Exercised: Why Something We Never Evolved to Do Is Healthy and Rewarding

by Daniel E. Lieberman

for what happens when cells compete with each other in a kind of twisted unnatural selection within the body.86 Think of your body as a giant ecosystem with nearly forty trillion cells from more than two hundred different cell lines. Normally, these cells cooperate harmoniously even as they acquire random mutations, nearly all harmless. Every once in a while, however, cells develop mutations that disrupt their function, and a tiny fraction of those mutations trigger cells to compete with each other. When such mutations occur, the cells become malignant. At this point, they divide uncontrollably, migrate throughout the body, and voraciously consume calories. If your immune system fails to kill these cancerous cells quickly enough, they overtake organs, disrupt their function, and starve other cells. The most common cancers occur in reproductive organs, intestines, skin, lungs, and marrow because cells in these tissues divide frequently and are exposed to external influences like radiation, toxins, and hormones that affect their likelihood of dividing or mutating.