Transcript: The idea of contingency was invented by Alfred Russell Wallace, a rival to Charles Darwin and co-discoverer of the idea of natural selection. In contingency, life diversifies according to genetic principles and then is pruned back according to adaptation to the natural environment, but the pruning or culling of life has an element of chance, of a lottery. Paleontologist Steven J. Gould, in his book Wonderful Life, wrote about the diversification of life in the oceans of the Earth during the Cambrian explosion. He argued that it was impossible with hindsight to decide which of the many body plans invented during this period would have or could have survived according to natural selection. The element of chance was strong. He went further and argued that if you replayed the tape of life on Earth under identical conditions, it would not be obvious that eventually you would get mammals or apes or Homo sapiens. We can see contingency in the Cambrian survivors and also in the more famous incident where the mammals and the dinosaurs coexisted. Rather than thinking of the mammals as natural and obvious successors to the more primitive dinosaurs, we should realize that mammals and dinosaurs coexisted successfully for over a hundred million years before the dinosaurs were wiped out by a chance event, the impactor from space.