Average life span is pretty short for contemporary foragers (30-35 years). For that matter, it was short for agricultural peoples until fairly recently. Most people probably don’t know this. Most of the people who do know fundamentally misunderstand it. Today most people die when they’re old, moderately close to the average age of death. Back in the day, a very large fraction died when young, due to infectious disease and food shortages. And subincision. Most people who know about those short lifespans in the past somehow can’t really believe that infant mortality accounts for most of the difference. If the average lifespan was 30, they figure that hardly anyone made it to 40. I’ve had a doctor explain to me that that hardly anyone lived to be 70 in 1900 in the US (!).
We know that this is not the case in contemporary hunter-gatherers. If you make it to 15, you have a pretty good chance of making it to 60. Life expectancy at 15 was about 48 among the Australian Aborigines and 51 for the !Kung Bushmen. In some other groups, expected lifetime at 15 was lower, in the 30s. Still – even so lots of people made it to 60 or later.
A high average paternal age was only possible if quite a few guys lived well over 50 – but that happened.
West Hunter 
The remedy to this misunderstanding is not to let anyone get off with the lazy expression “life expectancy”. Demand to know whether they mean at birth, at puberty, or whatever. You see the same cock-up in discussions of pensions: what matters is life expectancy at, say, 62 (the US) or 65 (the UK) or 50 (France – but perhaps I don’t have that spot on?).
This is an error that had bugged me for many years. So many people talk like “life expectancy” is a maximum age. But then the public is pretty ignorant of all things mathematical, interpretation of averages not least among them.
“The days of our years are threescore years and ten; and if by reason of strength they be fourscore years, yet is their strength labour and sorrow; for it is soon cut off, and we fly away”.
Just to be clear, when you say a Bushman at 15 has a life expectancy of 51, I’m assuming you mean he has an even chance of reaching 51, not 66.
I was always puzzled when I read things like “the average life span in ancient Rome was 35 years.” I knew this must include infant mortality, but I couldn’t understand why anyone would ever print such an obviously misleading number without clarification. Why would anyone even think that this was a number the typical reader would be interested in? Most readers would be teenagers or adults, and they would be much more likely to be interested in how long someone their own age could have been expected to last in ancient Rome (or wherever).
I thought though there were studies of Roman burial sites where they did indeed find mean ages in the late 20’s early 30’s – I recall Clark mentioning it in “Farewell to Alms”
Not clear at all how to interpret such a finding. There are obvious sources of bias of course but even in the simplest of all possible cases age at death distributions can mislead badly. Consider a hypothetical population with constant age-specific death rates b and d, so r, the intrinsic rate of increase, is b-d. The stable age distribution p(x) is proportional to exp(-rx) l(x) = exp(-bx +dx) exp(-dx) = exp(-bx): it depends on the birth rate only, nothing to do with death rate. The distribution of age at death is then proportional to exp(-bx) so the average age at death is 1/b . If populations sawtoothed over time, usually growth with occasional crashes, mean age of death from burials would be pretty misleading about long term dynamics.
That’s an interesting perspective, however, I don’t see how this would be useful for a hunter/gatherer population as much as for a modern “civilization” population. Hunter/gatherers have more exposure to life-ending environmental factors, so I imagine that the range of causes of death would be larger. Actually, big question there. It was recently found that S. Am tribes living away from western dietary influences had no increases in blood pressure and possibly much less cardiovascular disease even well into older age. Therefore, applying your formulas might be better after first separating those populations into segments of death from natural causes and other.
Just to be clear, I meant what I said: 51 more years after 15, which would be 66.
Can’t find data on !Kung, but, among precontact Hiwi, median life expectancy at 15 was rather pitiful 17 for males and 14 for females (half of all 15-year-old males lived to be 32, and half of all 15-year-old females lived to be 29). Even at 25, median life expectancy was still 18 and 13 respectively (http://courses.washington.edu/evpsych/Hiwi-hunter-gatherers-JHE2007.pdf, table 1). Among northern Ache, median life expectancy at 15 was around 15 for both genders. (“Aché Life History: The Ecology and Demography of a Foraging People”, Hill, Hurtado, pp. 196-198.)
For foragers, especially for “native” (precontact) foragers, it is typical for the population to decay exponentially with age, because adults experience high mortality rates due to assorted diseases, homicide/war, and predators. Postcontact foragers fare a bit better because of their unique relationship with the outside world (many even hunt with guns instead of their traditional weapons).
I looked at that paper. I believe you’re mistaken. It says ” life expectancy at adulthood (age 15) is only an additional 31 years”. I believe you’re mistaken about the Ache, as well. “In the forest period about 65% of all children born survived to adulthood (age 15), and life expectancy for those young adults was an additional 40 years on average. “
Ok, looks like I misinterpreted the data in table 1. We should be looking at table 2, column lx (odds of survival to age x?), and yes, it shows mean and median life expectancy at age 15 in the low 30’s.
