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】