5

SURVIVING IN HARSH CONDITIONS

Species arise and disappear in a continuous process without a preconceived plan. Survival must have been difficult for the first humans living in harsh conditions. We can study the processes of natural selection and ageing in their original form in Ghana, where people still live under the same severe conditions as our ancestors. Examining those processes provides us with important insights into resistance to infectious disease, the link between fertility and the immune system, the survival of women beyond menopause, and old age in men.

Several years ago, I was invited to give a talk in New Zealand on evolution and ageing. After my presentation was over, I was walking on the beach when I stumbled upon a monument marking the spot where humans first set foot on New Zealand soil. Historians believe they have found evidence that those humans made the crossing to New Zealand from the islands of Polynesia in canoes. Those adventurers must have been the first to survive the long journey of thousands of kilometres. They were called Maori, and are considered the original inhabitants of New Zealand. The inscription on the plaque has stuck in my mind: ‘There are no signs of human habitation in New Zealand prior to the year AD 900.’ That makes New Zealand the last landmass of any size to have been populated by humans. It is a far-flung corner of the world; it had taken me 24 hours to fly there from Amsterdam. But a single day is nothing compared to the millions of years it took us as a species to complete the gruelling journey from the middle of Africa, via the Polynesian islands, to New Zealand.

The further you look back into your family, the bigger it gets. This means that two (randomly chosen) people must always have some ancestors in common. One evening, I received a phone call from a Westendorp whom I did not know. He knew that I was married, and he was able to name my two daughters. The caller turned out to have carried out extensive genealogical research on our family history. He was hoping I could provide some information he needed to fill a gap in our family tree. Unfortunately, I was of no use to him; I simply did not know enough. It is not unusual to have an inkling about the identity of your grandparents’ parents, but what about their brothers and sisters? I daresay most people know nothing about their family four or more generations back. The purpose of a family tree is to fill that information gap — to trace your roots and your relations, sometimes for several generations, sometimes much further into the past.

Somewhere along the line, I must be related to the British nobility, and even to the Maori of New Zealand. Finding that relationship may, in the most extreme case, mean going back 250,000 generations. But there is no doubt that I can trace a single unbroken line of copies of my DNA all the way back to the middle of Africa.

AN EXTRAORDINARY FIND IN CHAD

Scientists who study the family tree of the human species are called ‘paleoanthropologists’. In order to reconstruct a family tree going back millions of years, they have to look at fragments of bone, teeth, and skulls, which can often be difficult to interpret. This means there are many open questions in this science: which species lived alongside each other; where and when did one species turn into another? It is no surprise, then, that a new find can turn ideas about lineages and relationships on their head. In 2002, a skull was discovered in northern Chad, which many believe belonged to the oldest human ancestor found so far. Proving a claim like that is difficult; after all, what constitutes ‘human’? If you were to meet this oldest of human ancestors walking down the street, you would probably be inclined to think it had escaped from the zoo. This new species was given the name Sahelanthropus tchadensis, meaning ‘Sahel man from Chad’. It is purported to be the ancestor of various later ‘hominid’, or human-like, species, of which the human species is the only survivor. S. tchadensis is thought to have been a tree-dweller that was common 6 to 7 million years ago in West Africa, and lived alongside the ancestors of modern apes. Some paleoanthropologists point out that the geographical location of this find is very far from the areas where most other human fossils are uncovered, in eastern and southern Africa. For this reason, they call the mainstream interpretation of this new find into question.

The systematic classification of plants and animals as developed by biologists places the great apes — which are large and tailless — as our closest relatives. The foundations of this classification system were laid down by the Swedish physician and botanist Carl Linnaeus, who gained his doctoral title from Hardewijk University in the Netherlands, and lived and worked in Leiden from 1735 to 1738. Linnaeus, who still believed in the divine creation of natural species, grouped plants and animals according to their outward characteristics. His method was very similar to that of modern-day paleoanthropologists, who have to make conclusions on the basis of bones, teeth, and skulls. Now, most scientists believe species are the result of evolution, and prefer to order them according to their descent — that is, their genetic relatedness. The classic Linnaean system often agrees with a classification based on genetic relationships, but not always, by any means.

