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Essay/Human Evolution

Sex makes babies

As far as we can tell, no other animal knows this. Did our understanding of baby-making change the course of human history?

Holly Dunsworth & Anne Buchanan

The Ain Sakhri lovers figurine. This is the oldest known representation of sexual intercourse in the world, dated c10,000 BCE. Photo courtesy of the Trustees of the British Museum

Holly Dunsworth

is chair of the department of sociology and anthropology at the University of Rhode Island. Her research has been published in Nature, Scientific American, and Proceedings of the National Academy of Sciences.

Anne Buchanan

is a research associate in the anthropology department at Penn State University, and author of The Mermaid’s Tale: Four Billion Years of Cooperation in the Making of Living Things (2009).

6,800 words

Edited by Brigid Hains

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Throughout most of evolutionary history, sex was just sex. Among vertebrates, fish were the first to do it, going back some 400 million years. While it might be fun for fish and all the other species that evolved to reproduce sexually, for most species, sex still is just sex. But for our own peculiar species of primate, sex is about something more. Sex is about babymaking. Contemplating sex and where we come from has played a fundamental role in human mating, partnering and raising children, and in forming families, communities and alliances, and more. Recognising this fundamental difference between us and the rest of Earth’s sexual beings overturns conventional evolutionary thinking, which has long understood human sex, reproduction and kinship as fundamentally the same for us as for any other mammal.

All sexually reproducing animals have a powerful ‘sex drive’. If they didn’t, they would quickly become extinct. Among most animals, this drive demands immediate attention. It’s the yowls of the tomcats in the alley who detect a female in heat, the bawling bull who smells a receptive cow. It can’t be ignored. But it’s not a ‘baby drive’ – at least it isn’t experienced as one. We know the two are intimately related, but the tomcat doesn’t. He just wants to find that female in heat. Sex can certainly make for high drama among manipulative social mammals, especially primates. Among many monkeys and apes, the alpha male often sires the most offspring during his tenure because he is granted the least fettered access to fertile females, and can foil the sexual devices of subordinates. But with our inventions of virgin worship, marriage, castration, contraception, fertility technology and genetic engineering, the human primate experiences sex in an entirely different way from any other animal, enmeshed in all kinds of cultural and emotional networks and significance.

Stories about what make humans unique glorify dexterous fingers, inventive minds and our habit of sharing complex ideas through intricate verbal cues. Our ancestors’ fabled intellects gave rise to art, technology and dynamic, large-scale politics. But there is an oft-overlooked plot in the human saga. It stars the ancient hominins who realised that they’re related to some people and not others, and that sexual intercourse might have something to do with that. The effects of this realisation are profound, and deserve some credit for our species’ widespread success on the planet.

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Pop culture is obsessed with sex, and science is no different. And for good reason: sex is fundamental to how and whether so much animal evolution happens. In conventional evolutionary science, ‘favoured’ genes cause themselves to be passed from one generation to the next, because they are responsible for traits that confer reproductive advantages in a particular environment. This is natural selection. When it comes to sex and reproduction, science takes a particular interest in sexual selection: that is, the evolution of traits involving mate choice and mating behaviours. Within this frame, scientists have tried to trace the origins of human mating, marriage and kinship to evolutionary ‘strategies’ that, conscious or not, were responsible for our survival and continued evolution rather than our extinction.

In other words, if you follow this mainstream or ‘Darwinian’ logic, there must be genes that underpin mating behaviours, which in turn cause animals (including the human animal) to be successful in reproducing, and thus those genes (and their associated behaviours) are perpetuated in populations. If that’s how simply things actually happen in nature, there will be genes ‘for’ mate preference, genes ‘for’ pair-bonding, genes ‘for’ polygamy and so on. 

We share many genes even with fruit flies, but we share far more with non-human primates. We share an especially large proportion of our genome with our closest relatives – chimpanzees and bonobos – so, if their mating behaviour is genetically driven, then we’ll learn a lot about ourselves by studying these apes. Although no one has actually identified genes for infanticide or for avoiding incest, for most evolutionary scientists, answers to questions such as why is infanticide so common among chimps and some monkeys, or why is the incest taboo so common in human societies, ought to be applicable interchangeably to all of us primates. Thus, evolutionary psychology and evolutionary theory more broadly has a clear theoretical box for human sexuality: the model of animal mating.

