Life Expectancy, Life Span, and the Size of Human Beings
Disease and Accidents kill many people: throughout history, most have not survived to live their full potential life span.
I was startled by this article I recently read at “ancestry.com” about what people in the USA ate in 200 years ago. I was not surprised because of the foods, but because of this: “…By one estimate, a white man who had reached his 20th birthday could expect to live just another 19 years. A white woman at 20 would live, on average, only a total of 38.8 years. If measuring from birth, which counted infant mortality, life expectancy would have been even lower. A white family in the early 19th century would typically have seven or eight children, but one would die by age one and another before age 21. And, of course, for slaves, childhood deaths were higher and life expectancy was even lower. About one in three African American children died, and only half lived to adulthood.”
Note — they actually are quite coy about listing the ACTUAL life expectancy — from birth. I looked up life expectancy in France during this period and it was little different.. in fact in the early 1700s, it was not much over 22 years. Digging further, it turns out that globally, that some figure under 30 was the average life expectancy at birth in all economic systems, from hunter-gatherers to early industrial nations, until vaccinations and antibiotics began to bring childhood mortality down. In fact, some further checking led me to some rather startling charts, showing global life expectancies not much higher than this until the last thousand years or so, even in western European countries where average wealth and were estimated to be gradually increasing over this period.. Check it out:
Even comparing the “west” with the rest — although the one (below) is just since 1770, after that estimated little bump up that started about 230 years ago.
I was previously aware of a very interesting blog by a a guy named Stephan Guyenet, a neurobiologist who has done some research on life expectancy among the Inuit. Here is a bit of his blog entry on this research: “One of the classic counter-arguments that’s used to discredit accounts of healthy hunter-gatherers is the fallacy that they were short-lived, and thus did not have time to develop diseases of old age like cancer. While the life expectancy of hunter-gatherers was not as high as ours today, most groups had a significant number of elderly individuals, who sometimes lived to 80 years and beyond. Mortality came mostly from accidents, warfare and infectious disease rather than chronic disease.
I found a a mortality table from the records of a Russian mission in Alaska (compiled by Veniaminov, taken from Cancer, Disease of Civilization), which recorded the ages of death of a traditionally-living Inuit population during the years 1822 to 1836. Here’s a plot of the raw data:
Excluding infant mortality, about 25% of their population lived past 60. Based on these data, the approximate life expectancy (excluding infant mortality) of this Inuit population was 43.5 years. It’s possible that life expectancy would have been higher before contact with the Russians, since they introduced a number of nasty diseases to which the Inuit were not resistant. Keep in mind that the Westerners who were developing cancer alongside them probably had a similar life expectancy at the time.”
I was quite taken with this material, since I had got very similar data on a population of hunter-gatherers in the Kalahari — in fact a 12 -15% mortality before age 15 seemed remarkably consistent over several generations of genealogical data. And, amazingly, I got about the same proportion of people living past age 60. I even got one woman who was at least 100 years old, because I verified her accounts of experiences she had in her mid-teens with historical accounts of events recorded in the archives of the Botswana Museum. I found a pair of twins over 80 (they were both widows with great grandchildren) still going strong: here they are — in a photo I took one spring day. The lady kneeling is searching for some truffles she had found, to show me they really existed… (long story).
None of this demographic data I got was exceptional. Nancy Howell had very similar findings from the !Kung San data collected in the 1960s.
In fact Dr. Howell had been married to Richard B.Lee while working on this demographic material, and Dr. Lee was my thesis supervisor for the research I was doing among the Kua. I was pleased that my data revealed nothing unexpected. In the intervening years since I was in the field, I went off to do studies of horticultural and pastoral groups in West Africa, and again did the usual genealogical and basic demographic analysis. Infant mortality was a bit higher in this data, over 20% in one village, but the birth spacing was shorter and overall age/sex ratio in the populations well within the range I expected. Again, there were proportions of people over 60 in the same ballpark. Life expectancy in the hunter-gatherer sample was about 25, a bit lower in some regions, higher in others, and I was not surprised when the West African data also indicated a life expectancy of about 30, despite the higher infant mortality. Life expectancy had gone up in the previous two generations as vaccination and other health care services were introduced to Burkina Faso. It was not as high as it had become in more developed countries in Africa, but it was clearly rising. So this was the state of my understanding when I first encountered the material concerning hunter-gatherers in other parts of the world.