Not everything that looks the same has the same origin. For example, we now know that, despite their remarkable similarity, swifts and swallows are not closely related to each other. Swifts are related to hummingbirds, and are not close to the passerines — or songbirds — which is the group that contains swallows. Another example is that of the auks in the northern hemisphere, which are as alike as two peas in a pod to the penguins of the southern hemisphere. This similarity should come as no surprise, since the physical circumstances are comparable in both polar regions, and necessitate comparable evolutionary adaptations for survival. This phenomenon is known as ‘convergent evolution’. But auks are not related in the slightest to penguins, and have more in common with gulls, terns, and skuas.

Our rapidly increasing ability to determine variations in DNA at the tiniest level, and the ever-increasing capacity of our computers to carry out complex analyses, are a great help in understanding patterns of descent. Thus, we now know that humans and chimpanzees share 98 per cent of their DNA. And we know for certain that humans are not descended from chimpanzees. Genetic similarities and differences allow us to conclude that humans and chimpanzees share a common ancestor. It must be the case that the earliest human-like creatures and the earliest ape-like creatures differed only very slightly. However, those differences increased with time. In the course of millions of years of evolution, we humans have learned to think conceptually, to talk, and to build machines.

Note that we are not the only species to have evolved in that time from those original tree-dwellers. Bones and skulls show that there have been more than twenty different hominid species in the course of human evolution, including such well-known characters as Java Man and the Neanderthals. However, all those other species have died out. Apparently, the others were not able to overcome the environmental challenges they faced. Homo sapiens, the species we belong to, is — as yet — the only exception. The appearance and disappearance of species is a constant and unstoppable process, and it is most probable that our own species will one day become extinct.

Surviving in the adverse conditions in Africa must have been a constant ‘struggle for life’ for those early hominids. Just like us, our early ancestors had no idea what the future would bring, or how they should prepare for it. There is no predetermined plan: evolution does not think rationally. Lives simply run their course. Only those newborn babies that manage to survive, reach adulthood, and have children can pass their hereditary characteristics on to the next generation via their genes. Since the chance of survival is generally limited, individuals can never invest enough in their offspring. Even today, death and extinction lie just around the corner — for you, your family, or the entire species.

Because it does not follow a predetermined plan, the erratic course of human evolution can only be understood in hindsight. Family trees are incomprehensible if you follow them upwards, from root to branch. Over and over, you end up following extinct branches, forcing you to go back and start again. Then, suddenly, the lineage you are following leads all the way through human evolution to the present day. It is as if it were suddenly obvious which route takes you to the centre of the maze, where a prize awaits you. Trying to work out the correct route beforehand is absolutely pointless.

The evolutionary development of the human species is a tangled web. Each generation produced offspring, but only some of those individuals will have descendants alive today. Many efforts led to nothing because the newborns were not well adapted to their environment, happened to die young, or failed to produce offspring of their own. Constantly producing more and more identical offspring will not help.

Sex, however, does help increase the survival chances of a lineage. Sex is a fantastic mechanism for creating new variants of the same species. This occurs in a two-step process. First, father and mother split the genetic material to be passed on — the number of chromosomes — in two, through a process called ‘meiotic cell division’. This occurs in the reproductive cells. The resulting egg or sperm cells each contain half the total number of the required chromosomes. When the sex cells merge during fertilisation, the material is recombined. This results in a reshuffled, complete set of chromosomes, and life can continue to develop.

Sex leads to the creation of countless new variants, and these can eventually evolve into new species. Some offspring that are the result of sexual reproduction are likely to be better equipped to survive in a new environment than those that result from asexual reproduction and are therefore identical to their parents. Looking within our own species, we may think of Java Man and the Neanderthals as examples of such variants; initially, they were successful as species, but they ultimately disappeared nevertheless. Other examples are aristocratic lineages in Britain, to which the current royal family, the House of Windsor, is a lucky exception; the dynasty has successfully managed to survive so far. Darwin was right when he remarked that it is only the survivors who compete in the struggle called ‘fitness’.

However, there are few people whose motivation for having sex is a conscious desire to perpetuate the human species. We have an instinctive urge to have sex, and it is usually done in the pursuit of pleasure. Both the urge and the pleasure involved in satisfying it are genetically determined, fixed in the code of our DNA. Sex is an evolutionary adaptation that has enabled us to survive to this day as a species that reproduces sexually, albeit to the detriment of our own bodies. It is no surprise that the British biologist Richard Dawkins gave his seminal 1976 book the title The Selfish Gene. Our entire genetic programme is geared to favour DNA, not humans themselves. We are nothing but temporary containers for a molecule called life, and the only thing that is thrown away is the packaging — which is us.

