People living in hunter-gatherer societies routinely reach middle age
AS A 42-year-old man born in the UK in the late 1960s, I can expect to live for about another 38 years. In other words, I can no longer claim to be young. I am, without doubt, middle-aged.
To some people that is a depressing realisation. We are used to dismissing our fifth and sixth decades as a negative chapter in our lives – perhaps even a cause for crisis. But it needn’t be like that. It is time to change how we think about middle age.
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This rethink has come about thanks to recent scientific findings that have shown just how important middle age is for every one of us, and how crucial it has been to the success of our species. Middle age is not just about wrinkles and worry. It is not about getting old. It is an ancient, pivotal episode in the human lifespan, pre-programmed into us by natural selection – an exceptional characteristic of an exceptional species.
Read more in our gallery: “Middle-aged prodigies: Seven poster children over 40”
Compared with other animals, humans have a very unusual pattern to our lives. We take a very long time to grow up, we are long-lived, and most of us stop reproducing halfway through our lifespan. A few other species have some elements of this life-plan, but only humans have distorted the course of their lives in such a dramatic way. Most of that distortion is caused by the evolution of middle age, which adds two healthy decades after the babies stop appearing – two decades which most other animals simply do not get.
An important clue that middle age isn’t just the start of a downward spiral is that it does not bear the hallmarks of general, passive decline. Most body systems deteriorate very little during this stage of life. Those that do, deteriorate in ways which are very distinctive, are rarely seen in other species, and are often abrupt.
For example, our ability to focus on nearby objects declines in a predictable way – long-sightedness is rare at 35 but universal at 50. Skin elasticity also decreases reliably and often surprisingly abruptly in early middle age. Patterns of fat deposition change in predictable, stereotyped ways. Other systems – notably cognition – barely change.
Each of these changes can be explained in evolutionary terms. In general, it only makes sense to invest in the repair and maintenance of body systems that deliver an immediate fitness benefit – that is, help to propagate your genes. As people get older they no longer need spectacular visual acuity or mate-attracting unblemished skin. Yet they did need their brains, and that is why we still invest heavily in them during middle age.
As for fat – that wonderfully efficient energy store which saved the lives of many of our hard-pressed ancestors – its role changes when we are no longer gearing up to produce offspring, especially in women. As the years pass, less fat is stored in depots ready to meet the demands of reproduction – the breasts, hips and thighs – or under the skin where it gives a smooth, youthful appearance. Once our baby-making days are over fat is stored in larger quantities and also stored more centrally, where it is easiest to carry about. That way, if times get tough we can use it for our own survival, thus freeing up food for our younger relatives.
These changes strongly suggest that middle age is a controlled and pre-programmed process – a process not of decline but of development.
When we think of human development, we usually think of the growth of a fetus or the maturation of a child into an adult. Yet development – and the genetic processes which direct it – does not stop when we reach late teens or early twenties. It continues well into adulthood. The tightly choreographed transition into middle age is a later, but equally important, stage of human development when we are each recast into yet another novel form.
“The tightly choreographed transition into middle age is an important stage of human development when we are each recast into yet another novel form”
That form is one of the most remarkable of all. It is an evolutionary novelty unique to humans – a resilient, healthy, energy-efficient and productive phase of life which has laid the foundations for our species’ success. Indeed, the multiple roles of middle-aged people in human societies are so complex and intertwined, it could be argued that they are the most impressive living things yet produced by natural selection.
The claim that middle age evolved faces one obvious objection. For any trait to evolve, natural selection has to act on it generation after generation. Yet we often think of prehistoric life as nasty, brutish and short. Surely too few of our ancestors lived beyond 40 to allow features of modern-day middle age, such as the deposition of a spare tyre around the middle, to have been selected for?
This is a misconception. Although average life expectancy may sometimes have been very low, this does not mean that Homo sapiens rarely reached the age of 40 during the past 100,000 years. Average life expectancy at birth can be a misleading measure; if infant mortality is high then the average is skewed dramatically downwards, even if people who survive to adulthood subsequently have a good chance of living a long, healthy life.
That doesn’t mean life wasn’t nasty, brutish and short at times – especially after humans underwent the transition to agriculture between 12,000 and 8000 years ago. But apart from this time, in which the longevity of adults may actually have decreased for a while, the evidence from skeletal remains suggests that our ancestors frequently lived well into middle age and beyond. Certainly many modern hunter-gatherers live well beyond 40.
The probable existence of lots of prehistoric middle-aged people means that natural selection had plenty to work on. Those with beneficial traits would have been more successful at nurturing their children to reproductive age and helping provide for their grandchildren, and hence would have passed on those traits to their descendants. As a result, modern middle age is the result of millennia of natural selection.
But why did it evolve as it did? The answer is inextricably bound up with the exceptional nature of humans. In prehistory, and still today, human survival is entirely dependent on skilled gathering of rare, valuable resources. Humans cooperate, plan and innovate so they can extract what they need from their environment – be that roots to eat, hides to wear or rare metals to coat smartphone touchscreens. We lead an energy-intensive, communication-driven, information-rich way of life, and it was the evolution of middle age which supported this.
