By Robert Martin
Special to CNN
Editor's note: Robert Martin is the A. Watson Armour III Curator of Biological Anthropology at the Field Museum in Chicago, as well as a member of the Committee on Evolutionary Biology at the University of Chicago. He is the author of "How We Do It: The Evolution and Future of Human Reproduction."
(CNN) -- 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.
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