The evolutionary rat race — which species will survive and which will die out? — is a fascinating phenomenon, and a good understanding of it is necessary if we are to get to the bottom of the question of why we age. Investments in development, sex, and reproduction have been strictly determined by natural selection, and are encoded in our DNA. As time passes, the costs of that programme accumulate, and that explains why our bodies deteriorate. Within an evolutionary framework, the development of children and the ageing of adults are two sides of the same coin. This is an essential insight for medical professionals.

If you ask a group of aspiring doctors what area of medicine they would like to specialise in, around a quarter of them will say paediatrics. That is a lot, since there are not that many sick children in developed countries these days. This desire to become a paediatrician is an expression of a genetically determined trait that raises the human species’ level of fitness. We are genetically programmed to love children. Only very few student doctors say they want to work with old people after they qualify, although doctors’ waiting rooms are thronged with droves of old patients. There is simply no evolutionary pressure to select for a love of old people.

For someone like me, who is responsible for training doctors in geriatric medicine, such student aspirations can be pretty frustrating, until you realise how our brains are wired. If people are likely to neglect their bodies in old age, as the disposable soma theory predicts, why would student doctors choose a medical career in gerontology? Is it logical to conclude that they would want to work with older people, just because everyone reaches old age these days? Of course not.

Knowing this evolutionary background, it is easier to understand the choices made by today’s medical students, and this has enabled me to develop a more realistic training programme. In 2000, I decided to change tack completely; I began teaching them about sex, reproduction, and evolution. I showed my students that ageing is a logical consequence of all those things. That set the ball rolling. As broad-based professionals, many of my students later forged careers in geriatric medicine.

THE GOLD COAST OF AFRICA

In 2002, a group of researchers from the Department of Gerontology and Geriatrics at Leiden University decided to study the ageing process among people in Ghana. Our motivation was scientific curiosity, but also, and more importantly, our aim was to gain a better understanding as medical doctors of the way our bodies and minds are structured, and how they eventually deteriorate. This meant we had to study the human body and mind in the unforgiving environment they originally evolved in — an environment of excruciating temperatures, food shortages, and infectious diseases — where medical intervention is not the order of the day. Those times are long gone in the West, and so we cannot study natural selection and the ageing process in their original form at home. However, there are still plenty of places in north-eastern Ghana where the conditions still pose a constant threat to the people who live there. So that is where we chose to carry out our ten-year study of young and old people.

The colonisation of Ghana, formerly the Gold Coast, began with the arrival of the Portuguese in the fifteenth century. They were followed by the Dutch in the seventeenth century, who were in turn followed by British overlords. Today, Ghana is a successful west African democracy. However, our research area in the north-east is still very ‘primal’. The area has remained practically untouched to this day, for the simple reason that it has virtually nothing to offer. Food is scarce, and the average income is estimated to be one dollar per person per day. There is no money for artificial fertilisers, and the rugged climate means that crop failures are common. People there work the land with traditional tools such as hoes. Extremely rarely, you might see an ox. Motorised agricultural machinery, such as tractors, does not exist there.

The Bimoba tribe, whose society is patriarchal, have lived there for centuries in a very primal way, close to nature. The head of a household lives with one or more women, including his mother if she is still alive, in a row of thatched huts. The huts are encircled by a wall, forming a compound. The bodies and minds of these people are adapted to this environment, which is characterised by scarcity and danger, and that is precisely why we chose to go there as researchers from the West.

We began by ‘mapping’ the area in a medical-anthropology sense. Who are its inhabitants? How many children do mothers tend to have? How many do fathers have? And how do these people live? The Bimoba were a blank slate as far as the scientific literature was concerned. As expected, the pattern of mortality on the ground was dominated by starvation and infection, with prosperous years alternating with periods of scarcity. Practically everything in the region is reddish-brown and dusty; precipitation and growth appear only during two short rainy seasons per year.