Back in 1997, the psychologist Steven Pinker wrote in How the Mind Works: ‘The human mating system is not like any other animal’s. But that does not mean it escapes the laws governing mating systems, which have been documented in hundreds of species.’ In Mutants (2004), the evolutionary developmental biologist Armand Leroi summed up this hardline argument with: ‘the psychologies of pheasants and Fijians are really much the same’. The idea here, the ‘law’ that governs mating, is that sexual selection is assumed to drive reproductive behaviour in similar ways in all kinds of creatures. Conventional theory describes the characteristics we use to choose our mates, be it the resplendent tail of the peacock or a man’s full beard, as indicators of good genes, that is, genetic predisposition for strength or good health, and thus we’re choosing not just a full beard, but a collection of favourable genes to pass on to our children. This strips away any uniqueness in our reproductive behaviour; we’re just like any other animal.There have been many human mating behaviours that have been anointed by hyper-Darwinians as ‘natural’ to the species, often by analogy with other primates – and often revealing as much about the preconceptions of their inventors as about any sound science. Thus we are told that men are genetically programmed to be dominant, women are programmed to seek the alpha male, monogamy is innate for women, polygamy is innate for men, and many other examples. Male violence is regularly interpreted as a programmatic legacy from human evolution, and violent stepfathers who hurt their partners’ children are understood to be acting out of the same impulses as male chimpanzees who kill infants in a troop. Thus the standard trope of ‘Demonic Males’ and choosy females.

These potent images are worth unpacking because they reveal the disorienting feedback loops between seeing ourselves as just like other animals, while interpreting other animals as being just like us.

According to conventional evolutionary theory, dominant male chimps and some other primates kill infants in the troops they join because they know that these babies aren’t theirs. This makes sense to mainstream evolutionary theory because every organism’s purpose in life is to survive to reproduce, but even better is when my genes outcompete yours. I win, you lose. Thus, a dominant male kills unrelated infants because this increases the chances that his genes, inside his babies, will outcompete, or outnumber, his rivals’. Survival of the fittest, indeed.

In eliminating the distinctions between human sexual behaviour and that of other primates, a murky anthropomorphism creeps in. The journalist Nicholas Wade wrote in The New York Times that male chimps and baboons ‘are prone to kill any infant they believe could not be theirs, so females try to blur paternity by mating with as many individuals as possible before each conception’. This suggests that non-human primates could know that semen transforms into a baby and that the act of sex, broadly, makes an infant. Further, it implies that they have a sense of relatedness, and that it extends to fathers. If not, then it’s deliberately narrating animal sex and violence like a scene from Game of Thrones, for our entertainment. And it works (it’s sensational and relatable) because a more scientifically grounded alternative – male baboons, gorillas and chimps might kill infants, but they’re less likely to kill ones clinging to females with whom they’ve mated because sexual relations between primates builds affiliation – isn’t nearly as scintillating.

It’s not just journalism that falls into this trap: scientists aren’t all that deft at escaping the temptations of anthropomorphising reproductive strategies either. Writing about male-male competition and the caretaking of infants by the male marmoset monkeys who sire them, the primatologist Sarah Hrdy quipped in Mothers and Others (2009) that ‘in the absence of DNA testing, it is impossible [for the monkey] to know who the father is’. But really, it’s the absence of the awareness that sex makes babies (which we’re calling reproductive consciousness) that makes it impossible for a monkey to know who the father is, or to have the concept of ‘father’ or paternity in the first place. Something else is driving marmoset fathers to care for their own biological offspring and not others.

So evolutionary speculation about the origin of human mating strategies not only rests on science’s tendency to ‘zoomorphise’ us. It also entangles science in a dizzying web of anthropomorphic assumptions about other animals.

Yet as the cognitive scientist and anthropologist Daniel Povinelli writes in ‘Behind the Ape’s Appearance’ (2004): ‘if chimpanzees do not reason about unobservable entities, then we would frequently need distinctly different explanations for human and chimpanzee behaviour – even in situations where the behaviour looks almost identical’. Chimpanzees deftly navigate a world with gravity without thinking about gravity, or rationalising about it and making rules. In an equally naïve way, they deftly navigate a world with paternity without thinking of the consequences of sexual intercourse. But with our fundamentally similar ape ways, humans navigate a world where males are known to be intrinsic to babymaking, where males and females can have descendants and siblings, where beliefs about relatedness shape society and politics, and where relatives can inherit immaterial and material wealth to a degree and complexity that is unparalleled in the animal kingdom.

When a dominant chimpanzee kills an unrelated infant, one set of explanations is needed. And it’s not enough to map those directly, with no recognition of the utterly distinct human experience of reproduction and family, on to human domestic violence.

Human horniness is tethered to beliefs, to knowledge, to conscious calculation, to past and to future

Influential ideas about the evolution of human pair-bonding also dismiss our distinctiveness. In Primeval Kinship (2010), the primatologist Bernard Chapais wrote a book-length hypothesis for the evolution of human pair-bonding, matrilineal and patrilineal descent groups, and exogamous (anti-inbreeding) marriage patterns. Like many other scientists, Chapais interprets the very peculiar mating habits and family structures of humans as outcomes of standard processes of natural and sexual selection that favour particular mating strategies for animals with particular niches and needs. Human mating strategies can, as far as Chapais and many others are concerned, be understood without any reference to our peculiar self-consciousness about them.

Our species’ habit of pair-bonding, for example, is widely considered an adaptation to having particularly costly babies. They’re so big (read: hungry) and so slow to grow to independence, that allomothering – infant care from other than the mother – not only from a baby’s grandmother, maternal aunts and sisters, but also from the baby’s brothers, father and his kin, enabled us to continue and maybe even thrive under these conditions. And that’s not a bad explanation as a starting point. It helps to explain why other primates, such as owl and marmoset monkeys, have invested fathers. Owl monkey fathers don’t ‘know’ who their offspring are in the sense that humans ‘know’. But natural selection has favoured (or at least, not frowned upon) owl monkey fathers who care for their offspring – they have a reproductive advantage and, insofar as this behaviour is heritable, whether biologically or because it’s learned, it will be reproduced in further generations. On the surface, this looks akin to human behaviour, minus all the elaborate consequences of human paternity. 

It seems reasonable to assume that at the proximate level of attraction and arousal we are close to other animals. But talk of ‘animal instincts’ is for dark and steamy trysts, not wedding vows. Human horniness is tethered to beliefs, to knowledge, to conscious calculation, to past and to future. What is more, humans arrange marriages – sometimes from birth – greatly shaping human sexual behaviour over a lifetime. And new genetic studies show that marriage is a very ancient human behaviour indeed. Because our species is inextricably steeped in sociocultural context, you could argue that all human marriage is somehow arranged. This is a whole new approach to mate choice on the evolutionary scene. Like other social animals, we do compete for mates and we are choosy, but it’s not just because we want to have sex with them, it’s also because we want to make babies with them, to merge families with theirs, to make a future together. Reproductive consciousness isn’t just an aftermath to human mating. It has shaped it profoundly.

In his book Why Is Sex Fun? (1997), Jared Diamond writes that human sexuality is unusual because we often have sex for recreation rather than procreation. But the popular emphasis on orgasms and acrobatics misses the mark. Animals have sex for a smorgasbord of immediate reasons, including pleasure, yet no species knowingly has sex for procreation except us. So it’s not just our Kama Sutra-ness but it’s also our reproductive consciousness that sets humans apart. Sex isn’t something that just happens to human beings. It’s the fact that we know what can happen when we do it that might be one of the turning points in making humans unique inhabitants of the planet. Because of reproductive consciousness, humans know that they are related to one another: grandparents, parents, siblings and children. And this knowledge – sex equals babies, and babies equals kinship – marks one of the turning points of the history of life.

So how did we work it out in the first place?

As far as we know, there is no animal that spends time dwelling on what it cannot perceive with its senses other than the human animal. Understanding where babies come from can’t simply be observed. It requires grasping that a rather routine activity today will have long-term consequences in the future – connecting a long-ago act to the baby mice, kittens, baby gorillas or newborn whales and elephants born 20 days, two months, eight months, or almost two years later. Among the few of us, including bonobos, that copulate while pregnant – which can shrink the time between cause and effect – being able to link the business and substance of sex to pregnancy and its outcome would still take the kind of wild imagination that only humans are thought to possess. That, plus language, helps us to think these sorts of abstract creations and to communicate them. Once we’re a few years old, humans begin to explain the unobservable. Soon thereafter, we’re weaving and repeating stories about where babies come from. And it’s not much longer until we’re seasoned gossips about tribe members.

