Human beings may owe their existence to nearby supernovas

IF A SUPERNOVA went off near Earth, that would be bad. From a distance of less than, say, 25 light-years, the resulting bombardment of fast-moving atomic nuclei, known as cosmic rays, would destroy the layer of atmospheric ozone that stops most of the sun's harmful ultraviolet light reaching Earth's surface. In combination, these two kinds of radiation, cosmic and ultraviolet, would then kill many forms of life. If a supernova went off not quite so close by, though, that might be interesting. It...