Humans, in particular women, have, on average, many decades left to live once their reproductive years are over. This is, of course, a great thing, and we’re all happy for every additional year we get; but I wonder how it came to be that way from an evolutionary perspective: Pressure from evolutionary selection favours those traits that maximise their chances of being passed on to the next generation. So you’d think that once offspring is there, there’s no reason for the individual to stay around much longer as far as nature is concerned. Is this just a result of improved medicine and nutrition, and nature “intended” for us to die shortly after we cease to be fertile, or is there another mechanism at work?
I don’t know if theres a factual answer, but my guess is grandparents are a help.
That was my thought. Humans take years longer to be self-sufficient, especially in a wild environment.
My guess is that a long human lifespan wasn’t specifically selected for by evolution, but rather, short lifespans (beyond reproduction and child-rearing) weren’t consistently selected against.
I wonder if we’re all that different from other mammals?
For very round numbers human fenale lifespan is 200% of reproductive senescence (40 vs. 80 years) while male lifespan is about 133% of reproductive senescence (60 vs 80). And both first achieve sexual maturity at about 15% (12 vs 80) of lifespan. Again these are very round numbers, assume decent lifetime nutrition, etc.
I don’t know how those percentages compare to cows, whales, mice, dogs, cats, etc. But IMO that would be a worthwhile line of inquiry.
I’m on my phone or I’d be digging for cites about now.
Agree in general it’s related to how long as a percentage it takes a newborn to become self-sufficient. I will WAG that we’ll find a correlation between that factor and post-reproductive lifespan across all the mammals.
I think the general theory is living long helps your grandchildren survive. Perhaps that also explains menopause, which other animals don’t experience ?
Largely because the end of reproductive age is accompanied by more decline in physical ability that would result in the death of most of other animals from starvation, predation, or simply an inability to recover from disease or injury. Humans have others to help us through. I don’t think that’s unheard of across all other animals, but we have much more capability to care for the aged and infirm.
I have actually read articles discussing the importance of grandparents in human history and society in a few different magazines (sorry, no cites).
The difference is that humans can and will contribute much longer than most animals to the survival of their offspring. Human lifespan in fact extended drastically in relatively recent times on an evolutionary scale; the bones of early “anatomically modern humans” show no signs of menopause, indicating that nobody back then lived that long. “A world without grandparents” is one way I heard it described.
Also, it’s very useful for at least some people to live a very long time as repositories of knowledge. An example I recall being given is the centenarian who recalls how as a little girl the last time they had a “hundred years drought” the tribe survived by eating a particular root that nobody has eaten in the intervening decades because of how foul-tasting it is. Before literacy the only way such knowledge was available was if somebody remembered it.
Civilization has done a great job eliminating or mitigating external pressures that would otherwise affect us.
Like all other animals, our bodies begin to fail not long after our peak reproductive years. But we have:
- Healthcare, both preventative and curative. We can slow the early impact of the aging process to have more productive years and we can actively fight against later impacts to eke out many more “unproductive” years
- Social nets. Retirement, family support, currency in general, all give us means to survive even when our health is failing.
Think about housepets. An “inside cat” has a longer lifesan than an outside one because living in comfort and with access to regular healthcare removes many of the dangers and pressures that lead to catastrophic health failures.
Yet some chimpanzees do experience menopause. It’s likely a complicated set of genes that have existed in our clades for a long time, and then selected for at some point.
Chimpanzees, gorillas, and orangutans have similar lifespans to us. They tend to live about 40 years in the wild, but can easily live up to 60 to 70 years or more in captivity, where they don’t have to face the dangers of the wild and where they receive medical care as necessary.
Monkeys don’t tend to live quite as long, but can typically still live for 30 or 40 years in captivity, which is a pretty lengthy time for a smaller mammal. Squirrels, for comparison, only live about 3 years or so in the wild, and up to 5 to 10 years in captivity. Humans evolved from a squirrel-like ancestor that presumably had a lifespan roughly similar to that of modern squirrels.