Abstract conceptual ability, what Povinelli refers to as a mind primed to think about ‘ghosts, gravity, and God’, is among the few exceptional human traits that primatologists, who are ever narrowing the divide between us and our closest non-human relatives, can embrace. To quote Povinelli: ‘The mental lives of humans and chimpanzees are similar, in that both species form innumerable (and in many cases, identical) concepts about observable things, but, at the same time, are radically different, in that humans form additional concepts about inherently unobservable things.’ As far as we understand non-human cognition today, there is little to suggest that other animals hold beliefs, material or spiritual, about pregnancy or babymaking, or that they understand that anyone is related to babies, especially males. Without a vivid imagination for the past and the future and the mysterious connections between them, such an understanding couldn’t exist.

We don’t know when our human ancestors first began to reason abstractly in a deeply imaginative way, and we certainly don’t know when any first moments of a-ha occurred, when anyone grasped that man + woman + whatever else they might imagine = baby. Scenarios for the origins of reproductive consciousness play out in fantastic works such as Jean M Auel’s The Clan of the Cave Bear (1980) and its sequel The Valley of Horses (1982) but, outside of speculative fiction, it’s unlikely that we’ll ever know what happened with any certainty. However, there are traces of the cognitive development of humans that afford interesting clues.

Surely the control of fire offered an opportunity for hominins to observe how organic matter can transform, and to transfer those rules to sex and babies. The best evidence for fire control doesn’t appear until 800,000 years ago at a Homo erectus site in Israel, which is half a million years before anything evolved that we’d call Homo sapiens. Perhaps it was cooking – we like to think of this as the ‘bun in the oven’ hypothesis – although bread itself is of course only a few thousand years old. Foraging for eggs would have revealed to ancient hominins how amorphous fluid transforms into baby birds – an observation begging to be transferred to baby humans. The earliest ostrich eggshells with a definite human use, ingeniously used as containers to carry and store water, don’t appear until the later Middle Stone Age of Africa around 60,000 years ago. Abstract thinking itself emerged long before this.

If reproductive consciousness is as old as the earliest archaeological evidence for our unique ability to think about the future, the past and present, and to link far out causes and effects together, then it might have originated as far back as 1.75 million years ago when Homo erectus began deliberately shaping stones in the form of tear-dropped hand-axes. But ultimately we can’t know for sure what erectus brains could think about sex and kinship. If reproductive consciousness is possible only with our species-specific brains, then it could have dawned on the first of our species, Homo sapiens, which means it could still be very ancient, going back some 200,000-300,000 years. Given that it’s likely that sapiens took some time to fire up its brain to modern capacity, then it’s possible that reproductive consciousness originated within the past 100,000 years when the archaeological record starts to boast the products of sophisticated abstract thinking with those engraved eggshells, and also in etched rocks, carved stones and painted caves.

Reproductive consciousness is a powerful context for boosting male-female cooperation, even beyond mates 

Whenever the cognitive ability arose, reproductive consciousness is almost certainly as old as 100,000 years, and so the cultural byproducts of it must be ancient too. Dowries, engagement rings and marriages are influenced by our unique cognition and culture, even if such traits have long been seen by evolutionary biologists as the bipedal ape’s versions of what bush crickets, bower birds and wolves do.

Let’s start with the consequences that might run very deep in our past. Humans around the world form pair-bonds within larger communities. We are generally not promiscuous, like chimpanzees and bonobos, despite the tremendous opportunity around us. Sure, we cheat, but most partnered humans are as reproductively faithful as physically isolated gorillas and gibbons, even with far greater opportunities to sleep around. And while many diverse cultures promote or tolerate polygamy, monogamy is far more common in reality, even within putatively polygamous societies. This human tendency to bond in pairs could have evolved, as Bernard Chapais and others have hypothesised, because of a long series of ecological shifts in the past several million years of our evolutionary history, including selection for allomothering males. Whether it’s an alternative or a complement to this idea, reproductive consciousness likely also played a role in the evolution of people’s ability and desire to form exclusive sexual partnerships within a larger community.

