Based on a News.YC recommendation, I'm currently reading "The Selfish Gene" right now and in the book, Richard Dawkins, says at some point billions of years ago a critical fundamental evolution happened - a particularly remarkable molecule was formed by accident, a "Replicator" with the ability to replicate itself. He said the chance of that happening was "exceedingly improbable" and "but in our human estimates of what is probable and what is not, we are not used to dealing in hundreds of millions of years". Improbable accidents (like the one that probably occurred around generation 20,000 of the bacteria) can shape the future.