The Questions of Developmental Biology

However, recent studies have indicated that there are genetic components to senescence,
and that the genetically determined life span characteristic of a species can be modulated by
altering genes or diet.
Maximum Life Span and Life Expectancy
The maximum life span is a characteristic of the species. It is the maximum number of
years a member of that species has been known to survive. The maximum human life span is
estimated to be 121 years (Arking 1998). The life spans of tortoises and lake trout are both
unknown, but are estimated to be more than 150 years. The maximum life span of a domestic dog
is about 20 years, and that of a laboratory mouse is 4.5 years. If a Drosophila fruit fly survives to
eclose (in the wild, over 90% die as larvae), it has a maximum life span of 3 months.
However, a person cannot expect to live 121 years, and most mice in the wild do not live
to celebrate their first birthday. The life expectancy, the amount of time a member of a species
can expect to live, is not characteristic of species, but of populations. It is usually defined as the
age at which half the population still survives. A baby born in England in the 1780s could expect
to live to be 35 years old. In Massachusetts during that same time, the life expectancy was 28
years. This was the normal range of human life expectancy for most of the human race in most
times. Even today, the life expectancy in some areas of the world (Cambodia, Togo, Afghanistan,
and several other countries) is less than 40 years. In the United States, a child born in 1986 can
expect to live 71 years if male and 78 years if female.*
Given that in most times and places, humans did not live much past 40 years, our
awareness of human aging is relatively new. A 65-year-old person was rare in colonial America,
but is a common sight today. Some survival curves for female Homo sapiens in the United States
are plotted in Figure 18.35. In 1900, 50% of American women were dead by age 58. In 1980,
50% of American women were dead by age 81. Thus, the phenomena of senescence and the
diseases of aging are much more common today than they were a century ago. In 1900, people
did not have the "luxury" of dying from heart attacks or cancers. These diseases generally occur
in people over the age of 50 years. Rather, people died (as they are still dying in many parts of the
world) from infectious diseases and parasites (Arking 1998). Similarly, until recently, relatively
few people exhibited the more general human sensecent phenotype: graying hair, sagging and
wrinkling skin, joint stiffness, osteoporosis (loss of bone calcium), loss of muscle fibers and
muscular strength, memory loss, eyesight deterioration, and the slowing of sexual responsiveness.
As Shakespeare noted in As You Like It, those
who did survive to senescence left the world
"sans teeth, sans eyes, sans taste, sans
everything."