Beliefs about relatedness and about babymaking could have factored into mate choice and competition for a long time, in a particularly human kind of sexual selection. Reproductive consciousness would have increased attraction to and competition for mates, male or female, who are observed to be good community members and good parents, or who have potential to be. It would have increased competition for mates or for families with resources they’re willing to share. Reproductive consciousness is a powerful context for boosting male-female cooperation, even beyond mates and into adult brother-sister relationships – effectively a uniquely human phenomenon. The understanding of paternity would also have boosted brother-brother bonds. Although chimpanzee males cooperate, brothers are not intentionally befriended. Humans take nepotism far beyond where other primates go because we know about relatedness, and we encode it with meaning and value.

A very rich example can be found in incest taboos that are, like pair-bonding, widespread in human societies, enough to be virtually universal. The standard evolutionary interpretation of human incest taboos runs like this: human incest taboos and animal inbreeding avoidance are general evolutionary strategies for preventing very rare, debilitating genetic traits in offspring. Many animal species practise matrilocality and patrilocality. That is, females or males remain in their natal group, and the opposite sex (sometimes both sexes) leaves their natal group as they reach reproductive age. This exogamy is common among non-human primates and other animals, and results in ‘inbreeding avoidance’. However, we’ve learned from genetic studies that the risks from inbreeding have been overblown; it doesn’t lead to harmful traits nearly as often as this view has led us to believe. Other species have mating strategies that don’t avoid incest: banded mongoose females, for example, breed readily with their brothers and fathers.

It’s possible that human incest taboos are rooted in some biological factors underpinning aversion and attraction, which seem to urge us to mate outside our familiar immediate family. However, such taboos can be just as robustly interpreted as a product of our reproductive consciousness, which in turn has influenced cultural norms. Reproductive consciousness enables a calculation of relationships and reciprocity that is impossible for other animals. Socially enforced incest taboos keep fathers from monopolising daughters who would do better, for the family, if they married and reproduced with someone outside the family instead. Incest taboos force individuals to marry into someone else’s family, which results in benefits to their parents and siblings through alliance-building. And such social norms themselves have a profound effect on reproduction, not just of individuals but also of culture. Rather than simply mapping animal models onto human reproduction, when we take into account the distinctive human achievement of reproductive consciousness, we can interpret incest taboos as operating to control sexuality in order to optimise social, political and economic outcomes, even at the expense of optimising genetic ones.

This is not to say that incest taboos are entirely cultural phenomena. Despite not being coded in DNA, a socially mandated incest taboo can lead to differential reproductive success for individuals who practise it, thus becoming an influence on human biology, not just a product of it. Incest taboos discourage instant gratification to favour the long-view and all its spoils. And so culture in turn is able to affect biological evolution.

Reproductive consciousness is just one element in the invention of human culture – a whole cluster of behaviours, knowledge, values and beliefs that unhooks human destiny from the standard evolutionary model of other species. We humans do many things that undermine our evolutionary interests. We practise religious celibacy, contraception, abortion, suicide bombing. We adopt infants who aren’t our kin, we go to war, we kill our siblings. A lot of this we do voluntarily, and none of it perpetuates our own genes. In fact, it actively does not.

Reproductive consciousness transforms human beings and families into human lineages, inextricably intertwined with other bloodlines, passing along and exchanging genes just like any other animal, but also transmitting immaterial and material culture across and down through the generations, which in turn affects the fates of the genes going forward as well. Understanding that sex makes babies has allowed us to create social norms and expectations about when and with whom we reproduce that biologically driven behaviour alone could never have done. But our understanding that sex makes babies has done much more than shape human destiny. It has changed the entire planet through our manipulation of other species to our own ends.

At the end of the last Ice Age, some 10,000-12,000 years ago, people began the transition from foraging to farming. This shift defines the beginning of the Neolithic and forms the foundations for the Anthropocene, the present age of visible human impact on the Earth’s ecosystems that might soon become an official geologic epoch. Archaeologists still argue about why this change happened where and when it did. Either farming offered advantages that foraging did not, or foraging became unsustainable, and people had no choice but to adopt a new way of life. Perhaps it was some of both. Whatever the cause, the transition was slow and patchy, taking hundreds or even thousands of years.

