help.

These are groups dominated by male coalitions that (almost certainly) generally swap females. So, as a human female ages she is surrounded by more and more people she is related to. The genetic return to investing in existing children (rather than having more children) increases. So, like toothed whales, human females develop lengthy female menopause.

Having grandmothers around makes it more possible for their daughters to successfully care for children. Increased replication means post-menopausal survival by human females is selected for and gets longer and longer. Expanded grandmotherly investment helps make it possible for human females to “stack” children rather than having them sequentially.

If living longer provides a (net) replication advantage, then natural selection will operate to retard ageing, to keep our level of adaptation higher for longer. Enough selection pressure will select for mechanism(s) through which the selection pressure can operate.

Longevity ups and downs

A possible mechanism for this response to selective pressures is that, as the subsistence strategy becomes increasingly skill-based, human males remain competitive mates at older ages. The older the paternal age at conception, the longer the telomeres in the semen tend to be, so (at the margin) the longer the life expectancy of the offspring tend to me. As there is no reason that this would extend female reproductive life-span, their post-reproductive life-span gets longer. Setting up the feedback effect described above.

Grandmothers helping to care for the offspring of their offspring enabled longer childhoods, so more capacity for brain-growth after birth, so ameliorating the selection friction against larger brains from risky childbirth. The effect only gets any real strength in aiding brain growth by providing a mechanism for the selection pressure to have bigger brains to have effect by aiding the lengthening of childhoods.

Longer life-spans also create increased possibilities for those cell-replication failures (specifically, defections from being cooperative cells) that we call “cancer”. But increased carnivory, particularly focused on fat (and so fat-adaption), likely ameliorated the effect, given that evidence suggests that cancer cells tend to prefer glucose to ketones. Fat adaption likely also helped economise on internal cell-growth signals (such as through increasing insulin resistance), decreasing cancer risks.

Conversely, if we largely eliminate infectious diseases (so live longer, creating more time for cell-replication failures to occur) and move away from fat-adapted ancestral eating (i.e. eat in ways — continual snacking — or foods — such as processed seed oils or high-fructose corn syrup — that we do not have long-term evolutionary adaptation to), we can expect cancer rates to increase.

That there is evidence for selection within (European) farming populations for alleles protective against metabolic diseases implies that a more plant-based diet is generally less conducive to metabolic resilience within humans. If a human population has no history of farming — e.g. belong to foraging cultures with no indigenous history of farming — then the adverse effect on metabolic health of shifting to an agricultural diet they will not be adapted to, let alone an industrial diet that no one is adapted to, is likely to be very strong.

The grandmother effect also impacted on mating patterns. As post-menopausal longevity increases, the ratio of fertile males to fertile females rises, increasing male competition over females. We could not become a harem species, or a solitary species (pair-bonded or otherwise), because of our cooperative subsistence strategy. So, we become a cooperatively pair-bonding species, based on males responding to increased competition for mates by mate-guarding.

Building from the cooperative subsistence strategy, pair-bonded males within the group came to respect each other’s pair bonds. This greatly reduces overall competition between males within groups. It also requires suppression of male dominance behaviour. Such suppression makes cooperative hunting easier. Prestige (bottom-up status), based on aiding the functioning of the group, develops as a currency of cooperation, expanding patterns that can be seen in chimpanzees and other primates where assisting in resolving disputes is a path to status.

To establish female compliance with pair-bonding within group living, males extend their provisioning to children. (Later likely extended to parenting effort in its own right.) This is done as part of a strategy of cooperation between males within the group sharing the results of hunts so as to ensure regular access to animal meat and fat and build mutual respect for each other’s pair-bonds. Hence, a cooperative mate-guarding strategy develops. One where relations between older and younger males can be managed while juggling connections with other males and connections with one’s spouse. A pattern that evolves into mutual acceptance and support for parental control of the mating of their children, particularly daughters.

The subsistence strategy becomes increasingly not only cooperative but sex-differentiated. Human females increasingly are no longer in a position to successfully raise children without male investment. Males become both provisioning Dads and the sex facing greater risks in human reproduction that ends up becoming rolling, near 20-year projects (given that a foraging child comes to take almost 20 years to regularly forage as many calories as they consume).

Pair bonding with male protection and provision turns presumptive (biological) paternity into (social) fatherhood. Human males end up adopting the most extreme Dad strategy among terrestrial mammals. So extreme, that it shifts the relative risks across the reproduction strategy so that human females become the ornamental sex. (Including the genetic-replication risk of investing in a child that is not his.)

To secure the prospects for her future children, and her place within the group, females have to get a male to take on the higher-risk Dad role for her children, including being a respected member within the group. All this flips the dynamic from intense male competition for females (due to post-menopausal longevity) to females seeking to attract long-term investment by a male.

Human females, now the lower-risk-facing sex, evolutionarily invest in features designed to signal long-term fertility (big breasts, curvy hips) so as to encourage and secure such long-term investment by a male. A prospect not merely for one child, but for a series of children. The higher fat content of female bodies also allows buffering both mother and child (and their energetically-demanding brains) against the variable food intake of a carnivory-dominated subsistence strategy. Something that the evolving ornamentalism of human females capitalises on.

