The evolutionary background of human reproduction can shed
much-needed light on some of life's most intimate mysteries.
In my book, I trace our
reproductive lifeline back to its earliest roots, revealing the hard science --
and the primate origins -- behind sex cells, mating behavior, gestation and the
way we care for our young.
My main aim is to deal with
issues that are of practical importance in everyday life. In that light, here
are some of the questions that I tackle.
1. Why
are a man's testes located outside
the body?
In most mammals, including all
primates, the testes descend from their initial position next to the kidneys
into a pouch-like scrotum outside the body cavity.
Descent of the testes has been
explained in various ways.
The most widely accepted
explanation is that descent of testes is connected with the raised body
temperature that is typical of mammals. It has often been baldly stated that
sperm production cannot take place at body temperature. We know, for instance,
that infertility results if a man's testes remain in the body cavity. Yet sperm
production clearly can take place inside the body, because some mammals -- such
as dolphins, rhinoceroses and elephants -- produce sperm with testes that do
not descend.
Instead, it seems that a lower
temperature is better for sperm storage. Mammals with undescended testes have
various special adaptations to store sperm at sites where the temperature is
lower.
2. How
many sperm are needed to ensure fertilization?
The short answer is that only one
sperm is needed to fertilize an egg. Indeed, it is disastrous if an egg is
fertilized by more than one sperm as only one set of paternal chromosomes
should be provided.
There are special mechanisms to
prevent more than one sperm from fertilizing an egg. Yet the average human
ejaculate contains a quarter of a billion (250 million) sperm.
Why so many? Several surveys have
shown that men who have ejaculates containing fewer than 60 million sperm tend
to have fertility problems. Moreover, fertility improves as sperm counts
increase from 60 million to about 200 million sperm. But any further increase
seems to have no effect on fertility.
We do know that the large number
of sperm initially ejaculated is whittled down and that only a few hundred end
up close to the egg. Part of that reduction in numbers clearly serves to remove
unsuitable sperm. For instance, deformed sperm are filtered out by the mucus in
the neck of the womb (cervix), and it seems likely that further filtering
occurs in the oviduct (a tube allowing for the passage of eggs from an ovary).
So large numbers of sperm are presumably needed to ensure that one high-quality
sperm reaches the target.
3. Is it true that human sperm
counts are declining?
Since 1974, various surveys have
indicated that human sperm counts have been reduced by about half in several
industrialized populations over the past 60 years or so.
But that conclusion has been
challenged for various reasons, notably because some studies found no evidence
for a decline in sperm counts. It has turned out that this is because there are
marked differences in occurrence and timing between regions. It was also
suggested that reductions in sperm numbers were an artifact arising from
changes in counting methods.
However, the same methods have
been used to assess sperm counts in farm animals such as cattle, sheep and
pigs, and no decline in numbers has occurred. Human sperm counts have not yet
declined to levels where fertility is severely threatened, and decreases may be
leveling off.
More worryingly at this point,
declining sperm counts are accompanied by a marked rise in abnormalities of the
male reproductive system -- undescended testes and abnormal development of the
penis -- in tandem with higher rates of testicular and prostate cancer. The
evidence suggests an environmental effect of some kind that is only affecting
humans. Several factors have been implicated, but prominent candidates are
environmental toxins that mimic the action of steroid hormones.
4. Is there a distinct fertile
window in a woman's cycle?
The "egg-timer" model
of the human menstrual cycle, with ovulation and conception regularly occurring
close to midcycle, has formed the basis for medical thinking and intervention
since the 1930s. This model needs radical revision.
The notion of regular midcycle
ovulation is itself just a statistical abstraction. Menstrual cycles show
considerable variation both in length, routinely ranging between three and five
weeks, and in timing of ovulation relative to menstruation.
More important, however, several
lines of evidence lead to the revolutionary conclusion that sperm are stored in
the human womb, probably in crypts in the cervix. This means that intercourse
leading to conception can occur up to 10 days or more before ovulation takes
place.
This raises major problems for
the "rhythm method" of contraception, which relies on the accepted
wisdom that sperm and eggs have strictly limited maximum life-spans, surviving
for only two days and one day, respectively. In practice, the rhythm method
(even with refinements) is very unreliable.
Worse yet, deliberate avoidance
of intercourse around the time of ovulation can be confidently expected to
increase the risk of fertilization with a time-worn sperm or egg and hence the
probability of fetal abnormality. Various studies have indicated that this is,
indeed, what happens.
5. Why is human birth such a long
and painful process?
Human birth is a challenging
process usually lasting several hours, rather than just an hour or two as in
monkeys and apes. This is because our newborns are unusually large -- twice as
big as babies of great apes -- and have particularly large brains.
During birth, the human baby
rotates in a complex manner, with the result that its head normally ends up
facing toward the mother's back rather than to the front, as in other primates.
Measurement reveals that the size of the newborn's head is pushed to the limit.
Biological dimensions generally fit a standard bell curve, which statisticians
call a normal distribution. But the curves are truncated at the upper end for
the newborn's head and at the lower end the size of the pelvic canal.
In a process called "genetic
pruning," natural selection has acted against over-large heads and small
pelvic canals. The fit is still so tight that the human brain also has to grow
an unusual amount after birth. In other primates, the newborn's brain is about
half its adult size, but in humans it is only a quarter of adult size.
So, despite the large size of
human newborns and their brains, additional rapid brain growth after birth is
needed.