While I think the idea of adult Paleolithic hunter-gatherers regularly dying at age 30 can be laid to rest (sadly, I reckon that particular misconception has an impressive life expectancy), last week’s post on the Gurven-Kaplan paper brings up another question: if the human potential lifespan of 68-78 years, or roughly seven decades, is an evolved, inherent, even genetic trait, what is the evolutionary justification for its selection? Where is the advantage?
The classic Darwinian view is that selection of traits revolves almost entirely around fertility. Once an animal can no longer produce offspring, it has no “reason” to go on living. Since a genes’ survivability ultimately comes down to reproduction, whether an individual can have kids is the primary determinant of viability.
This makes sense when you look at the life cycles of various organisms. Lesser animals reproduce en masse, like spiders with hundreds of eggs in a single go or salmon with 2,500 eggs per kilo of bodyweight – quantity over quality. There’s very little, if any, mothering occurring with these types of animals. It’s just winning through numbers. They don’t have the capacity for socializing or organized communal child rearing; they just pump out as many kids/eggs as they can and hope for the best. Spiders typically die after spawning, and for Pacific salmon, spawning is usually the last thing they ever do. Their purpose in life – as laid out in their DNA – has been fulfilled. Their genes has been propagated, and there is, in the grand scheme of things, no “reason” for the reproducer to continue living. Further up the spectrum, reproduction becomes more of an ordeal and a sacrifice. It’s a general rule that mammalian pregnancy takes a lot out of the mothers; they must bear the fetus(es) for weeks or months, take in extra calories to sustain its growth and development, and eventually have their sexual anatomy ripped asunder by the act of giving birth. Larger or more complex mammals require more resources, more sacrifice, and more time committed. Canine pregnancies run about two months, chimpanzee pregnancies last around 220/230 days, and elephant pregnancies last about 22 months. Puppies nurse for ten weeks or so, while human infants (pre-formula) nurse for two or three years.
Humans represent the pinnacle of mammalian complexity. If they are regularly living past reproductive age and consuming resources that could otherwise go to younger, fitter, more virile members of the group, there must be a reason for it. The original assertion – that an organism’s fitness depends on its contribution to its genes’ survival – holds true, but the complexity of the human animal requires an expansion of the original idea. These old folks must be contributing to the success of the gene through non-reproductive means.
The obvious answer is the “grandmother hypothesis” – the idea that post-menopausal women can provide their children and grandchildren with a better chance to breed, and thus further contributing to the survivability of their own genes. Unable to reproduce themselves grandmothers care for little ones while the immediate parents are out gathering or hunting (or working a job in modern terms). Once that last batch of kids has reached young adulthood, at around age 15 when they can begin fending for themselves, the grandparents are no longer “needed.” The young adults are ready to start producing for the community and having their own kids, thus ushering in the new wave of grandparents (the young adults’ parents). The first set of grandparents, now beginning to hit age 70, can pass away without negatively affecting the genes’ survival. They’ve done their part and contributed to the survivability. There’s no longer a need for even more longevity. It all sounds pretty morbid, but that’s how this stuff is hypothesized to work. There’s also something called the “mother hypothesis,” which is similar to the grandmother hypothesis, except it explains human longevity by positing that if mothers have their last child by age 45 or so, they’re stuck raising the child until age 60 or 70. Both are valid and viable ideas, and I think both can coexist.
But what about grandfathers? A long-standing criticism of the mother and grandmother hypotheses is that no explanation is given for the longevity of the elderly male. What can Gramps contribute to society, and why does he live past the viability of his sperm?
Kaplan, Lancaster and Robson propose the “embodied capital” model of human longevity to encompass both sexes. Simply put, it suggests that male and female human longevity is necessary because of the slow, long development of human children. We aren’t like most mammals, who tend to spring forth from the womb with the ability to walk (or swim) and avoid embarrassing themselves; our infants are immobile fleshy bundles. Our children need guidance and instruction from our elders. They need support – the community needs material support, since the children consume resources without providing any. And both grandma and grandpa are involved in the teaching process. The emphasis here is on passing on knowledge and wisdom. They’re not just chasing little ones around.
We can’t survive on instinct alone. Our physical gifts aren’t sufficient. And we don’t pop out of the womb with knowledge and wisdom pre-installed. We come out with empty heads full of potential. We have to learn, or, more accurately, we have to be taught. And who teaches us? Experience is a stern, proven tutor, but exogenous instruction from experienced adults, parents, and grandparents – from society, really – is even more crucial. We need adults to live past reproductive age because human children are unproductive members of society for at least the first 12-15 years of their lives. They are either totally helpless babies (unable to walk, talk, and procure food), extremely annoying toddlers (now able to walk, babble, and get into trouble), or haughty mischief-making pre-teens. And all the while, they are students. They’re learning, watching, observing, and filling their big empty brains with the knowledge and experience that will help them be productive, resourceful adults. But they couldn’t do that if all the adults were dying off by age thirty.
While kids learn, work must be done. The animals must be hunted, the food must be gathered, the crops (if they’re horticulturalists) must be tended. The daily chores for a hunter-gatherer community require physical strength and know-how. Hunting requires endurance, precision, and fearlessness, which kids have little of. Gathering means carrying heavy loads, digging, climbing, and walking long distances. Kids undoubtedly accompany adults on outings (to learn, remember), but they cannot be expected to provide for the group. Adults aren’t just teachers, then. They’re also material producers. They hunt, fish, gather, build, carry, defend, and explore – all the nuts and bolts stuff that makes a society go.
Humans are animals, true, but a special kind of animal, one that has expanded the standard definition of evolutionary fitness. And no, I’m not trying to imbue our species with some sort of cosmic or spiritual significance; we create our own significance, our own meaning, by virtue of our massive brains. That’s the point. Our brains provide our consciousness, and, for all intents and purposes, set us apart from even our closest, brainiest cousins in the animal kingdom. Our tendency toward higher thought also allows us to exert mastery over nature. We plan. We study. We learn. We can develop the ability to spear a moving target – an ability that relies on our mastery of the physical and the mental realms. Hand eye coordination, spatial visualization. Memorization of the properties of thousands of wild plants – which are poisonous, nutritious, medicinal? Instinct and the subconscious save our rear ends in times of acute trouble, but our careful, measured intelligence and rationality puts us at the top of the food chain. We couldn’t take advantage of our brains without the security and dependability of having older humans around to teach us, train us, and support us. The kids may be the future, but adults set the path.
To summarize:
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