The low level of polygyny in foraging societies likely represents a response to the importance of male investment in the prospects for children interacting with the risks of the Dad strategy. Australian foragers likely developed an unusually high level of polygyny because males accrue more connections as they age, connections that matter due to the high level of active engagement with the environment the locally evolved subsistence strategy entails, while the lack of local big game animals means that there are few prospects for younger males to disrupt such status patterns.

It is a general pattern that grooms are older than brides. As more information is revealed about the skill, wealth and status value of a groom as they age, but this is not true of the fertility of a bride, such a persistent age gap is not surprising. The more that is true, and the more marriage is decided by parents or other close kin, the greater the age gaps at marriage can be expected to be. For parents and kin can be expected to be less concerned with age gaps than brides while being more concerned about connections, wealth accumulation, and (particularly) willingness to bid for the marriage. Conversely, the greater the role of wealth inheritance, the earlier a bride or groom’s value is revealed, so the younger the age of marriage can be expected to be. (Royal brides and grooms were often very young.) Another value of earlier marriage for brides is that it maximises the length of access to (so expected value of) the bride’s fertility and minimises the chance of something happening to lower or lose the value of her fertility to her family or kin.

Evidence suggesting long-distance trade turns up very early in origins of Homo sapiens. The development of exchange between groups represents an extension of the cooperative subsistence strategy. One made possible by the evolution of normative behaviour needed to sustain high levels of cooperation in a cognitively sophisticated species. Deliberate, even systematic, exchange of brides between groups was also likely an relatively early feature of human societies.

The evolution of a sex-differentiated, cooperative subsistence strategy leads to another very unusual feature of humans among great apes: maternal infanticide. There has been no recorded observation of infanticide among bonobos, presumably because of the presence of females in their dominance coalitions. Both gorillas and chimpanzees engage in killing infants of rival males to induce fertility in the previously-nursing female.

While humans can display male-pattern infanticide, maternal infanticide is a recurrent feature of human societies. This is so precisely because human reproduction is a long-term cooperative project. When an infant is born, the human mother has had to decide whether they had the social support required to raise the child. If not, the child is likely to be abandoned or killed. Human babies babble to attract attention and have been selected for cuteness to maximise the chance that both mother and (presumptive biological) father will bond with the infant as quickly and as completely as possible.

There is evidence that both human males and human females engage in both short-term and long-term mating strategies. Human males may attempt the Cad strategy of mating without providing. Human females may attempt to “gene shop” with willing Cads. To be stackers of sexual partners rather than stickers with one.

The more parents are after sons-in-law, not sperm donors, the more they can be expected to attempt to block short-term mating with their daughters. Similarly, the parents of grooms want daughters-in-law who are reliable transmitters of their lineage, typically enforced by being seen to be such, so blocking even the suspicion of any short-term mating with their son’s bride(s).

As human males have not been selected to be physically sexually attractive to human females, a majority of males are likely to find the Dad strategy (and its possibility of a series of children with one mate) a better bet than trying to be a Cad. Moreover, given the lengthy biological expense of human children, the Cad strategy became parasitic on the Dad strategy. (In some societies and circumstances, also raising the children of Cads can be an accepted cost of the Dad strategy: this is not a general pattern, however.) It is not surprising, therefore, that human societies have often sought to block short-term mating.

Evolutionary novelty

The less a society is structured to transfer risks to males, the more the specifically female risks of childbirth and infant care will structure interactions between the sexes. Sex that is substantially or wholly insulated from reproductive risks will also display very different patterns than those set out above. It is noticeable, however, that women, having had fun with (and so rewarded, and advertised their acceptance of) Cad strategies when young, can find it difficult to find a suitable Dad later. Especially given public policies that undermine Dad strategies (such as requiring men to pay for children while getting none of the other benefits of being a Dad). But these are a whole other level of complexity, and (human) evolutionary novelty.

Our evolutionary history explains how we became the very weird species with:

  • Ornamental females, despite human childbirth being unusually risky.
  • The most extreme Dad-strategy males among all terrestrial mammals.
  • The only non-eusocial terrestrial species with strongly sex-differentiated subsistence strategies.
  • The only group-living, cooperatively pair-bonding, great ape.
  • The longest-living ape due to being the only ape with females with long post-menopausal survival.
  • The most fat-obsessed carnivorous omnivore.
  • Having the most biologically expensive children in the biosphere.
  • While being by far the most fertile of the great apes and the only global primate.

Weaving our way through the evidence and the scientific literature, it becomes very clear what an odd species Homo sapiens are and what a very unusual evolutionary path our genus has been on. So odd, it is not surprising that it took so long for evolutionary processes to evolve a profoundly technological and cultural species. Our genus having become such species long before we became Homo sapiens.

(This is a bit of a work in progress, so has been updated to reflect further information and thought.)