A piece of the puzzle emerged in the 1970s, when physicists pointed out the apparently coincidental alignment of fundamental properties of the universe and the requirements of living organisms. We live, for example, in a cosmos that makes and disperses plenty of the element carbon, the central piece of known biochemistry. We also live in a galactic landscape of stars that's neither too sparse nor too crowded, and we live at a time when the universe has cooled but not yet succumbed to thermodynamic extinction. If nature's forces were tuned just a little differently, all these things would change, and life might not have ever occurred.