Well… you figure that humans under primitive conditions show traits of an r strategy, producing many children, but also having high rates of infant and child mortality. And those same humans have a short life expectancy – women dying young from the stress and fatigue of producing so many babies, relatively few members of the group reaching old age.
And then along comes modern medicine, in which infant and child mortality are reduced, and adult life expectancy is increased, and traits of a K strategy begin to emerge, i.e. people choose to have fewer children, but invest more in the children they do have.
And then you look at the countries where the population is burgeoning the fastest, and they tend to be what are known as “emerging” countries – infant and child health and adult life expectancy are beginning to improve, but the birth rate has not yet gone down to compensate.
Another reason for producing many children is they’re workers from pretty young age, so they’re producing wealth for parents in agricultural societies.
The one generalization that does seem to hold is that the demands of flight (in birds and bats, anyway–I don’t know about insects) lead to very efficient metabolisms and longer lifespans than would be predicted by rate-of-living theory.
This guy’s books have rather a lot to say about the nuts and bolts of why this is, without just defaulting to “Oh, it’s evolution”, which is obvious and not really the question, right? Spoiler alert: how organisms deal with oxygen has a lot to do with it, being the foundation of (most) life as well as arguably the most reactive and destructive element there is.
This generalization makes a fair bit of sense, but is hard to test empirically. The simple reason is that flight has evolved only twice among the living vertebrates, so we have n=2. Those two cases support the hypothesis, which makes sense because the hypothesis was generated specifically to explain those two examples. The rate-of-living theory is a useful straw-man. So many things violate it in so many ways that one can almost always point to a divergence and say, “Aha! We found something that doesn’t follow the rate of living theory!” without mentioning the thousands of other counterexamples. Again, the rate-of-living theory mostly only works for those organisms it was generated to explain, and even then only sometimes.
Also, a Laysan Albatross has been recorded to live over sixty years. So many species of birds have greater longevity than most mammals, and their bodies are smaller and their metabolism is greater. So there are selective pressures going on as the evolution of all living organisms continues. Evolution has not stopped, so as humans have continued to evolve, our life spans have increased. There is an evolutionary advantage for us individually, but whether or not that is beneficial for our species, time will tell.
My 2 cents is that it’s many factors. The quest for “the fountain of youth” has resulted in no one silver bullet. I’ve read a book by John Robbins, Dan Buettner, and others about human lifespan. They don’t really get into any biology of it but the answers they found are several factors. At least some of those can relate to other species such as chronic vs acute stressors. You would have to do something similar they did and create a list of contributing factors and how those play off of one another.
I would differ slightly with your interpretation of that study. They predict, based on a variety of measures and a model, that menopause is evolving to occur later in life. They did not actually show that this change is occurring across generations. As my mother-in-law likes to say, you can predict in one hand and spit in the other and see which one fills up faster.
Severely complicating any real-world measurement of heritable change in age at menopause is the fact that their are many environmental factors that can have mild effects on menopause, and the environment is changing over time. Since the Framingham Heart Study was not designed to detect that kind of inter-generational selective effect, the best they can do is predict, with a lot of built in assumptions.
I wonder if there are any epigenetic changes pushing longer lifespans?
Actually, I don’t think there’s any compelling evidence anywhere to suggest that human lifespans are actually increasing, is there? People say that all the time, but it’s just a failure to grasp statistics. Decreased infant mortality is the big change, which skews average human lifespan way up. People didn’t used to all die at 30:
“The years of our life are seventy, or even by reason of strength eighty” – Psalm 90:10
“Life expectancy at birth for males was 75.1 years in the first half of 2020, representing a decline of 1.2 years from 76.3 years in 2019. For females, life expectancy declined to 80.5 years, decreasing 0.9 year from 81.4 years in 2019” – CDC
All that really changes is that more people manage to hit that maximum age range thanks mostly to improved nutrition and medical technology that offers some people (especially babies) protection from diseases that kill us young.
Yes, you’re right, average human lifespawn is prolongated now, from archeological evidence we see only a few very old skulls without teeth (especially if we talk many thousands years ago), most adults died in 40-50s, depending on age it could be less for many men who died in battles and women died a lot while giving birth too, so people could and did live to 70s, but they were if not unique but unusual. Plus it may be a safe assumption less percent of deaths were long illnesses and not fast ones from injuries or deadly deseases, now other than accidents and catastrophes you can be cured pretty much from everything.
Well, in a sense, that explains why animals in zoos live so much longer than the same species in the wild – but then there is that rare animal in the wild that is documented to be as old as the majority in zoos. But that doesn’t explain why this species has a longer lifespan than that one – why a pet rat can go almost three years, but a pet mouse will hardly make it past two, yet the cat can go fourteen or fifteen, and the parrot fifty.
Sure, but we also should take in mind pets die a lot from cancer, mice and rats when they’re 1-3 years old and it’s very common, cats and dogs when they’re 10+. And it’s only one of major causes of death, so we need to discuss what natural death is, is it heart failure only?
There are enormous quantities of data from a huge range of populations showing that adult lifespans are increasing greatly. One of the central questions the academic field of Demography can’t stop studying from every possible angle is the increase in human life expectancy.
You are correct that for several decades most of the gains in life expectancy derived from decreases in infant and child mortality. Most of the gains for the last several decades in many countries came mostly from adult mortality. The standard demographic lifetable, of which tens of thousands have been published for humans, includes a column for residual life expectancy at each age that makes studying this question relatively straightforward.