American Scientist

HOW WE DO IT: The Evolution and Future of Human Reproduction. Robert Martin. xii + 308 pp. Basic Books, 2013. $27.99.

One of the great paradoxes of modern biology is that scientists now have the means to take control of human evolution through genetic manipulation and cloning, yet we know precious little about how and why our existing traits arose in the first place. Some of that ignorance is self-inflicted: Misconceptions, cultural taboos, misguided assumptions about gender, and general prudishness have held back research on sex and its influence on the evolution of our species.

Robert Martin’s How We Do It provides a refreshing account of what we do know about the subject, how we got to this stage of awareness, and where we go next. Starting with an overview of sperm and eggs and ending with birth control and in vitro fertilization, Martin, who has been researching these subjects for decades as curator of biological anthropology at the Field Museum in Chicago, takes off the blinders. He puts human sex into the broad context of the genetic, morphological, and behavioral variation that exists in the animal kingdom.

An impressive, recurring theme in Martin’s account: How many unanswered questions remain about why humans are the way we are, sometimes in ways that are distinct from other mammals. No other mammal potty trains its babies, and it remains unclear why people do. (In fact, most mammal mothers swallow their babies’ waste, so perhaps those who are potty training a toddler will not feel evolutionarily shortchanged here.) Breasts are not necessary for effective suckling nor are they reliably correlated with women’s health or nutrition, and no one agrees on why women have them. Some mammalian males do not have nipples. Science has yet to explain why men have retained them.

Most male mammals have a penis bone, but not humans; again, we don’t know why. Men’s sperm counts around the world have mysteriously fallen by about 30 to 40 percent since the 1950s. Not all mammals menstruate, and a slew of difficult-to-test hypotheses attempt to answer why women do so. Another head-scratcher is why women, along with other higher primates, sloths, armadillos, and several bats, have a single-chambered womb, when the evolutionary advantage of this development is unclear. Primate and especially human newborns stand out among mammals for the magnitude of care they require, for unknown reasons—not that that has kept biologists from speculating (an issue Pat Shipman recently tackled in the November–December 2013 Marginalia column).

Throughout How We Do It, Martin shares his fascination with scientists’ repeatedly misguided and bungled attempts to understand our own reproductive biology and how the research unfolds in sometimes alarming, sometimes amusing ways. The first man to observe his own semen under a microscope in 1667, Dutch scientist Anton van Leeuwenhoek, thought his ejaculate was infected by parasites when he saw his sperm. In addition to sex, Martin talks about infant care as an important part of human and primate evolution, a topic rife with culturally influenced misconceptions. One disturbing example is the forced bowel-release schedule for infants that was advised by Victorian physicians. This method involved sticking soap up a baby’s bottom, a practice that was later shown to result in emotionally disturbed children and adults. Martin explains that the timing and mode of potty training varies substantially across cultures, making it difficult to pin down the fittest way to do the job.

Equally unsettling, some of the birth control advice regularly condoned by sexually conservative religions, such as abstinence during ovulation, actually seem to promote fertilization with an old sperm, which studies indicate may result in higher rates of abnormalities and miscarriages. And prudishness has hobbled progress numerous times; for instance, the National Institutes of Health prohibited funding birth control research until 1959.

Cultural squeamishness has led to a series of roundabout and even ridiculous means by which we have learned about the biology of sex. In one instance, Italian priest and biologist Lazzaro Spallanzani outfitted frogs with tight-fitting taffeta pants to show that their semen is necessary for tadpoles to emerge from eggs. In another series of experiments, fertility researcher Derek Robinson outfitted men with insulating underwear or asked them to hold their scrotums over a lightbulb for 30 minutes to establish that heating the scrotum is an effective, if somewhat sultry, form of birth control.

Today some of the old taboos are finally lifting. Martin’s work makes it clear that evolutionary biology is revamping what we think of as “natural” for humans, at times debunking long-held culturally influenced ideas about the most appropriate ways for humans to have sex, procreate, and raise children. These insights are not just scientifically illuminating. The overall well-being of the human population will be improved through better informed decisions about preventing sexually transmitted diseases, raising healthy infants, managing fertility, and keeping reproductive parts shipshape.