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Time - Man of the Year
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1992-10-19
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SCIENCE, Page 47COVER STORIESIs Sex Really Necessary?
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.
By J. Madeleine Nash/Chicago.