Arriving in the region as a Western researcher, it appeared at first glance that poverty pervaded everything. Later, we began to notice subtle signs of rank and position: a pig perhaps, and the occasional rectangular hut with a corrugated iron roof and an electricity connection. Such attainments indicated higher socio-economic status. And, as they do everywhere and all the time, these differences in wealth translate directly into a higher or lower risk of mortality. Accordingly, we found that the people with the lowest socio-economic status were twice as likely to die at any given time as the highest-status individuals. A higher income offers the possibility of more food, of vaccinating your children, of digging a well for clean drinking water. Money raises individuals’ life expectancy, because wealth offers more possibilities for survival in a hostile environment.

From this it is clear that the economic principle of the distribution of limited resources is fully applicable to this community in north-eastern Ghana — not only in the real economy, but also in human life-trajectories. Here, too, the disposable soma theory stands up: these people must choose between investing in reproduction and investing in maintaining their own bodies. Having children and bringing them up requires great (financial) investment on the part of the parents. But, at the same time, children are an insurance policy for a parent’s old age. However, when conditions are bad, one set of parents in every five in Ghana will lose all their children before they reach adulthood.

This is probably the reason that having a large family is a source of prestige and status. Men aspire to the taking of as many wives as possible. Each wife requires investment in the form of a dowry, which only a few men can afford. Generally, all women have a large number of children, and childlessness is a social taboo. The number of children a man has within the same family can reach thirty or even forty. There are also men in this polygamous society who cannot afford a wife. They are excluded from the evolutionary game; the number of offspring they father, notwithstanding extramarital sex, is zero. And so everyone is part of the subtle game called ‘fitness’, in which human behaviour, property, and reproduction all play a role.

Over many generations, the Bimoba have developed a culture that enables them to survive in this unfavourable environment. Their entire social order is geared towards prioritising having children, and so maximising the tribe’s chances of survival. The limited resources on which the local population depend for a living have left a lasting mark on their social structure and in their genetic material. In this polygamous society, it is the dominant and wealthy compound landlord, the male head of the household, who fathers the most children. Parallels with the alpha males of primate societies are obvious. The dominant and submissive behaviour of apes, the strict hierarchy within the group, grooming behaviour — everything that evolutionary biologists study — has a strongly genetic background. Just like apes, humans are full of emotions that have an evolutionary basis. Our behaviour is coloured by the environment we live in, but the ‘hardware’, the character traits that underlie that behaviour, is also determined to a great extent by specific genetic characteristics. This is the human behaviour that has been selected for in the polygamous society of the Bimoba.

In various places around the world, scientists research the genetic basis for human behaviour. They often focus their studies on the differences between identical and fraternal twins. Identical twins share all their genes, while fraternal twins share half, just like any two siblings. By studying the differences between these two types of twins, it is possible to determine which human traits have a genetic basis and which do not. Dominant behaviour, tenacity, intelligence, but also sociability, affectionateness, and optimism are determined to a great extent by our DNA. All these skills are necessary to gain and maintain a place in society. This does not mean that everyone possesses every trait in the same measure. Although the average height in our society may be 1.8 metres, some people measure 1.6 metres; others; 2 metres. Such variations — whether greater or smaller — are all part of the normal range of variability.

Natural selection has adapted the way we function socially to the environment we live in, and, conversely, we attempt to mould the environment to suit us by means of our behaviour. We are not simply slaves to our genes. People in Africa can choose not to get married, or to remain childless. Such decisions are influenced by their upbringing, by the people they associate with, and by the people they turn to for advice. And, of course, their behaviour also depends on their circumstances and the (in)tangible resources available to them. Does a man have enough money for a dowry, so that he can marry a woman?

The life trajectories of those of us who live in modern developed societies can be explained by our parentage — what genetic basis did I inherit from them? — and by the circumstances we live in. It is nonsense to try to explain the biological events in our lives, such as illnesses, and our behaviour, which is based on our emotions, on the basis of our genetic makeup or our environment alone. Both are important. Even someone who has been an optimist his whole life can be so upset by the death of his beloved wife that he slides into depression. And the opposite is also true. Someone with a rather bristly personality who has never been able to form an attachment to a partner throughout her life can find herself falling head over heels in love, and getting married in old age.