If all these populations had “low” life expectancy, then, it still did not prevent them from having a demographic pattern that included a relatively high proportion of people over sixty. It appears that there were definitely some evolutionary drivers here, favouring longer life span. This has some important implications for the use of the “life history” theory. Life history models replaced the “r” strategies (high litter sizes, massive mortality, fast maturation, low parental investment, short life span, small body size) and K strategies (tendency towards singleton births, lower infant and juvenile mortality, slow maturation, high parental investment, long life span, large body size) that had been developed. I had another look at a well known paper that seems to have become one of the major exemplars of life history theory.
If you read this, it is at first quite persuasive: “Now, scientists Andrea Migliano, Lucio Vinicius, and Marta Lahr have performed a study on two pygmy groups from the Philippines, the Aeta and the Batak, and concluded that there may be a better explanation for pygmies’ short stature. Their study is published in a recent issue of the Proceedings of the National Academy of Sciences.
The researchers point out that one characteristic unique to but common among many pygmy populations is their short lifespan compared to other humans. With this in mind, the researchers suggest that pygmies represent the “fast” extreme of life history strategies, with short stature being a side effect.
“We first thought that we would find a relationship between small body size and increased fitness in pygmies — for example, that the shorter pygmies would have more advantages, such as higher fertility, than the taller ones,” Migliano told PhysOrg.com. “However, the smaller pygmies had lower fertility than the taller pygmies. So this gave us the idea that perhaps there was no advantage in being short among pygmies.
“Then, when I went to the field, and started to interview them, I noticed the very high mortality rates — really high compared to any other population,” she said. “So when we checked that different pygmy groups followed the same pattern, we thought that these facts should be linked. Also, life history theory has been used for a long time to understand body size diversity among mammals, and we thought it should also apply to the understanding of human diversity.”
Because of their short life expectancies, the researchers speculate that pygmies have had to shift their reproductive years forward. The average life expectancy at birth for different pygmy populations ranges from just 16 years to 24 years. Very few pygmy women reach the end of their reproductive period, as only a small percentage survive past age 40.” (source: Migliano, Andrea Bamberg, Vinicius, Lucio, and Lahr, Marta Mirazon. “Life history trade-offs explain the evolution of human pygmies.” (Proceedings of the National Academy of Sciences. December 18, 2007. vol. 104, no. 51, 20216–20219.)
The Batak, of course, are not foragers; they are horticulturalists, like those in my West African samples. They live in the Philippines. Like the Bwaba and other West African horticulturalists, they supplemented varying amounts of their cultivated crop with wild foods — plants, game and fish. They are of an ethnic group called “Negrito” characterized by small body size. They are “pygmies” — like the central African groups of hunter-gatherers, like the Mbuti and the Baka.
Well I am just over five feet tall myself. I fit right in among the Kua women in the Kalahari, was a bit shorter than average women in West Africa, and am shorter than both parents. I was, in fact, the same height as my maternal grandmother.
So I have always felt that being short was not such a bad thing, really.
What about the idea that there are long term evolutionary trends in “life history”?
Humans do appear, in fact, to have gotten smaller for at least the past 30,000 years, which, as John Hawks suggests, “may have resulted from Late Pleistocene and Holocene subsistence strategies, which replaced close-contact ambush hunting of large mammals with projectile weapons, intensive collection of small animals, fish, and shellfish, and ultimately sedentary pastoralism and agriculture”. Neanderthals and other early humans, according to many experts, were likely to have been “ambush predators”, who needed to get close to their prey in order to dispatch them. When they changed to using “distance hunting” technologies, they could have undergone a shift in selection pressures, away from robust bone and muscular mass: energy efficiency alone would favour the survival of smaller and leaner hunters.
Chris Stringer (Natural History Museum in London) also suggested ago that modern humans in Africa 60,000 years ago had begun to hunt in a “new way”. There is evidence of use of bow and arrow technology (as well as spear throwers) from sites in southern Africa dating back well before 40,000 years ago, so the trend toward smaller body size in the late Pleistocene might well have started earlier than than Frayer proposed: see also page 60–61 here.
Can we assume, then, that this decline in body size is evidence of a shift to shorter life spans, higher infant mortality, earlier maturation, lower parental investment, and so on? I doubt that this interpretation is valid.
I wonder if the more copious data set indicating an even more precipitous decline in body size during the Holocene, especially among populations that took up farming, might be confusing the issue?
I say this because of secular trend. Especially in the last 200 years the increase in life expectancy has been associated with an increase in body size; most noticeable in European populations. The increase seems to be connected to recent trends in industrialized economies toward reduced rates of mortality at all ages and improved nutritional status. This does appear to fit the life history model: body size — and other aspects of tissue development (including brain size) — in humans is fairly plastic. Changes in these physical aspects, especially change covering the vast majority of individuals in the same population, often reflects changes in environmental conditions, especially nutrition and disease incidence, rather than changes in underlying genetic potentialities.