From an evolutionary perspective, you can therefore conclude that our longer lifespans started out in the “planet of the apes” days of the Earth, when there were a lot more ape-like species than there are today (about 10 to 30 million years ago, IIRC). Since all great apes share our longer lifespan, I think it is safe to assume that our lifespan had already evolved long before modern humans had even existed. Early hominids probably could have had at least a 30 to 40 year lifespan if they had been properly cared for, but of course that was probably reduced quite a bit in the wild, just as lifespans for great apes are reduced in the wild today.
Does any of this take into account that men can father children much later in life — e.g. guitarist Andres Segovia fathered a child while in his nineties? Or is he an outlier?
It doesnt seem likely that there was much evolutionary pressure there. Men werent living that long.
There is a well-known relationship between typical adult body size and lifespan in mammals.
While humans live longer than would be predicted by our size, we aren’t off-the-charts. (Bats are, they live enormously longer than mice and shrews and whatnot, probably because they avoid a lot of predation by flying.)
Human infants require a lot of care, and at some point it makes more “sense” to invest in the offspring you already have than to invest in making more offspring. So living perhaps 10 years longer than we reproduce would seem to increase reproductive success. That doesn’t explain women who hit menopause at 40 and live past 70, but it might be that bearing children is so expensive that removing that physical stress adds a lot of extra lifespan?
The other animal that experiences menopause is the killer whale:
Why did menopause evolve? New study of whales gives some clues
https://www.reuters.com/science/why-did-menopause-evolve-new-study-whales-gives-some-clues-2024-03-13/
Interestingly, their reproductive and total lifespan is similar to ours, at least for the females:
Studies if Miller whales living in the Pacific off the U.S. West Coast show that females stop reproducing around age 40, but often live into their 60s and even their 80s. Male killer whales typically die before age 40.
(Note that i typed that quote by hand, so there might be typos.)
I see two reasons:
First, the grandmother hypothesis is a common theory. Grandparents can be a help when caring for the young - particularly for humans, where children require help to survive for years - unlike, say, cattle where the offspring hitthe ground running so to speak. Even animals like cats, dogs, rabbits, etc. the offspring are capable of looking after themselves fairly soon. In that case, there is no need for outside assistance. Plus consider many such animals are solitary and do not live in cooperative packs where they share the spoils of the hunt.
This follow on the problem that pregnancy and childbirth is a major strain on the female body - particularly, evolution could only go so far to accomodate the baby’s big brain. However, the hips cannot be much wider without the risk that torso weight would result in hip fractures. One analysis I recall said we are almost reverting to marsupial stage, where the baby is born premature and requires a year of care before it even wwalks or eats normal food, followed by a decade of further care. Add to this the deterioration of the human body with age making later pregnancies greater risk, and the odds of birth deformities rising with age - making older pregnancies not only riskier but less productive. As a result, women who had children later in life risked dying while still needing to care for previous children, thus endangering the children’s survival too. Evolution favoured those who stopped early.
Also note there is less issues with fathering a child at a later age, and widows could find another man to help provide for them and their offspring. So less evolutionary pressure on men. Plus the tendency to monogamy means that older men would have less offspring anyway, unless they somehow appealed to younger women.
Or, “it takes a village to raise a child…”
Rich older men were attractive husbands for young women in some societies.
My personal thought, which seems to be supported by the two sources below, would be that longevity past sexual maturity would largely correlate with the socializing factor of the animal. If it’s a pack animal then the whole group is foraging and hunting together, and there’s strength in numbers. However, that requires a bit more brain power, so you also need longer to bring the children up to strength.
For animals that are less pack-oriented and which simply try to go after whatever is in front of them, without much thought about it, they just need to create another generation and then they’re largely useless. Their continued existence, to some extent, just creates more competition for the next generation.
Hmm, this is a new concept that hasn’t occured to me. 
But seriously, the creation of wealth that could be stored is part of the reason to keep older people around. Back in the day wealth may mean you had more furs to trade due to your abilty to make good trades over the years, but if no one needed your furs you were out of wealth. Now we have money that older people can accumulate and used to obtain numerous desireable things.
Ya beat me to it. Grandparents increased the likelihood that children would make it to adulthood. To put it another way, a child who had the genes of a grandparent was more likely to become a grandparent themselves.