Domestication is essentially predation with forethought and evolutionary intent. Early farmers were the first to replace natural selection with artificial selection. They had to decide which seeds to plant (if in fact to plant them at all), which animals to cull from wild herds that lived nearby, to corral animals for meat, wool, hides, milk, blood and so on. By all professional accounts, cultivation – that is tending animals and plants without choosing to breed subsets for specific traits – preceded domestication, but even without practising artificial selection, cultivators, like their hunter-gatherer ancestors, possessed a wealth of knowledge about nature. Farmers must have known that pollination and mating were prerequisites for the next generation of plants and animals, and this must have made them better at gardening and animal husbandry. Palaeolithic peoples had sophisticated symbolic language and extensive knowledge of animal and plant behaviours. This would certainly have included an understanding of sex and reproduction.

Domestication of goats, sheep or cows reflects an understanding of the role of sex in producing animals with desired characteristics, such as docility, herd behaviour and fatty milk. Farmers realised that parent animals with these traits would be likely to produce offspring with the same traits, and this would have informed their decisions about which animals they would allow to reproduce. At some point people also understood that selective pollination could drive a food crop in a particular direction as well – seeds that all ripen at the same time, seeds that stay on the plant until the farmer wants to harvest them, seeds that taste better, and so forth. Whether early farmers understood that pollination was essentially plant sex is unknown, but domestication – purposeful, human-driven artificial selection rather than simply taking advantage of what natural selection or chance delivered – was a fundamental part of the transition to agriculture, and that couldn’t have happened without reproductive consciousness.

The transition to agriculture brought the beginning of settled life, the growth of villages and towns, and eventually cities, with dramatic population growth and ensuing environmental, epidemiological, cultural and political consequences. Without reproductive consciousness, human history would have been utterly different.

Now, it’s true that humans aren’t the only animals that farm. Ants farm aphids, damselfish farm algae, leafcutter ants grow fungi on the leaves they collect. However, as far as we know, they aren’t consciously breeding selected fungi or aphids specifically to engineer the traits of future generations. This isn’t domestication, and it’s not farming as we know it. Domestication requires reproductive consciousness and, without it, the ecological leviathan of agricultural and industrial human society would not have been possible.

Someone left a cuneiform note in the Sumerian city of Eridu in Iraq, describing how the penis, specifically the god Enki’s, is a creative force

Perhaps it’s not surprising, then, that early agricultural civilisations have a wealth of stories and myths about fertility. More than 2,000 years ago, inscriptions on an Egyptian wall reveal the life-creating abilities of the penis. Hieroglyphs tell of how the male god Atum masturbated the universe into existence. A thousand years before that, someone left a cuneiform note in the Sumerian city of Eridu in Iraq, describing how the penis, specifically the god Enki’s, is a creative force, with Enki’s bearing the rivers Tigris and Euphrates. The Code of Hammurabi, as translated into English, suggests that Babylonian men were very aware that they help to create offspring via sexual intercourse.

In the Bible, when God told Adam and Eve to ‘be fruitful and multiply’, he was transferring creative power to their capable loins. Later in the script, the Ten Commandments demand that we honour our father and mother, that we not commit adultery, and that we do not covet our neighbour’s wife. From a similar time, in The Twelve Tables (450 BCE), the earliest coded Roman law that has survived recorded history, appears to demonstrate a fairly sound empirical understanding of reproduction. For example, in Table IV.5 it says: ‘A child born after 10 months since the father’s death will not be admitted into a legal inheritance.’ Tracking gestation length to legitimise human children was part of the law by then, despite it predating any scientific understanding of reproduction by 2,000 years.

The fundamental arithmetic of human reproduction might be ancient knowledge, but an accurate understanding of the detailed logistics of conception and embryonic development were more elusive. We have a great deal of evidence from ethnographic sources that, while the male role in reproduction seems to be universally acknowledged, the precise details of this vary widely from culture to culture. Among lowlands peoples in the Amazon Basin, there has been a widespread belief that multiple men can contribute to producing a single child, their semen building up or anointing the foetus during pregnancy. At the turn of the 20th century, anthropologists such as Bronislaw Malinowski in The Father in Primitive Psychology (1927), and Douglas Lockwood in I, the Aboriginal (1962), recorded the stories of peoples of the South Pacific and Australia who believed that penetrating intercourse opens up a woman or makes ‘the road’ for conception.

