For over 40 years, biological anthropologist Robert Martin has studied sex. Martin, the A. Watson Armour III Curator at the Field Museum in Chicago -- one of the world's leading natural history museums -- is fascinated by the evolution of mating. He began by studying mating patterns in primates, completing his PhD on the mating behavior on tree shrews (work which led biologists to remove the animals from the primate category) and has more recently focused on the pregnancy, birthing and nursing lessons women today can learn from our ancestors.
Rock campaigned heavily to have the Church accept the pill as a natural form of birth control. But the Pope decided against it, and that has been the position ever since.
Martin's new book, How We Do It: The Evolution and Future of Human Reproduction, touches on a whole range of issues related to sex and mothering in humans. He explores the context--and consequences--behind everything from declining sperm counts to birth control to IV-fertilization. I spoke with Martin about why he sees the rising rate of cesareans as a cause for concern, the Catholic Church's support of the rhythm birth control method as potentially harmful, and allowing nursing women flexible schedules should be a priority for employers.
There's no hard evidence that humans are adapted for either a monogamous or polygamous system. But what's the most common scenario?
About 85 percent of human societies around the world are polygamous. You have a system of one man with two or more wives. A minority, about ten percent, are monogamous. The predominant condition is polygamy. But what happens in practice is that although a society may allow polygamy, a man must have enough resources to have several wives. In a lot of societies that are polygamous, most men are, in fact, monogamous because they can only afford one wife.
How does social structure affect mothering?
The social system doesn't have too much to do with mothering. Primates all share intensive mothering but you can find all kinds of different social systems. You get pair-living primates, monogamous primates, harem-living (one male with several females) and you get multi-male groups.
What makes humans different from other primates?
In terms of mating systems, I don't see where we're truly unique. One thing is you don't have restriction of mating to a two or three-day period which is typical of mammals generally. But monkeys and apes often mate throughout the cycle. I don't see anything unique about us compared to monkeys and apes.
But human babies, compared with other mammals, are particularly helpless. Is there an evolutionary reason for this?
It's because of our big brains. In most primates, a baby develops in a mother's body until it has half of its brain size. A lot of brain growth occurs in the womb. At birth, human brains are a quarter of adult size. We can't give birth to babies any later because the brain will just fit through the pelvis as it is. It takes about a year for us to get to the position where monkeys and apes are at birth. That makes human babies particularly dependent on care for their first year of life, which has implications for social organization. Special social support is necessary.
So a monkey emerges from the womb in a higher-functioning state?
Do you see specific ways humans should raise children, given the helplessness of infants?
A baby can't move around independently like a rhesus monkey or a chimpanzee. Other primates carry the baby around on her fur, but it can move away and explore. Our babies really can't do that. For the first year of life they require special attention. Social support is needed, whether it's provided by men, women, or other members of the group.
Can you talk about the decline of sperm counts?
The evidence is overwhelming: there is a serious problem with declining sperm counts in industrialized countries. The average human ejaculation is 250 million sperm. Why do we need so many? You only need one for the job. The answer is that if you have below 60 million in an ejaculate, you have declining fertility. This is happening to more and more men. We don't have good data from Africa or South America or areas where people are living in non-industrialized conditions, and it would be interesting to see if this is happening there.
Why is it happening in industrialized countries?
Many factors in how we live now are contributing, but it's definitely environmental. There are chemicals in the environment that behave like steroid hormones. BPA is now used extensively. If you get a plastic container, there's probably BPA in it. It hardens plastic and is used for making parts in vehicles and all kinds of things. It also lines food containers--tin cans have it. I recently thought that water pipes might be lined with this: and they do. The water we drink every day is carried through pipes that may contain BPA. Every single person in the United States has BPA in their blood. It's particularly high in babies, especially premature babies. There are some factories in China that have very few controls and have BPA levels that are 50 times higher than they are in the US. One study shows that sperm counts of people working in factories decline as the amount of BPA increases. That's a pretty direct indication.
What are the implications of the widespread use of birth control? Are there specific kinds that are more "natural" or line up with the natural rhythms of a woman's body?