Daniel wants to know why a mayfly lives one day, a dog lives fifteen years, a giant tortoise lives over a hundred, and a Greenland shark might live four hundred.

The answer starts with something you can feel right now if you put your hand on your chest.

The pattern scientists noticed is this: in many mammals, the faster the heart beats, the shorter the life. A mouse's heart beats around five hundred times a minute and lives two or three years. An elephant's heart beats around thirty times a minute and lives sixty to seventy years. Many mammals end up going through roughly the same number of heartbeats over a lifetime -- somewhere around one to one and a half billion -- just at very different speeds.

For many years, scientists thought this was a big part of the explanation. Faster metabolism, shorter life. Slower metabolism, long ...