Encyclopedia of Evolution.pdf - Online Reading Center

since their tiny seeds could not grow well in the shade of
the trees; now the K or C plants have the advantage.
Just as the human economy has evolved a great diversity
of different ways of making a living, so the “economy
of nature” has produced communities of plants and animals
that make their living by means of many different life history
strategies.
Charles Darwin, following the lead of economist Thomas
Malthus, noted that all organisms have the capacity to reproduce
far in excess of the space and resources available (see
Malthus, Thomas; origin of species [book]). Darwin
calculated that even a pair of elephants, if they bred as much
as possible and all the offspring survived, could produce 19
million individuals in 750 years. The elephant represents the
K end of the life history spectrum. Species at the r end of the
spectrum have almost ridiculously large potential reproductive
outputs, because the vast majority of their offspring die.
If resources were unlimited and all offspring survived, a pair
of houseflies could produce a population of 200 quintillion in
five months, and a pair of starfish could produce a population
in 16 years as great as the number of electrons in the visible
universe. A single bacterium, dividing every few minutes,
could cover the Earth two meters deep in bacterial slime in
less than two days.
Organisms may increase or decrease their reproductive
investments, depending on the immediate circumstances.
During stressful years with inadequate resources, plants may
produce fewer seeds, and animals smaller litters, than during
years of abundance. Adjustments can be made even after
the next generation has been initiated. Under conditions of
inadequate resources or pollination, plants may abort seeds
or fruits. Spontaneous abortions are common in animals. In
golden hamsters, a mother may even eat some of her own offspring
if resources become inadequate.
The life history of Homo sapiens and its immediate evolutionary
ancestors is a particularly striking example of the
K strategy. One of the major features of the evolution of the
hominin lineage has been the prodigious increase in brain
size; both relatively and absolutely, the modern human brain
is outstanding in the animal kingdom. The first hominins to
Life Histories of Primates
experience the evolutionary explosion in brain size were the
early members of the genus Homo (see Homo Habilis). Once
large brains and their associated intelligence had evolved,
they provided an advantage that justified their tremendous
cost. Much of this brain growth occurs during fetal development
and continues well past childhood (see neoteny).
A human fetus, because of its large head, is born only
with great difficulty. The passage of the head through the
birth canal is the most prolonged, painful, and dangerous part
of the birth process in humans. The birth canal cannot evolve
to be much larger than it is, for otherwise a woman would
not be able to walk. Therefore natural selection favored an
increase in brain size, but also put constraints upon it. The
evolutionary solution to this dilemma was for the fetus to be
born early, while the head was still small. In chimpanzees, the
ratio of the gestation period (in weeks) to the total life span
(in years) is about 0.8. If the human gestation period were the
same length of time, relative to the life span, as it is in chimps
primates, the human gestation period would be about 15
months and birth would be impossible (see table). By being
born after nine months’ gestation, the baby’s head is small
enough to emerge, but the baby is still, in many ways, a fetus,
having undergone only a little over half of the prenatal development
that is normal for primates. Anthropologists estimate
that this aspect of human life history (premature birth) began
with the hominin species Homo ergaster.
This is why babies are born in such a helpless condition.
As a result, they require continual protection and care.
The level of reproductive investment of a human mother is
among the highest in the animal kingdom, for not only is the
baby large when born (up to one-tenth the mother’s weight),
but it requires nursing and continual attention. The consequences
do not stop there. Human mothers can take care of
their babies by themselves, but the baby is much more likely
to survive and be healthy if taken care of by both parents,
and even more so with the help of grandparents (see below).
A human male would be able to produce more offspring by
having multiple mates but might produce more surviving
and healthy offspring by staying home and helping out with
just one mate. This may be the evolutionary basis for the
human family unit. The cascade of evolutionary causation
Characteristic
Ratio of gestation (weeks)
Lemur Macaque Gibbon Chimp Human
relative to life span (years) 1 1 1 0.85 0.54
Infancy* 6% 8% 10% 10% 10%
Subadult* 11% 12% 17% 16% 14%
Adult* 83% 80% 73% 74% 76%
Years to end of reproductive life 18 24 30 40 50
Post-reproductive years 0 0 0 0 20
Ratio of post-reproductive years to life span 0 0 0 0 0.3
*Percentage of years to end of reproductive life
life history, evolution of