Chapter 1. Longevity
Today, almost every society contains old individuals, and children soon
learn, through language and family, that they have elderly relatives,
who one day will die. This was not always the case, because when
humans first found themselves in natural environments, lifespans were
very much shorter. The major causes of death were disease, attack
by predators and shortage of food or water. A human population can
sustain itself if infant mortality is about 25 per cent (as it is today in
the great apes in their natural environments) and annual mortality is
about 7 per cent thereafter. Under these circumstances the expectation
of life at birth is less than 20 years. Females who reached reproductive
maturity would expect to live to about 28 years and bear on average
about 6 children. These figures are similar for one of the most primitive
tribes in existence today, namely, the Yanomami Indians that inhabit
a forest region of South America. In such societies, aged people are
not very common, and death from old age is certainly the exception
rather than the rule.
In Western societies with good health care the expectation of life
at birth is now about four times higher than in primitive ones: more
than 80 years for females, and just a few years shorter for males.
There is not much infant mortality, and most subsequent mortality
in the following decades is due to accidents, suicide, homicide, or
occasional intrinsic disease, such as cancer. For a population, the shape
of the survival curve is like that shown in Figure 1A. It is clear
that the force of mortality starts to increase only quite late in life.
With constant annual mortality, the survival curve is ‘exponential’ as
shown in Figure 1B. Through history a major factor that increased
longevity was the development of agriculture, which ensured a much
more reliable supply of food. Even so, civilisations as advanced as the
ancient Romans and Greeks had survival curves that were not very
different from exponential, as shown in Figure 2. There was high infant
mortality and expectation of life at birth was only about 25 years,
and at one year was about 34 years. Infectious disease was the main
cause of death, and it was not until most of this was eliminated in the
twentieth century that the expectation of life rose very substantially.
This was due to several major advances: the first was the discovery that
disease was caused by infectious agents, usually bacteria or viruses.
Then hygeine was greatly improved so that the chances of infection
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