For example, hunter-gatherer societies often have complex and difficult techniques for finding and processing food that take a long time to learn. There is evidence that many hunter-gatherers take decades to learn their craft and resource-acquiring abilities may not peak until they are over 40.
Gathering sufficient calories is crucial for the success of a human community, especially since young humans take so long to grow up. Indeed, for the early years of life they devour calories without contributing many to the group themselves. Research suggests that a human child requires resources to be provided by multiple adults – almost certainly more than two young parents. For example, a recent study of two groups of South American hunter-gatherers suggested that each couple requires the help of an additional 1.3 non-reproducing adults to provide for their children (). Thus, middle-aged people may be seen as an essential human innovation, an elite caste of skilled, experienced “super-providers” on which the rest of us depend.
Culture conveyers
The other key role of middle age is the propagation of information. All animals inherit a great deal of information in their genes; some also learn more as they grow up. Humans have taken this second form of information transfer to a new level. We are born knowing and being able to do almost nothing. Each of us depends on a continuous infusion of skills, knowledge and customs – collectively known as culture – if we are to survive. And the main route by which culture is transferred is by middle-aged people showing and telling their children what to do, as well as the young adults with whom they hunt and gather.
“Each of us depends on culture to survive, and the main route by which culture is transmitted is by middle-aged people telling children and young adults what to do”
These two roles of middle-aged humans – as super-providers and master-culture-conveyers – continue today. In offices, on construction sites and on sports pitches around the world, we see middle-aged people advising and guiding younger adults and sometimes even ordering them about. Middle-aged people can do more, earn more and, in short, they run the world.
This has left its mark on the human brain. As might be expected of people propagating complex skills, middle-aged people exhibit no dramatic cognitive deterioration. Changes do occur in our thinking abilities, but they are subtle. For example, response speeds slow down over the course of adulthood. However, speed isn’t everything, and it is still debated whether other abilities deteriorate at all.
One recent study of more than 7000 civil servants in London suggested that there is a small but measurable cognitive diminution during middle age (). However, cognitive tests gauge relatively simple, low-level elements of thought, and it is possible that middle-aged people no longer depend on those elements much. Anecdotally, middle-aged people tend to be better at developing long-term plans, selecting relevant material from a mass of information, planning their time and coordinating the efforts of others – a constellation of skills that we might call wisdom.
To carry out their roles in society, middle-aged people need not necessarily think better or worse than younger adults, but they may have to think differently. Indeed, functional brain imaging studies suggest that they sometimes use different brain regions than young people when performing the same tasks, raising the possibility that the nature of thought itself changes as we get older.
A central and related feature of middle age is the many healthy years we enjoy after we have stopped reproducing. Female humans are especially unusual animals because they become infertile halfway through their lives, but male humans often also effectively “self-sterilise” by remaining with their post-menopausal partners. Almost no other species does this.
The possible benefits of the menopause are not immediately obvious – after all, natural selection favours individuals who rear the most offspring. Yet there are other, rare examples of reproductive cessation in the animal kingdom which may provide some clues. Orcas also undergo menopause, and it is striking how much their lives mirror ours (). They are long-lived, slowly developing, intelligent and vocally communicative. They invent and apply a complex array of techniques for communal food acquisition and they are extremely widespread.
Thus, humans can be seen as members of an elite club of species in which adulthood has become so long and complicated that it can no longer all be given over to breeding. Just like long-sightedness and inelastic skin, the menopause now appears to be a coordinated, controlled process. Recent research suggests that it is not a meandering, stumbling deterioration but a neatly executed event which is a key part of the developmental programme of middle age. It liberates women and their partners from the unremitting demands of producing children, and gives them time to do what middle-aged people do best – live long and pamper.
Middle age is fascinating because it links our species’ history to individual people’s experiences. Each person is destined to pass through a phase of life for which they frequently do not feel prepared. The sudden end of fertility challenges their self-image, their appearance alters before their eyes, and even the ways their brain works change. The midlife crisis, middle-aged parenthood, the empty nest syndrome and new, unexpected urges all beckon, but at last science has started to tackle these once inexplicable forces.
Few people look forward to middle age. Some fear it, some joke about it. Yet recent advances in palaeoanthropology, neuroscience and reproductive biology are revealing the truth about this long-neglected phase of human life. Without the evolution of middle age, human life as we know it could never have existed.
Crisis, what crisis?
One of the most enduring concepts related to middle age is the midlife crisis. It is usually associated with men in their early to mid-forties, and is often said to include a psychological crisis related to self-worth, a tendency to seek the romantic attentions of inappropriately young women, and a reversion to childish interests and pastimes such as sports cars. However, it may not be a real phenomenon at all.
None of the three elements stands up to scrutiny. There is no evidence that men are more likely to experience emotional flux in their forties, their preference for younger women does not undergo a step-change in middle age, and there is no evidence that their desire to behave childishly increases. In fact, men in their forties are no more likely to claim they are experiencing a crisis than men or women in their thirties or fifties, and many researchers now believe that the concept of the midlife crisis should be consigned to the psychological dustbin.
- This article is based on David Bainbridge’s new book (Portobello)
