In the early 1900s, Carrel’s chicken-heart cells supposedly proved that all cells had the potential for immortality. But normal human cells—either in culture or in the human body—can’t grow indefinitely like cancer cells. They divide only a finite number of times, then stop growing and begin to die. The number of times they can divide is a specific number called the Hayflick Limit, after Leonard Hayflick, who’d published a paper in 1961 showing that normal cells reach their limit when they’ve doubled about fifty times.