However, Malinowski had a narrow view of these belief systems. He was set on explaining the origins of puzzling societies where there was a matrilineal pattern of ancestry within a patriarchal power structure. This, he argued, was because these cultures lacked an understanding of paternity, of how babies are made. Of the peoples of the Trobriand Islands, part of Papua New Guinea, he wrote that they ‘have a well-established institution of marriage, but are quite ignorant of the man’s share in the begetting of children, the “father” has for the Trobriander a purely social definition: he is the man married to the mother, who lives in the same house with her and forms part of the household’. To Malinowski, ‘the idea that it is solely and exclusively the mother who builds up the child’s body, while the male does not in any way contribute to its production, is the most important factor of the social organisation of the Trobrianders’. So, in a patriarchy without paternity, if the mother cannot carry the status, her brother does, and these ideas had intellectual impacts far and wide. It is now popularly assumed that there were people – at least as recently as 100 years ago – who didn’t know where babies come from. This is probably incorrect.

Malinowski was fixated on his informants’ ‘absence of any idea of the fertilising value of semen’, using it to generalise that they ignored the male’s role. Yet they had already told him that sex with men is what primes a woman’s fertility. So, regardless of the details, sex and men are still helping women to make baby Trobrianders. He also records a belief that spiritual action is, to quote his informants, ‘the real cause of child-birth’. But this is not so odd, nor incompatible with reproductive consciousness. After all, many Americans, in polite or unfamiliar company or in earshot of children, won’t talk about sex; God makes babies. Perhaps the problem was, in part, Malinowski’s own fixation on semen. He quotes one informant’s apparently incorrect explanation for reproduction to be: ‘The mother feeds the infant in her body. Then, when it comes out, she feeds it with her milk’; and ‘The mother makes the child out of her flesh.’ Yet if we wanted to be purely materialistic, this could be considered more correct than the genome-centric view of babymaking. Egg and sperm and the genomes they carry are next to nothing compared with the food, flesh and milk that build a baby. Babies are certainly more mother’s blood and milk than they are egg or sperm, but a person’s intellect might be questioned if they said so today.

Until we can make synthetic babies from scratch in vats, we are stuck with much of the old apparatus of parenting

In the Western world, theories about the relative contributions of man and woman to the making of a baby have also been the subject of much speculation and debate. ‘Preformation’, the pre-Enlightenment idea that we were formed as we look now from the start and merely grow up, imagined something like every woman containing an infinitely stacked Russian doll of her future progeny. Once sperm were first seen by Anton van Leeuwenhoek under his microscope in 1677, a similar concept was transferred to males, reinforcing the image of the womb as a vessel that welcomes impregnation from a man. Around 1870, explanations of conception drew much closer to what we have today. What the anthropologists Steven Beckerman and Paul Valentine call the ‘One Sperm, One Fertilisation Doctrine’ had its origins in 19th-century Austria when Gregor Mendel obtained experimental evidence that a single pollen grain introduced into an ovule produced a well-developed seed.

Recently, The New York Times reported that scientists were on the verge of making ‘people without biological parents’. This describes hypothetical, synthetic human genomes grown inside embryos in vitro. But unless scientists are also synthesising eggs and sperm (both of which contribute more than DNA to offspring), and wombs, and women too, then they haven’t removed parents from their required role in reproduction. The same is true of cloning living or recently extinct species. Any synthetic human genome will be constrained by what works for Homo sapiens and what has worked for more than 3.7 billion years. A synthetic genome is part of a larger process that is influenced by the humans who built it and the agents they used in the laboratory, as well as the woman who carries the pregnancy to term, and all who influence her biology while she is pregnant, and while her own mother was pregnant with her, and all the biological influences from the child’s caregivers. Until we can make synthetic babies from scratch in vats, we are stuck with much of the old apparatus of parenting.

By contrast, scientists have successfully removed some other old-timey necessities from human reproduction. In-vitro fertilisation removes sexual intercourse from the equation, and it also removes seminal fluid because sperm is plucked and washed prior to meeting an egg. But even now, our beliefs are evolving about the necessity of seminal fluid since it’s been recently discovered that its absence likely influences the biology of the placenta and the progeny, particularly the metabolic traits of the offspring.

