If "short people got no reason to live", then why exactly do they live longer than tall people?
This study (abstract) from 2003 reviewed evidence for the relationship between stature and longevity, finding that short people have a substantial record of living longer in many large-population and some more select samples:
Findings based on millions of deaths suggest that shorter, smaller bodies have lower death rates and fewer diet-related chronic diseases, especially past middle age. Shorter people also appear to have longer average lifespans. The authors suggest that the differences in longevity between the sexes is due to their height differences because men average about 8.0% taller than women and have a 7.9% lower life expectancy at birth. Animal experiments also show that smaller animals within the same species generally live longer.
The paper is one of those reviews of dozens of studies, so there are lots of references. Here's an interesting factoid:
In addition, centenarians tend to be quite short and light [14, 63 and 69]. Japanese centenarians averaged about 10 cm shorter than 75 year olds and Hungarian centenarians averaged 154 cm. In an unpublished analysis, we compared 14 European countries divided into taller and shorter halves based on heights during youthful years and found the shorter countries averaged 77 centenarians per million vs 48 per million for the taller half. Short Sardinians and Okinawans, not included in this analysis, also have exceptionally high percentages of centenarians (136 and 340 per million respectively).
They also did a survey of famous dead people to see if the taller ones died sooner (they did). I wonder if that included the relatively short and long-lived John Quincy Adams, incidentally, the first president to wear long pants instead of knee britches (!). In fact, looking over the numbers, the dividing point for "short" famous people is 173 cm, or 5 feet 8 inches, which is a bit taller than average height for men.
The authors also suggest that the added longevity due to caloric restriction in experimental animals may actually be a reflection of small body size, rather than of caloric restriction per se. Their preferred explanation is called the "entropy theory" of aging, which essentially is the argument that the bigger you are, the more things can go wrong (Samaras 1974).
My major complaint is that more of the original data are not plotted -- there are a lot of plots of group means. But without seeing the variance in the data, it is impossible to tell if the effect of size comes from a large effect on a subset of very tall people, or a broad regression of longevity against size. In fact, I would strongly suspect that there is some optimum height that has the longest lifespan, with shorter lifespans on either side. This is almost certainly true, since congenital dwarfs do not have as long an average lifespan as nondwarfs. But the issue is where the optimum size may be -- is it relatively high within the range of, say, 5' 5" to 5' 8"? Or is it low, say, 4' 6" to 4' 10"?
Evolution of height?
Of course, this is one of those basic size questions that has a lot of impact on human evolution. For one thing, people today really vary in height in different populations. Do relatively tall populations pay a cost in longevity. The study suggests that at least in developed societies, they do. But how do pygmies compare? This kind of question may not currently be answerable, considering that differences in mortality rates in human populations owe much more to infectious agents than to anything else.
Rephrase the question, then: considering that mortality rates during recent human evolution were much higher (and average lifespan much shorter) than today, do the height differences in longevity that we now observe have any evolutionary relevance at all? It is hard to argue that their effect should have been the same as today. But on the other hand, it is hard to believe that radical size differences between populations had no effect on life history evolution (and longevity) at all. The question is how much effect, and how relevant the example of recent human demography.
Then there are the body size changes in the fossil record. Conventionally, we think that the increase in body size from Homo habilis (or a like-sized variant of early Homo) to early humans was accompanied by a substantial increase in body size (evidenced in particular by early human fossils like Nariokotome [KNM-WT 15000], KNM-ER 3228, and KNM-ER 1808). Conventionally, we also tend to think that lifespan increased at this time, along with body size. But the negative relation of body size on longevity in living humans (and possibly other mammals) creates an evolutionary challenge: if both longevity and body size increased (and substantially so in both instances), then some of the genetic changes accompanying this shift must have been under strong antagonistic selection -- pressure to find ways to eke greater lifespan out of larger animals.
Or how about Neandertals and later Europeans? The Neandertals were not taller, but they were bigger. Which is more important? The evidence suggests that Upper Paleolithic people had much higher lifespans than Neandertals. Could this have been a consequence of their smaller bodies? Or did it occur in spite of their greater height? And did the non-European contemporaries of Neandertals, many of whom were presumably smaller than Neandertals, possibly live longer? If so, such a life history difference may have been fundamental to the evolution of modern humans.
Lots of interesting possibilities. It is unclear right now to me whether the size of the size-longevity effect is great enough to account for many of these things (or in the case of early Homo, to affect it markedly). But an 8 percent difference in both size and longevity between men and women does indicate that the effect may well have been on the scale of evolutionary importance.
Samaras TT. 1974. The law of entropy and the aging process. Hum Develop 17:314-320. Abstract
Samaras TT, Elrick H, Storms LH. 2003. Is height related to longevity? Life Sci 72:1781-1802. PubMed