Early humans had to move frequently to find food, it is thought, and those who moved the most and found the most food were most likely to survive. Over eons, this process led to the selection of genes that were optimized by abundant physical activity. Physical activity also appears to drive several gene-controlled cellular processes that help promote health. In this way, evolution favored the more active tribesmen, who tended to live longer and could later step in to help with grandchildren, encouraging the survival of active families.
In other words, exercise is good for us, they point out in their new article, because long ago, the youngest and most vulnerable humans needed grandparents, and those grandparents needed to be vigorous and mobile to help keep the grandchildren fed.
Crucially, the new Active Grandparents document also delves into what it is about physical activity that makes it still so necessary for healthy aging today. For one, moving consumes energy that might otherwise be stored as fat, which, in excess, can contribute to diseases of modern life such as type 2 diabetes, Dr. Lieberman and his co-authors write.
Activity also triggers a cascade of effects that make us stronger. “Exercise is a type of stress,” Dr. Lieberman told me. It slightly tears the muscles and strains the blood vessels and organs. In response, a lot of exercise science shows, our bodies initiate a variety of cellular mechanisms that repair tears and strains and, in most cases, overbuild the affected parts. “It’s like you spilled coffee on the floor, wiped it up, and your floor is cleaner than it was,” said Dr. Lieberman. This inner overreaction is likely to be especially important when we’re older, he continued. Without exercise and the repairs that go with it, aging human bodies function less well. We wear out. We can’t take care of the grandchildren.
Crucially, Dr. Lieberman said, lack of exercise during aging explains why there is a difference between human life span (how many years we live) and health span (how many of those years we remain in overall good health) ).
“They used to be the same thing,” said Dr. Lieberman. An inactive early human would not stay healthy and would probably die early. Today, many of us can remain inactive and survive into old age, but we may not remain completely healthy if we do. Our genetic heritage and our history as humans require exercise and movement, Dr. Lieberman said. “Retirement is not the time to slow down.”
This idea that we can, should, and even should stay active as we age, thanks to human evolution, is at the heart of the Active Grandparents Hypothesis. However seductive as the hypothesis is, it is only a theory and almost impossible to prove.
