Why Do We Not Live Forever? The human body is composed not of perishable materials like wood or metal, but of living cells that can grow and replenish themselves. However if the body's 【M1】________ individual units are renewable, why do we not live forever? Theologians explain that Adam and Eve were refused 【M2】________ mortality and expelled from Eden because they disobeyed God. 【M3】________ Evolutionary biologists hold that natural selection favors genes that promote having many offsprings to those that 【M4】________ might insure longer life. In their view, life span is a trade-off 【M5】________ between fertility and longevity, and infinite life would be allied with childlessness and rapid extinction. A chance discovery that a Texas biologist has provided a 【M6】________ different kind of explanation. The biologist, Dr. Lawrence Donehower of the Baylor College of Medicine in Houston, has found that age seems to be a necessary cost of suppressing 【M7】________ cancer. The finding concerns with an intensively studied cellular 【M8】________ agent that is the body's first line of defense against tumors. The agent is known to biologists with the 【M9】________ humble name of p53-p for protein, 53 a measure of its weight. In fact, p53 is one of most important hubs in the living cell's vast 【M10】________ network of interacting components. Many circuits that monitor different aspects of the cell's integrity report back to p53. 【M1】