The Science of Sex: Adam and Eve Exposed

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Adam and Eve Exposed index

According to evolutionary psychologists, men will launch a thousand ships, bludgeon one another to death in single combat and do whatever else it takes to get laid. Women are supposedly more cautious and pragmatic in their sex lives. This difference lies in our genes, argue the evo-psych squad — because, in the unending struggle to reproduce, men have stood to gain or lose a lot more than women. It’s a controversial notion, but one that receives some support from a study of our remote ancestors published in this month’s Nature Genetics.

Simply put, the evolutionary psychological argument is this: women generally have about as many offspring as their physiology allows; men, on the other hand, are physiologically capable of leaving huge numbers of offspring, but they also face a significant likelihood of leaving none. Thus, it seems likely that during human evolution, there has been much more variability in the reproductive success of men than that of women. Over thousands of generations this difference could have fostered the spread of genes for gung-ho risk-taking among men, and for caution among women.

Of course, many critics claim that this is so much simplistic mumbo-jumbo. Male-female differences are largely the product of socialization, they claim, or at the very least, it’s difficult to tease apart the influence of nature and nurture. What’s more, sex roles have changed radically over time: How could our ancient genetic heritage play such an important role? These critics also point out, correctly, that we don’t actually know much about what happened during human evolution. “Where’s the data?” they ask.

This month, some data finally showed up, hidden away in a highly technical paper by an international research group led by Peter Underhill of Stanford University. The paper dishes up some evidence that men were indeed more variable than women in their reproductive success, and variable over thousands of generations.

But first, some background. Back in 1991, researchers at U.C. Berkeley coined the term “mitochondrial Eve” to describe a woman who lived in East Africa about 240,000 years ago, a “common maternal ancestor” of modern females. Eve was the source of a tiny part of the human genome, known as mitochondrial DNA, that women inherit exclusively from their mothers. In other words, if any woman on Earth traced her ancestors back up the female line, through her mother, her mother’s mother and so on, for about 5,000 generations, she would eventually come to Eve. (Of course, many other people, both male and female, in Eve’s generation also passed on elements of their DNA — just not their mitochondrial DNA, and not through an all-female line.)

Cut to today — and to a new character in this genomic drama, Y Chromosome Adam. Underhill’s group studied another small part of the genome: a portion of the tiny Y chromosome that men inherit exclusively from their fathers. Using markers on this chromosome, they deduced that all living men have a “last common paternal ancestor” who lived in East Africa about 60,000 years ago. In other words, Eve died some 80,000 years before Adam was born.

A December/May romance indeed. But the fact that Eve lived so many years before Adam is highly significant. It tells us that, during recent human evolution, reproduction has been a far higher-stakes game for men than for women.

To understand this, we need to clarify what “reproductive variability” means. Consider two extreme scenarios. In the zero-variability scenario, each woman in Eve’s generation leaves exactly one daughter, who herself leaves exactly one daughter and so on. In this scenario, obviously, no woman is ever going become an ancestor of all the world’s women. In the maximum-variability scenario, on the other hand, it’s all or nothing: Eve mothers all the women of the next generation, and other women mother none. In this case, Eve becomes a world-ancestor in a single generation.

The longer it took an individual to become the prime ancestor, the more stable their reproductive odds. The fact that we have to reach back 80,000 more years to find a common female ancestor than to find a common male ancestor means that, during human evolution, the reproductive success of men has been much more variable than the reproductive success of women.

It doesn’t matter for the evo-psych argument why reproductive success was more variable for men, just that it was so. Men were always facing the danger of having no offspring and the hope of having many. It seems plausible that this would cause genes for risk-taking behavior to spread among men, just as the existence of huge lottery prizes encourages gambling. Women, on the other hand, had less to gain (reproductively speaking) from risk-taking behavior, so genes promoting such behavior did not spread among women to the same degree.

What were the risky behaviors that men engaged in during human evolution? Some may have been directly concerned with sex, ranging from plying a woman with valuable gifts (risking depletion of resources) all the way to rape (risking death at the hands of the woman and her allies). The main risks, though, were probably those associated with competing with other men for dominant positions and for the access to females that these positions offered. Fighting and the threat of fighting, in other words. It’s no accident that the very molecule that plays the key role in making fetuses male — testosterone — also promotes the development of brain circuitry for aggression. Indeed, much of the aggression that we see today, from war and gang violence to contact sports and other forms of competition, could fit within a broad definition of “unsafe sex.” It’s reasonable to speculate that genes concerned with aggressive behavior were among those that conferred success on Adam and his progeny.

The findings on “Adam” and “Eve” help put the fuzzy science of evolutionary psychology on a sturdier footing. And in the process, perhaps they may help us understand ourselves a little better.