RESISTANCE TO INFECTIOUS DISEASE

The women in our research area in Ghana gave birth on average to between six and seven children. When circumstances are harsh, it is necessary to produce that number of offspring to guarantee a constant population in the long term. If women have even more children, the survival rate of their offspring begins to drop. The first and most obvious explanation for this is that there is less food and fewer (financial) resources available per child. But starvation is not the only life-threatening danger in Ghana. Infections are also an ever-present hazard. If we look back at the evolutionary development of the human species, we see that the risk of contracting an infection increased around ten thousand years ago, when we gave up a hunter-gatherer lifestyle in favour of a settled life in social groups, working the land, and keeping livestock. This change meant we had access to a better and more reliable source of food, but it also brought with it a sharp rise in the risk of infections spreading from animals to humans, or from humans to humans. In response to such infections, humans developed a versatile immune system to keep invaders in check.

Only those offspring with sufficient defences against infection survived the transition from hunting and gathering to agriculture and animal husbandry. The development of agrarian societies progressed in fits and starts. There are countless examples of communities that were wiped out by epidemics. Sometimes, a few individuals managed to survive the catastrophe, and they became the founders of new, thriving communities. This is how modern humans gradually developed their powerful immune defences against infection.

The body’s resistance to infectious diseases takes a fundamentally two-pronged approach. The first line of defence is a lookout function. Its job is to identify pathogens as hostile agents. Pathogens do everything they can to remain unnoticed in the human body. They disguise themselves, making it difficult for them to be to identified as foreign bodies, so that they are not targeted by the immune system. This is how certain tropical worms, once they enter the human body, can remain in the bloodstream for thirty years or more. Other pathogens are able to change their disguise repeatedly, so our immune system is taken by surprise again and again, and we get sick every time we encounter them. The flu virus, for example, is slightly different every season, which explains why it can infect so many people each year.

Neutralising pathogens is the second part of our immune defence system against infection. Once the intruder has been identified as alien and hostile, it must be killed and removed. This is achieved by means of an inflammatory response. In the case of flu, inflammation is associated with the familiar symptoms: aching muscles, fever, fatigue. Another example of inflammation is that familiar red, painful, throbbing finger that only gets better when the festering splinter is removed.

It seems logical that the stronger an individual’s immune system is, the better, since then her chance of survival is the greatest. So it is quite remarkable that human beings are still susceptible to infections, despite so many generations of selection in favour of resistance. We have managed to acquire other biological traits through natural selection in a very short space of time. For example, when humans began herding livestock, milk became part of our staple diet. By means of natural selection we developed the ability within just a few generations to produce the enzyme lactase, which enables us convert lactose (the sugar that is so plentiful in dairy products) into ordinary, more easily digestible sugar.

It is only when we look at human lives as a whole that it becomes clear why further selection for resistance to infection becomes problematic. If recognition mechanisms for pathogens are never allowed to fail, and inflammatory responses can never be too powerful, undesirable side effects can develop, such as the rejection of an embryo from the womb.

An embryo that needs to stay in its mother’s womb long enough to grow into a full-term baby is an equal mixture of its father and its mother. Of course, the mother thinks it is her child, but that is not quite the case. Fifty per cent of its genetic material has come from the father, and those genes have an impact on the child’s development from the outset. A direct consequence of this is that the mother’s immune system has a tendency to identify the embryo in her womb — half mother, half father — as a foreign body, leading it to want to reject the embryo in a ‘spontaneous abortion’. To prevent this from happening, proteins are present at the interface between the placenta and the womb that prevent the immune cells of the mother and the child from attacking each other. Nevertheless, such an immune response does sometimes occur, and the result is a pre-term birth. In such cases, the differences between mother and child are too great, and her ‘tolerance’ too little. The chance of this happening is greater if the mother’s immune defences are ‘stronger’.

In our study area in north-eastern Ghana, we were able to identify the DNA variation that explains hereditary differences in immune defences between different human individuals. Those who carry certain variations are better equipped to fight off infections and survive them; but, at the same time, pregnant female carriers of that variation have an increased risk of rejecting their embryo. This shows why evolution cannot simply go on forever selecting for ever better defences against infectious disease: the resulting reduction in reproductive fitness leads to those evolutionary variations dying out. Conversely, a ‘weak’ immune system cannot be selected for limitlessly, simply because that reduces the risk of pre-term birth. If a woman falls prey to a fatal infection before she can ever become pregnant, there can be no question of fitness at all. This results in a constant wavering between the two extremes. This is what we call ‘balanced selection’.

