Transcript: The history of life on Earth is not a simple linear progression from simple to more complex, from bacteria to us. There have been many twists and turns in this tale, many evolutionary dead ends. Chance effects are important on the history of life, for example, the role of giant impactors from space causing mass extinctions. The arrival rate of these huge impactors is utterly random and cannot be predicted. Chance also plays a role in the microscopic process of mutation which leaves the diversification of genetic material. As the physician Lewis Thomas has said, “The capacity to blunder slightly is the real marvel of DNA. Without this special attribute we would still be anaerobic bacteria, and there would be no music.”