WHY DO WE GROW OLD?
You may have imagined that all living things must inevitably wear out. Cars and vacuum cleaners in daily use eventually stop working. It is easy to suppose that animals age and die in a similar way. But professor of zoology Steven Austad explains: “living organisms are very different from machines. The most fundamental defining character of living organisms, in fact, may be their ability to repair themselves.” The way your body repairs itself after an injury is marvelous, but the routine repairs it makes are, in some respects, even more remarkable. Consider your bones, for example. “Seemingly inert when viewed from the outside, bone is a living tissue that ceaselessly destroys and rebuilds itself throughout adult life,” This remodeling essentially replaces the entire skeleton every 10 years.” Other parts of your body are renewed more often. Some cells in your skin, liver, and intestines may be replaced almost daily. Every second, your body produces about 25 mi