While
it is absolutely clear why women experience menopause from biological
perspectives, when you start looking from the global viewpoint, the answer to
this question becomes blurred and distorted. After all, chimpanzees and baboons,
our closest evolutionary relatives who we share 99% of our DNA with, are
reproductive throughout their lifespan but human women can spend the last third
of their lives infertile. Actually, there are just three species on the planet
— humans, killer whales and pilot whales — where females routinely stop
breeding years before the end of their lives.
According
to evolutionary theory, the goal of any organism is to procreate—to pass
on its genes. Thus, it has always been an enigma to evolutionary theorists why
human females live so long after they have lost the ability to reproduce. They
are the only ones in the primate family that have a long post-reproductive
life. It is confusing in and of itself that they lose the ability to reproduce
during their lifetime, as it is quite rare in the rest of the animal kingdom.
Female
Lifespan Increase Hypothesis
One hypothesis to explain menopause is that humans didn’t live as long as we do now with modern medicine and that, in our natural environment (i.e. the African savanna 100,000 years ago), women never would have lived long enough to reach menopause. It is a known fact that modern indigenous people (many of whom live in environments similar to that of our distant African ancestors) have an average lifespan between 35-40. But the high infant mortality in these populations is skewing that statistic to make it seem that adults are dying younger than they are. Most modern hunter-gatherers, if they make it through childhood, will reach a ripe old age. So that can’t be the reason for menopause.
Grandmother
Hypothesis
Dr.
George C. Williams offered the first reasonable evolutionary explanation
in 1957. He hypothesized that menopause is adaptive because it keeps older
women from being exposed to the risks associated with childbirth (which were
much higher for our ancestors). This allows them to remain alive long enough to
ensure their children are raised to maturity to have their grandchildren (thus
continuing the original mother’s gene line). This became known as the
grandmother hypothesis.
Biologist
Virpi Lummaa, whose recent work on evolutionary theory and birth control
has conducted numerous highly regarded studies verifying the role of natural selection
in human populations. Recent Lummaa’s work has also looked at a key prediction
of evolutionary theory, namely, that an individual’s fitness will always be a
trade-off between reproductive and somatic investment. In other words, the
fitness differences (and by fitness we’re talking reproductive success) of an
individual that has numerous offspring at an early age versus focusing instead
on their somatic interests (physical health and growth) in order to reproduce
at a later time.
This
is what is known as Life History Theory.
Many species go through what is called an r-selection strategy and put all of
their investment in as many offspring as possible, many of whom will never
reach sexual maturity. Others take a K-selection approach and produce fewer
offspring but help ensure that each one grows to reproduce. What’s more, these
strategies are not fixed but will frequently shift depending on available
resources.
Utilizing
life history theory, Lummaa suggests that reproductive senescence is actually
an adaptation, and a fitness enhancing adaptation at that. So how exactly would
cutting off 1/3 of an individual’s reproductive potential result in higher
reproductive success? Therein lies the grandmother hypothesis and a key
contribution of Dr. Lummaa’s work.
The
grandmother hypothesis suggests that humans have “given up” their reproductive
potential in later years in order to invest in the children they already have
as well as their grandchildren. Naturally, this is an unconscious, biological
adaptation that emerges over many generations and is not the result of
individual decision-making. For such a hypothesis to be confirmed it would have
to be demonstrated that children are significantly more likely to survive when
a grandmother is present than when she isn’t.
Dr.
Lummaa has done just that in her study published in the journal Nature,
demonstrating that children are 12% more likely to survive to adulthood when
they have a grandmother’s support than when they don’t. That may not seem like
a lot, but consider all of the descendants from that surviving 12%, each
carrying the trait for reproductive senescence, and you can see how it wouldn’t
take long for the trait to become fixed in a population. Furthermore, one of
the key innovations of her study was her choice of sample set. By using Finnish
records dating from the 18th and 19th centuries she could ensure that any
modern health benefits wouldn’t influence the results and would therefore
accurately pinpoint the grandmother’s role.
Absent Father
Hypothesis
While
we are talking mainly to the process, where female is involved, the male
influence should also be considered since you need “two for tango”. The absent father hypothesis proposes that reduced
paternal investment linked with increasing maternal age was an additional
impetus for the evolution of menopause. Reduced paternal investment may have
been linked with increasing maternal age for two reasons: men’s defection from
their middle-aged mates and men’s relatively earlier death. The absent father hypothesis
is not an alternative to the grandmother hypothesis but rather a complement. It
outlines an additional cost—reduced paternal investment—associated with
continued reproduction by ancestral middle-aged women that could have been an
additional impetus for the evolution of menopause.
One
selective pressure associated with the high level of paternal investment in
humans compared to other primates is the relatively helpless nature of human infants.
Without prolonged investment from both mothers and fathers or related kin,
infants and young children were more likely to die before they reached
reproductive age. Children, whose fathers die, for example, suffer a generally higher
death rate than children with fathers.
Reproductive Conflict Hypothesis
A different approach suggests that humans might in
part have evolved this strategy in order to decrease competition between
generations of reproducing women in one family and increase child survival in
times when resources for childrearing were scarce.
Previous research had hinted that competition for
resources, such as food, time and help with childcare between generations
living under the same roof may have been a key factor in the evolution of
menopause—a theory called the Reproductive Conflict Hypothesis. Based on the
same Finnish data, mentioned earlier, researchers noted on potential reduced
competition between the older women and their daughters-in-law, along with
childcare provided for grandchildren, the fact which may explain the evolutionary
benefits of "menopause".
In fact, more often during this time period,
mothers and daughters-in-law lived under the same roof and thus shared the same
resources, according to Lummaa. Daughters, however, typically married off and
moved to live with a husband and his family.
Supporting their idea further, the researchers
found that children born to older woman who were pregnant at the same time as a
daughter-in-law were 50 percent less likely to survive to age 15. Meanwhile,
children born to younger women pregnant at the same time as a mother-in-law,
were 66 percent less likely to survive to adolescence. The researchers suggest
competition for resources may explain this dip in offspring survival. However,
simultaneous pregnancies between mothers and daughters had no significant
effect on child survival.
Patriarch Hypothesis
The Patriarch
Hypothesis is one more theory to explain the occurrence of menopause in
human females as substantial evolutionary advantage. It is an alternative
theory to the grandmother hypothesis which tends to ignore male
benefits of continued spermatogenesis and their roles in assistance.
Patriarch hypothesis suggests selection pressure on
male longevity extended the female lifespan; whose adjustment of life
history has been constrained by the size of the ovaries – resulting in
human females surviving beyond the age at which they can reproduce. Frank Marlowe first
put forward the patriarch hypothesis, suggesting that if women survive beyond
an age at which they can reproduce and men continue spermatogenesis, then
old males stand to benefit greatly if they can copulate with younger females.
It is theorized that increased use of tools and weapons compensates for the
decline in natural fighting ability with age. This serves to produce a more
stable male hierarchy, where attainment of high social status and reproductive
access is less reliant on physical strength.
With such a scenario older males are able to retain
a competitive ability with younger males, thereby asserting a selection
pressure on extending longevity in males that could retain social status.
Higher ranking males may also be a more attractive mate choice. One mechanism
that could extend the lifespan is delaying the age at maturity. Offspring with
a slower life history would exhibit a protracted period of dependence. If
depletion of oocytes occurs at age 50, females should selectively
counter this as it reduces their fecundity.
Recruitment of help from kin and husbands may
compensate by enabling females to reduce birth intervals by weaning offspring
at an earlier age. In addition, by passing on longevity to her sons, a female
would stand to gain inclusive fitness.
Sources and Additional
Information: