I found this quote from Vaupel et al. (2003) very interesting:
Death of the frail alters the composition of a cohort, lowering subsequent mortality and possibly offsetting increases in mortality resulting from cumulative damage.
Their paper is a commentary on a study of dietary restriction and longevity in Drosophila (Mair et al. 2003). Dietary restriction is known to promote longer lifespan in many model animals, including Drosophila. Mair et al. (2003) found that the effect of dietary restriction was not necessarily a long-term phenomenon -- instead, they found that the death rates of the flies were strongly determined by what they had been eating in the 2 previous days (!):
These investigators show that when flies fed a restricted diet are switched to a full diet, mortality soars to the level suffered by fully fed flies. Conversely, when the diet of fully fed Drosophila is restricted, mortality plunges within 2 days to the level enjoyed by flies that have experienced a lifelong restricted diet.
Vaupel et al. (2003) press the issue to consider the effects of lifestyle change on human mortality rates:
Following unification of East and West Germany (1989-1990), mortality in the East declined toward prevailing levels in the West, especially among the elderly (8, 9) (see the figure). Although conditions early in life do significantly influence human health and survival late in life (10), the German example--and other demographic data--provide strong evidence that such effects are of less importance (at least in more recent decades) than changes in current conditions (2, 11, 12). The second half of the 20th century saw a radical (and continuing) decline in old-age mortality in most developed countries: in Western Europe, for instance, from 1950 to 2000 the probability of surviving from age 80 to 100 increased 20-fold (13). Most of this increase is due to improvements in economic and social conditions and to ongoing medical advances (14).
Epidemiological and clinical research provide further evidence of the malleability of old age. For example, the risk of death for elderly smokers who quit falls, within 1 or 2 years, to a lower level than that suffered by recalcitrant smokers (15). There is a growing appreciation that even octogenarians and nonagenarians can substantially benefit from medical interventions such as cataract surgery (16) and hip replacement (17). Low-tech interventions at advanced ages can have an important impact. For instance, Fiatrone et al. found that physical training leads to significant gains in muscle strength, size, and functional mobility among frail residents of nursing homes: The oldest person studied (and helped) was 96 years old (18).
The bottom line is that mortality rates are malleable. They are changed not only by improvements in health, nutrition, and environment across the lifespan; they are also improved by short-term changes in older adults. That shouldn't really be a surprise (yes, I mean beyond the fact that this is a 2003 paper!), since most of us know somebody who, thanks to modern medicine, is living with some condition that would have killed them 40 years ago. But somehow medical changes seem to be exceptions, not "natural" in some way. The point of the experimental work and Vaupel et al.'s (2003) commentary is that medicine is not the only late-acting life-extender; broader changes in behavior, stress, diet, or social relationships.
Aristotle contrasted premature death with natural death due to old age--he asserted that nothing could be done about old age (19). More than 23 centuries later, many still believe that death rates at older ages are intractable (20). This view is reinforced by evolutionary theories of aging, which emphasize that senescence is inevitable because the force of selection against deleterious, late-acting mutations declines with age (21). Research over the past decade strongly supports an encouraging alternative--that aging is plastic and survival can be substantially extended by various genetic changes and nongenetic interventions (1-2, 5, 7-18, 20). For most species, damage to cells and tissues accumulates with age, and mortality rises. Nonetheless, aging is so remarkably pliable that interventions do not have to be lifelong. As illustrated by the Mair et al. report and other studies, interventions even late in life can switch death rates to a lower, healthier trajectory.
I'm thinking about this in the context of the new study by Rachel Caspari and Sang-Hee Lee (2006) in AJPA. Here's the abstract:
Increased longevity, expressed as the number of individuals surviving to older adulthood, represents a key way that Upper Paleolithic Europeans differ from earlier European (Neandertal) populations. Here, we address whether longevity increased as a result of cultural/adaptive change in Upper Paleolithic Europe, or whether it was introduced to Europe as a part of modern human biology. We compare the ratio of older to younger adults (OY ratio) in an early modern human sample associated with the Middle Paleolithic from Western Asia with OY ratios of European Upper Paleolithic moderns and penecontemporary Neandertals from the same region. We also compare these Neandertals to European Neandertals. The difference between the OY ratios of modern humans of the Middle and Upper Paleolithic is large and significant, but there is no significant difference between the Neandertals and early modern humans of Western Asia. Longevity for the West Asian Neandertals is significantly more common than for the European Neandertals. We conclude that the increase in adult survivorship associated with the Upper Paleolithic is not a biological attribute of modern humans, but reflects important cultural adaptations promoting the demographic and material representations of modernity.
This would seem to indicate that recent human cultural innovations, as encompassed within the Upper Paleolithic of Europe but presumably shared by all living and recent people, were responsible for a vast increase in the survivorship of younger adults. More people were reaching older ages. This doesn't necessarily mean that these early modern people matched the longevity of historic populations, but it does mean that they must have escaped or delayed many of the major sources of mortality affecting earlier modern and archaic humans. And rather than being an aspect of human phylogeny, specifically the cultural changes indicated by 25,000 years ago appear to have been key.
The work on experimental animals and recent human demography comes together to indicate that nongenetic, environmental changes in mortality can be substantial. Not only can they be great, but they may be highly sensitive -- small changes in parameters may have a large impact on the chance of death. It doesn't take much figuring to realize that a small decline in age-specific mortality may have a large effect on average lifespan.
The death of the strong alters the composition of a cohort also. When a key person dies, it has ripple effects on small human groups. For people living on the edge, when a key person takes a risk, it impacts the entire group's chances of survival. When risks are less necessary, the entire group may benefit.
The secret to understanding the evolution of longevity is to show the channels through which changes in human culture allowed reductions in mortality. These changes are technological, organizational, and -- very possibly -- spiritual. All ways that humans deal with risk.
Vaupel JW, Carey JR, Christensen K. 2003. Aging: It's never too late. Science 301:1679--1681. DOI link
Caspari R, Lee S-H. 2006. Is human longevity a consequence of cultural change or modern biology? Am J Phys Anthropol (in press) DOI link