One of the often unspoken assumptions of this model, however, in terms of genetic trends and evolutionary trends within species, is that shorter life spans, faster maturation rates, lower parental investment in more numerous offspring, and so on… would be associated with lower intelligence. This model holds up in many species of animals, the proxy for this being smaller brain sizes proportional to body sizes. It is the famous “K” strategy vs “r” strategy, as a key to selection models, introduced by the ecologists Robert MacArthur and E. O. Wilson in 1970.
Recently, of course, data on smaller mammals like rats, and particularly on birds, has indicated that the relationship between brain size and cognitive ability might have been overemphasized in the past: the density of neural tissues might be more significant.
However that did not stop Philip Rushton from speculating that Africans, Europeans, and Orientals formed a “less-to-more K-selected” range within the human species; setting off “race-realism” advocacy among researchers so inclined to contort the original model as proposed by MacArthur and Wilson, to apply it within rather than among species.
It seems to me that this idea confounds “K” vs “r” strategy evolutionary model with the effects of “within species” plasticity. It would be relatively benign had it not also been extended to imply that that differences in average intelligence in human populations, like differences in height, were mainly a result of genetic changes — of evolutionary pressures — rather than a fairly plastic aspect of human biology, responsive to environment during gestation and maturation.
There has been extensive testing using standard IQ questionnaires — as well as proxies like impulse control and reaction time tests. The taller, healthier populations in “developed” industrialized countries seem to come out with the higher scores. In fact, the average scores, on standard IQ tests, have been increasing over the past hundred years.
Some people take this to mean that populations in industrial countries are either getting more intelligent, as their health and nutrition improves, or else are simply getting better at taking tests. Others wonder if this is due to more intelligent people having more children…and worry about the recent trend to small family size among educated women.
This last point is controversial because it assumes that IQ is largely genetic in causation. As such, it is a fine system-justification meme for colonialism and continued predatory resource extraction in regions where brown and black people can be pushed aside. An evolutionary “explanation” for the data showing that the fastest industrialization and modernization has occured among populations that are taller and have higher average IQ. Thus, the implication is that these populations are more evolved — more “K-selected” — in short, they are biologically superior. This is, perhaps, why Nicolas Wade called his book A Troublesome Inheritance.
Besides the queasiness such models evoke, at least in me, there is an obvious flaw: these theories seem to me to be reverse the direction of the causal arrow between improved health and nutrition and increased intelligence and body size shown so clearly in studies documenting recent and ramarkably rapid secular trends, which are certainly not limited to any one country of origin. It also leaves us genetically shorter people a bit defensive. In my case, I was annoyed at the implication that small people might be stunted in brains as well as height. Not just for myself, either.. I felt this did not fit my experiences among the Kua, who tend to be short. Nor did this fit what I had experienced over a lifetime of meeting short, as well as tall, people. Brilliant people come in all sizes; so do doofusses.
Meanwhile, a massive research effort indicated that incidence of parasitic disease was the single most significant variable correlated with variation in IQ worldwide.
Despite this, Greg Cochran, in a blog he shared with the late Henry Harpending, referred to the model suggested by Magliano et. al. about why Pygmies are short, then combined it with what is known about their IQ from reports of testing among such populations as the Baka, and suggested that these people would be unlikely to survive long into the next century because they were too unintelligent; their brains were too small. He further implied that this was genetic and had been caused by a natural selection pressure brought about by low life expectancy in an environment with high disease incidence.
Indeed, there is evidence that many pygmies have a genetic difference — a higher frequency of a polymorphism that curtails the post puberty growth spurt.
I probably have that same trait… even though I reached puberty quite a bit later than many young people today, most of whom seem to tower over me.
This is the other aspect of secular trend, observed in all populations as they become “westernized” and exposed to contact with the products of the current juggernaut of globalization: a trend toward earlier puberty as well as greater adult height and larger body size. See an extended discussion of this aspect of height by Patrick Clarkin at the Evolution Institute webpage.
My own conclusion, so far, is simple. I suspect that data on secular trend — and very likely, the data on height, skin colour, and many other physical and behavioural differences among populations of humans — appears to be an expression of plasticity within a single species. The original “life history” model was the “K” vs “r” selection that was used to compare species like elephants, humans, and whales (at the extreme “K” end) with species like rabbits, mice, and mosquitoes (at the “r” end): it was about trends in the evolution of species niches and diversity within ecosystems.
Let’s please not confound secular trends with natural selection.
