Is Sex Really Necessary?

By J. Madeleine Nash/Chicago

Birds do it. Bees do it. But dandelions don't. The prodigious spread of these winsome weeds underscores a little-appreciated biological fact. Contrary to human experience, sex is not essential to reproduction. "Quite the opposite," exclaims anthropologist John Tooby of the University of California at Santa Barbara. "From an engineer's standpoint, sexual reproduction is insane. It's like trying to build an automobile by randomly taking parts out of two older models and piecing them together to make a brand-new car." In the time that process takes, asexual organisms can often churn out multiple generations of clones, gaining a distinct edge in the evolutionary numbers game. And therein lies the puzzle: If sex is such an inefficient way to reproduce, why is it so widespread?

Sex almost certainly originated nearly 3.5 billion years ago as a mechanism for repairing the DNA of bacteria. Because ancient earth was such a violent place, the genes of these unicellular organisms would have been frequently damaged by intense heat and ultraviolet radiation. "Conjugation" -- the intricate process in which one bacterium infuses genetic material into another -- provided an ingenious, if cumbersome, solution to this problem, although bacteria continued to rely on asexual reproduction to increase their numbers.

Animal sex, however, is a more recent invention. Biologist Lynn Margulis of the University of Massachusetts at Amherst believes the evolutionary roots of egg and sperm cells can be traced back to a group of organisms known as protists that first appeared some 1.5 billion years ago. (Modern examples include protozoa, giant kelp and malaria parasites.) During periods of starvation, Margulis conjectures, one protist was driven to devour another. Sometimes this cannibalistic meal was incompletely digested, and the nuclei of prey and predator fused. By joining forces, the fused cells were better able to survive adversity, and because they survived, their penchant for union was passed on to their distant descendants.

From this vantage point, human sexuality seems little more than a wondrous accident, born of a kind of original sin among protozoa. Most population biologists, however, believe sex was maintained over evolutionary time because it somehow enhanced survival. The mixing and matching of parental genes, they argue, provide organisms with a novel mechanism for generating genetically different offspring, thereby increasing the odds that their progeny could exploit new niches in a changing environment and, by virtue of their diversity, have a better chance of surviving the assaults of bacteria and other tiny germs that rapidly evolve tricks for eluding their hosts' defenses.

However sex came about, it is clearly responsible for many of the most remarkable features of the world around us, from the curvaceousness of human & females to the shimmering tails of peacocks to a lion's majestic mane. For the appearance of sex necessitated the evolution of a kaleidoscope of secondary characteristics that enabled males and females of each species to recognize one another and connect.

The influence of sex extends far beyond the realm of physical traits. For instance, the inescapable fact that women have eggs and men sperm has spurred the development of separate and often conflicting reproductive strategies. University of Michigan psychologist David Buss has found that men and women react very differently to questions about infidelity. Men tend to be far more upset by a lover's sexual infidelity than do women: just imagining their partner in bed with another man sends their heart rate soaring by almost five beats a minute. Says Buss: "That's the equivalent of drinking three cups of coffee at one time." Why is this so? Because, Buss explains, human egg fertilization occurs internally, and thus a man can never be certain that a child borne by his mate is really his. On the other hand, because women invest more time and energy in bearing and caring for children, they react more strongly to a threat of emotional infidelity. What women fear most is the loss of their mates' long-term commitment and support.

The celebrated war between the sexes, in other words, is not a figment of the imagination but derives from the evolutionary history of sex -- from that magic moment long, long ago when our unicellular ancestors entwined in immortal embrace.