If oxygen is vital for humans, why does it comprise only 20 percent of the atmosphere? Studies of ancient charcoal formations show that the atmosphere contained an even greater (and more variable) fraction of oxygen a few hundred million years ago. However, the increased oxygen content led to a greater threat of massive fires. Humanity arrived on the scene when the oxygen fraction stabilized around the optimal value (for human life) of 20 percent. This just-right timing points to a supernatural Designer.