This mechanism can offer a better explanation of our previous findings concerning infant mortality in the research area. If the children of mothers who have borne many babies die of infections more often than the average, a lack of sufficient food and money is only part of the reason. The fact that they have a more-than-average susceptibility to infectious disease is a sign that their immune system is less effective at fending off pathogens. Their less ‘strong’ immune system is inherited from their parents. It is this very genetic makeup that made it possible for their mother to have so many babies.

Our findings were not totally unexpected. This is an example of what scientists call the quality-quantity trade-off — that is, the fact that members of a species will produce either a few offspring of high quality, or many offspring of lower quality. This phenomenon is found throughout the animal and plant worlds, and is in line with the disposable soma theory. In this explanatory model, fertility — quantity — is exchanged for sustainability — quality — as evolutionary choices are made about which properties should be afforded more investment than others.

This discovery also provides an answer to another question. When Tom Kirkwood and I were studying the British nobility, we were puzzled to find that there appeared to be no correlation between the number of children born to modern-day aristocratic women and their life expectancy. This can be explained in the context of an immunological quality-quantity trade-off. The cost of exchanging fertility for a weak immune system is much lower today. The chance of dying from an infectious disease has been reduced to a minimum. To put it another way, investing in sex and fertility has become less expensive.

THE BENEFIT OF GRANDMOTHERS

Women have a remarkable life trajectory. Girls develop into young women, at which point they become extremely attractive to men. Their fertility reaches its highest when they are in their twenties, then declines rapidly. When they reach the age of 50 or so, their fertility comes to a definitive end when menopause sets in. However, post-menopausal women still have considerable chances of surviving, and many become grandmothers. The remarkable thing about this is that their ovaries are already ‘dead’, while the rest of their bodies and their minds may well still be perfectly fine. From an evolutionary perspective, the fact that women survive far beyond childbearing age appears to defy logic. According to the disposable soma theory, the survival of women after the menopause should be seen as an investment in longevity at the expense of fertility. The mortality rates for women in old age are actually lower than those for (still fertile) men, which accounts for the fact that most women outlive their male partners. That seems crazy, too. After all, there is no evolutionary pressure to select for longevity, is there? So how can we explain this?

Grandmothers often play a distinctive role in young families with growing children. Grandmas look after their children’s children, increasing the (survival) chances of those offspring by doing so. This is the case in modern families, where they provide mainly intangible advantages such as attention and education; but when conditions are severe, the presence or absence of a grandmother may mean the difference between life and death. When a grandmother provides childcare, her daughter is freed up for the production of offspring, and the number of ‘copies’ of grandmother’s genetic material in subsequent generations will increase. This ‘grandmother hypothesis’, as it is called, explains why women enter the menopause when they are about 50 years old. Since they can no longer produce any children of their own, postmenopausal women are free to fulfil the useful role of caregiver for their grandchildren. In evolutionary terms, postmenopausal survival was selected for because it contributes to the fitness of the species.

A clue to the evolutionary importance of grandmothers in contemporary society can be found in the work of the Dutch sociologists Fleur Thomese and Aart Liefbroer. They studied the involvement of grandparents in the care of young children, and the impact this has on subsequent births in families where both spouses are in paid work. They used data from three generations of men and women in contemporary Dutch families. Their findings showed that maternal grandmothers provided childcare more often than paternal grandmothers did, and that grandmas took care of their grandchildren more often than granddads did. Involvement of grandparents in raising their grandchildren increased the likelihood of more children being born in that family. These results led the researchers to conclude that, from an evolutionary standpoint, childcare by grandparents can indeed be seen as a successful reproductive strategy. Families in which grandparents play a part in the children’s upbringing have a greater degree of fitness in our modern age. The outcome of this study is an excellent example of how the processes of natural selection and evolution are not confined to the distant past or faraway Africa, but also apply to us, in the here and now.

The influence of grandmothers on the survival of their grandchildren can no longer be examined in contemporary Dutch society. The favourable conditions we now live in mean that mortality rates among small children are practically zero. This, in turn, means that death in early childhood is no longer a factor in natural selection. So in order to study the impact of grandmothers on infant mortality, researchers have to turn to historical church records. These registers of births, baptisms, marriages, and deaths have been a useful tool for reconstructing the histories of families all over the world. It is a hellish job, due to the major problem posed by the fact that individual people may be recorded in different registers, and those records are not cross-referenced. Some people lived their entire lives in one parish, where all their rites of passage took place. But others moved around during their lives. What about their histories? This type of research requires a long period of data collection and collation in advance, with genealogists and archivists playing a crucial part.

