In the early moments of the big bang, two elements were forged: hydrogen and helium. Technically, the tiniest traces of light elements such as lithium were produced as well, but nothing of consequence. The early universe was about three parts hydrogen to one part helium. All the heavier elements, from the oxygen in your lungs to the iron in your blood, were created through astrophysical processes—fused in stars, formed in supernovae or scattered by the collisions of neutron stars. Even in the present cosmos, heavier elements are so cosmically rare that astronomers group them all as metals.