Natural selection is not natural perfection. Living creatures
have evolved some remarkably complex adaptations, but we are
still very vulnerable to disease. Among the most tragic of those
ills--and perhaps most enigmatic--is cancer. A cancerous tumor
is exquisitely well adapted for survival in its own grotesque
way. Its cells continue to divide long after ordinary cells
would stop. They destroy surrounding tissues to make room for
themselves, and they trick the body into supplying them with
energy to grow even larger. But the tumors that afflict us are
not foreign parasites that have acquired sophisticated
strategies for attacking our bodies. They are made of our own
cells, turned against us. Nor is cancer some bizarre rarity: a
woman in the U.S. has a 39 percent chance of being diagnosed
with some type of cancer in her lifetime. A man has a 45 percent
chance. These facts make cancer a grim yet fascinating puzzle
for evolutionary biologists. If natural selection is powerful
enough to produce complex adaptatio