Still, low 30’s is not 51.
From that paper
“We suggest that reported modern African hunter-gatherer rates of violence may be low because of interference by powerful state-level societies prior to demographic study”
In terms of environment i wonder if there may be a correlation in Africa between the pleces where hunter-gatherer men had to help the most in providing food – which i think is likely to produce lower average levels of violence – and the places least suitable for the Bantu expansion? i.e. the more marginal the terrain and therefore the more help needed from the males the less likely the terrain was suitable for cattle so the groups who survived the expansion were disproportionately among the least violent ones?
Life expectancy at a given age is the mean future time to death. If life expectancy at age 15 were 51 years that is the mean future time to death. It would not necessarily be the case that the probability of living at least 51 more years is 50%.
For example suppose we start at age 0 with a hundred people and 99 die immediately and one
lives then for 100 years. Then the life expectancy is .99 x 0 + .01 x 100 = 1 year . However the
probability of living more than 1 year is 1%.
“and 51 for the !Kung Bushmen”
That’s kind of the point though. If you divide the ancestral environment into three logical categories
Type1: where a female can gather enough to feed her offspring on her own
Type2: where a female can’t gather enough on her own and the extra can come from either hunting or gathering
Type3: where a female can’t gather enough on her own and the extra can *only* come from hunting
Then extreme violence as the method of selection on the males is *only* really viable (optimal?) in the Type1 environment i.e towards the Yanomani end of things.
It’s the !Kung in particular who gave me the idea in reverse. What happens when a population from a Type1 environment moves into a Type3 environment?
If they move from Type1 to Type2 there are “not males help” options e.g. late weaning, grandmother effect, developing better ways to gather etc, as well as various “males help” possibilities but if one premise of this argument is “males hunt” then if a population moves from Type1 to Type3 there isn’t a survivable “not males help” branch. In that case it seems to me certain adaptions become logically necessary to provide the necessary calories i.e. either
1) Food, females and children become viewed as being held in common and all the hunted food goes into a communal pool and shared out to all the kids (for some kind of selection i think this requires the best hunters who bring the largest amount into the common food pool to get lots of sex).
2) Monogamy. If a male has to help (and critically if the environment is difficult enough so he could only hope to ever feed one set of children at a time) then monogamy provides the payback – and the more difficult the environment and the higher the percentage of calories required from hunting then the more likely it seems to me the population will develop monogamy e.g. the !Kung, Eskimos (i think?) etc.
Also if there is extreme selection on males on hunting ability then there doesn’t need to be extreme selection for violence – if anything the opposite – so you might expect populations like that to be less violent.
3) Some way of combining a bit of both.
One of the predictions of this (obviously very simple model) is people like the !Kung would be more monagamous and less violent.
(Another might be populations who went 1 to 2 to 3 might have greater longevity than those which went 1 to 3 directly etc).
#
The model would predict four recurring states (assuming the root is Type1) of populations as they move (or stopped) at each environment
1) Type1
2) Type1 moved to Type2
3) Type1 moved to Type2 moved to Type3
4) Type1 moved to Type3 directly
I think this is important because i think the biggest step wasn’t Out of Africa it was Out of the Tropics but a lot of that adaptation which would have once been physically and culturally visible radiating out from the Tropics was swamped by the later Bantu expansion.
In extremely harsh enviroment, even monogamy is not enough, polyandry is necessary for children survival (at some himalaya region)
yes, two brothers, makes perfect sense.
Infant/early childhood mortality is only part of the reason for the very low life expectancies in southern Africa. Due to rampant HIV infection there actually are many people dying in early adulthood.
Peter
Life expectancy averages are just that. Measures of central tendency. If anyone has been idiotic enough to think that folk typically live to 35 then keel over then they are nitwits whose opinions can be safely ignored. Life expectancy is a useful way to compare populations–its a really good index of how civilised a group is. The disrecpancy between boys and and girls can say useful things as well–for example about how competetive a culture is. Is there really a problem here? I mean a problem among people who know what the numbers refer to?
This is not exactly the time frame or the kind of people Greg was discussing in his post, but you can see the same phenomenon he describes by looking at life expectancy in the colonial United States (approximately 25 to 30 years at birth, according to some sources) and comparing it to the ages of the first several U.S. presidents who grew up in that period: Washington, 67; Adams, 90; Jefferson, 83; Madison, 85; Monroe, 73. I could also include Ben Franklin, who lived to the age of 84.