Much of the historical data used in such ‘bio-demographic’ research comes from Scandinavia. In the eighteenth and nineteenth centuries, stable communities arose there, which were connected by their shared religion. It is not difficult to imagine how these communities were structured: small groups of farmsteads clustered around a church, but otherwise cut off from the outside world by topographical features such as rivers, mountain ranges, or ravines. Life must have been hard, with long winters, failed harvests, and grinding poverty. Child mortality was high, and many women died in childbirth, as a late but nonetheless ‘direct’ result of having sex.

On the basis of the life histories of these people, researchers have been able to show that the presence of a grandmother did play an important role in raising the chances of their grandchildren’s survival. It is also not difficult to imagine that, for families living in such conditions, the availability of help when a mother dies in childbirth can be a key factor. Under these circumstances, the grandmother hypothesis is confirmed.

On an evolutionary scale, however, those Scandinavian church communities are a mere drop in the ocean. It is unlikely that a few centuries of natural selection in the developed world can entirely explain the genetic background to the remarkable development in women’s lives that is menopause. This female peculiarity must have its origins in a much more distant past, when families were structured very differently. Anthropological descriptions of past societies show that the structure of the overwhelming majority of them was polygamous. New techniques of DNA analysis confirm that, in the past, most men had more than one sexual partner. The position of (grand)mothers in a polygamous relationship is very different from that in a monogamous marriage, and it can have a powerful impact on the results of natural selection.

To really understand women’s lives, to get to the bottom of the function of the menopause and to shine a light on the benefit of grandmothers, we would need to carry out a statistical analysis of the birth rates and death rates of the members of polygamous families. However, the historical records necessary for undertaking such work do not exist. Instead, my research team studied the present-day life histories of the Bimoba tribe. Its polygamous structure, high mortality rates, and large number of offspring appear to reflect quite accurately the circumstances under which humans evolved. We reconstructed countless family trees, and determined women’s fertility rates and the survival chances of newborn babies.

For mothers in north-eastern Ghana, there is a considerable risk of dying in childbirth. It appeared that the presence or absence of the landlord’s mother, the grandmother of his children, had no impact either on the children’s survival chances or the number of children that were born. In a polygamous family there is a considerable age difference between wives — a man usually marries several young women over an extended period of time — and when one of them dies, any of the other wives can step in to provide childcare. The wives’ mothers played no part in the family structure; they often lived far away, in the women’s native villages. The conclusion from our study was undeniable: the presence of a grandmother had no significance for the number of children born in a family, or for the survival rate of those children.

The conundrum was how to reconcile these findings about the life histories of the Bimoba tribe with those based on the Scandinavian church records. The circumstances in which the two sets of grandmothers live(d) can be considered more or less the same, but their social positions were not comparable. Scandinavian grandmothers played a completely different part in the affairs of the family than the mothers of Ghanaian landlords. The role of a grandmother is much greater when a woman in a monogamous marriage dies.

What is beneficial in one environment is not necessarily beneficial in another. Our life trajectories in particular are the result of natural selection within polygamous societies that had to survive in a world of scarcity and ever-present physical dangers and infectious hazards. And that is precisely when the grandmother hypothesis turns out to be an inadequate explanation of the strange postmenopausal lifespan of women.

An alternative explanation has been proposed recently — taking into account the fact that men also exist. Unlike women, with their more or less abrupt menopause, male fertility declines much more gradually. Men often father children well into old age, and that is certainly true of a wealthy Bimoba landlord with the money to take several wives. We calculated that around 20 per cent of Bimoba children were fathered by men over 50. Other researchers have identified comparable percentages in similar circumstances elsewhere. Women cannot become pregnant spontaneously at the age of 50 and above. By contrast, men who father children in old age are still very much competitors in the evolutionary rat race. Since these men have more children on average, they leave an extra-large number of copies of their genetic material behind — they have an above-average number of offspring, so their fitness is greater. But in order to do this, they must reach a great age in the first place. These old fathers pass on the attribute of longevity to their sons, but also to their daughters. This is an alternative, plausible explanation of the fact that women continue to live for a long time, even when they have stopped investing in being fertile, and have entered menopause.