As ever, our cultural and scientific beliefs are evolving about the necessity of intercourse for making babies. The ‘seminal-priming’ hypothesis suggests that exposure to semen improves fertility for women and couples who, for example, are at risk of developing a pre-eclamptic pregnancy which risks foetal and maternal survival. So although some reproduction is now feasible without sex or semen, it seems not all of it is. And, while all our new-style means for babymaking can affect culturally prescribed kinship, the relationships that arise with a new baby are generally based on knowledge of the provenance of the egg and the sperm, which boil it all back down to that familiar fundamental equation of man + woman + wild imagination = baby. No matter how much we tinker with the specifics, reproductive consciousness remains a constant of human identity, from the Trobriand Islands and ancient Egypt to modern-day New York.

Just as we don’t know when our ancestors acquired language, we don’t know when reproductive consciousness arose. But we do know that each is ancient, and was present worldwide when European explorers first encountered and reported on isolated indigenous peoples. Because we are human, our ability to explain the unobservable, to understand that men help to make babies and that we are related to one another, has profoundly affected the social structures we’ve devised, the rules about who can have sex with whom, and formed a basis for wide-ranging cooperation on large-scale projects. It has driven our relationship with the plants and animals we’ve domesticated for food and labour, and this in turn has altered the land on which we live. If it didn’t spark the Anthropocene, reproductive consciousness has certainly deeply affected its trajectory. Knowledge is evolutionary power.

Acknowledging the centrality of reproductive consciousness to so much of our past as well as our present gives us a novel way to reframe how we explain much of human behaviour. Culture is something truly extraordinary about our species, and it is intellectually presumptuous to dismiss its role in human evolution. Evolutionary theory doesn’t explain virginity, the use of contraception, or ‘until death us do part’ as convincingly as the simple fact that culture is powerful, and culture is informed by reproductive consciousness, and a whole suite of institutions and conventions built out of it.

There are scientists hard at work on fitting culture into the framework of human evolution. In Not By Genes Alone: How Culture Transformed Human Evolution (2005), the anthropologists Peter Richerson and Robert Boyd suggest that we have evolved unique tribal social instincts, on top of our ancient primate social instincts ‘that allow us to interact cooperatively with a larger, symbolically marked set of people, or tribe. The tribal social instincts result from the gene-culture co-evolution of tribal-scale societies.’ And they suggest that this is why ‘humans are able to make common cause with a sizeable, culturally defined set of distantly related individuals, a form of social organisation that is absent in other primates’. But one aspect of human sociality is desperately missing from these sorts of conversations – reproductive consciousness.

When the creative power of our own bodies dawned on our ancestors, we seized a powerful role in our own behaviour

Our argument is not that culture is more powerful than biology, or vice versa. We simply wish to acknowledge that it belongs alongside biology in our evolutionary reconstruction of sex, sexuality, reproduction and kinship. If culture has been a factor in hundreds of thousands, if not millions, of years of hominin tool-making, then it has most likely also been there for the babymaking. Perhaps it’s just been easier to elevate and to contrast our material culture against all other species’ than it has to incorporate the importance of our immaterial culture. But what’s more material than the manufacture of flesh and blood offspring?

Early on in anthropology, studies of procreative beliefs and kinship such as Malinowski’s were explicitly concerned with understanding human evolution and imagining the lives of our ancestors. However, this style of research fell far from fashion as cultural anthropologists sought to distance themselves from research on human evolution. And rightly so: living peoples are not relics of the Stone Age and are not identical with our extinct ancestors.

Now that the dust has settled somewhat, however, scholars are reviving the connection between uniquely human kinship and its importance for the understanding of our ancient past. Putting it simply, humans have families in ways that no other animals do. In Early Human Kinship (2011), the anthropologist Wendy James encourages thinkers from science and the humanities to come together ‘on the very important question of how evolutionary theory could or should take account of the ordered character of human organisation, specifically … how we try to manage patterns of male-female and parent-child relations, and thus the purposeful outcomes of our own reproduction’.

Just how far back to push that purposeful impact we make on ourselves might never be known. But when the creative power of our own bodies dawned on our ancestors, we seized a powerful role in our own behaviour, and sometimes a powerful role over our descendants, future tribe members, fellow humans, and kindreds of all kinds on Mother Earth. Or putting it in truly human terms: kindred of all kinds on Mother and Father Earth.

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