One might counter that this just shows how white men of privilege and wealth have always been able to live longer than the masses. But it’s interesting to note that over a century later, when the life expectancy at birth in the U.S. had risen to fifty years of age – nearly double what it was in the colonial period – the U.S. presidents of the early twentieth century were frequently dying earlier than their peers who had lived more than a hundred years before: TR, 60; Taft, 72; Wilson, 67; Harding, 57; Coolidge, 60; Hoover, 90; FDR, 63.
Of course this is a small sample size and I wouldn’t try to make any overarching claims with this data. But it is interesting to note how privileged men in the early United States frequently lived as long as, or even longer than, the most privileged Americans in the early to mid twentieth century – even as life expectancy for most other Americans was rising rapidly.
Your second list is interesting: only one clever chap, and he outlived the rest by 20 or 30 years.
TR and Wilson weren’t “clever” chaps?
Not remotely as clever as Hoover. He really was impressive. He’d have been reckoned clever not just in the US but in countries where clever fellows often went into politics.
Hypothesis 1 -It is due to reduced infant mortility rate, which allowed less genetic fit people to survive but still their weak genes showing up somehow.
Hypothesis 2- Lack of social mobility or wider social gap allowed only upper class people to become president. Social equality allowed some middle or underclass people becoming presidents who still lacks genes for longevity.
Life expectancy in colonial New England was 43 for males, 42 for females – much higher than it was in England. The average 20-year old New England man died at 69. Generally, in the rural North, life expectancy was high and child mortality was relatively low (10-15%). Things were much worse in the South, particularly in the tidewater regions.
I wonder if life expectancy was much higher for the rich back then. The best way to make cash money was owning a Tidewater plantation – but life was short there. Cities were much less healthy than rural areas. Worse yet, wealthy people back then could afford medical care, which was undoubtedly bad for you.
“I wonder if life expectancy was much higher for the rich back then”
If the rich had more arranged marriages and those arrangements were based on wealth then although – depending on how the wealth was obtained – that might lead to assortative mating on cognitive ability it wouldn’t neccessarily be assortative on health.
Rich men marrying physically attractive but non-wealthy women might partially counteract that.
1) *If* the genetic load idea is correct (and it certainly makes sense to me)
2) and *if* the northern euro marriage model is/was better at shedding genetic load
3) and *if* the people in the 1600s and 1700s who had most assiduously followed the northern euro marriage model for the longest time were from the “middling sort of people” in northwestern Europe, and especially the nonconformist Protestant sects of Holland, Northern Germany and Eastern England
then
might you expect settlers to America from those populations at that particular time to have low genetic load and therefore possibly both high longevity and high fertility?
And do the extant versions of those populations – if they still exist in the same form – still have above average longevity?
Young animals die in large number are quite natural (less than 50% lion cubs survive to adult, crocodiels have less than 1% survival rate for new hatched). The entire sexual reproduction is intended to weed out bad genes through such massive death. If you consider how much gametes actually made into fetus, the mortality rates are even higher.
Why there is such strong physical attraction in sexual reproduction? The first step to weed out bad genes is to judge physical beauty. Ugly people are the first causualty in order to keep next generation with less mutations.
>Young animals die in nature quite naturally.
They taste good. Predators notice. Bambi is tastier than old Buck. I gigged a big old 1′ diameter body bullfrog once- awful tough compared to young stuff. Even grass- young meristem-heavy shoots taste good, old tall tough acidic grass, meh.
Off topic but something I am curios about. What was the evolutionary force causing humans to lose their body hair? Could a male preference for less hairy women do it because the offspring of both sexes be less furry? If so, could a preference for less fur be a side effect of selection for a better brain?
This is off topic, but I just read Germs Are Us and was struck by the quote from Martin Blaser ““[causing nothing but harm] isn’t how evolution works” […] “H. pylori is an ancestral component of humanity.”
That seems to be the assumption that the inevitable trend is toward symbiosis that I recall from Ewald’s “Plague Time”. My recollection is that it’s false if the pathogen can easily jump to another host. The bulk of the New Yorker article reminded me of Seth Roberts.
I have corresponded at length with Blaser. He’s a damn fool, not just on this. Speaking of, would there be general interest in listing all the damn fools in public life? Of course making the short list would be easier.
Here’s an L.A. Times article about an 87-year-old man in Nigeria who has married 107 women:
http://articles.latimes.com/2011/may/12/world/la-fg-nigeria-much-married-man-20110512
Yes, Greg, give us your list of damn fools in public life.
Most everybody, of course. They fall into different categories of useless, ranging from merely boring to those for which I would pay hard cash for the privilege of working them over with a baseball bat.
Columnists and pundits are mostly cheerleaders for their side – ugly cheerleaders. Hardly any (<< 1%) of them have any predictive power, or anything to say that is original and true. And they don't know jack. I once noted how Matt Yglesias keeps opining about alternate energy, but hasn't the faintest idea what a Carnot cycle or a band gap is.
They are not seekers of truth they are seekers of power and status.
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