WHAT IS HUMAN MORTALITY? Part 1

WHAT IS HUMAN MORTALITY?

Comparing low-mortality human populations to the hunter-gatherer profile reveals dramatic reductions in the probability of death at all ages (4, 5) (Fig. 1A). The percentage improvements have been greatest at younger ages, but the absolute improvements are larger at older ages when death rates are high. For instance, the expected annual probability of death for a 65-y-old hunter-gatherer is about 5.3%; in contrast, for 65-y-olds in Japan today, the chance of death is only abou

t 0.8%. Although mortality drops at all ages, the age at lowest mortality changes very little and the shape of the mortality profile has not changed much despite the huge reductions in its actual values.

Fig. 1.

Life expectancy is increasing in most countries and has exceeded 80 in several, as low-mortality nations continue to make progress in averting deaths. The health and economic implications of mortality reduction have been given substantial attention, but the observed malleability of human mortality has not been placed in a broad evolutionary context. We quantify the rate and amount of mortality reduction by comparing a variety of human populations to the evolved human mortality profile, here estimated as the average mortality pattern for ethnographically observed hunter-gatherers. We show that human mortality has decreased so substantially that the difference between hunter-gatherers and today’s lowest mortality populations is greater than the difference between hunter-gatherers and wild chimpanzees. The bulk of this mortality reduction has occurred since 1900 and has been experienced by only about 4 of the roughly 8,000 human generations that have ever lived. Moreover, mortality improvement in humans is on par with or greater than the reductions in mortality in other species achieved by laboratory selection experiments and endocrine pathway mutations. This observed plasticity in age-specific risk of death is at odds with conventional theories of aging.