Raymond Pierrehumbert, holder of the Halley Professorship of Physics at Oxford, gives the 2017 annual Wolfson Haldane Lecture. The lecture is introduced by Hermione Lee, College President. The Proterozoic is the period of Earth history extending from approximately 2.5 billion years ago to 550 million years ago, and makes up something over half of all Earth history to date. It begins with a dramatic rise in oxygen in the atmosphere, global “snowball” glaciations, and major disturbances of the carbon cycle, and ends with another period of carbon cycle fluctuations accompanied by the two Snowball glaciations; shortly after the exit from the second of these, the first multicellular life appears in the fossil record, and not long thereafter comes the Cambrian explosion. However, between the two eras of great climate disruption extends a period of about a billion years in which nothing much is happening, either from the standpoint of evolutionary innovation (insofar as visible for single-celled life in the fossil record) or from the standpoint of glaciation or biogeochemical cycling. This is the “boring billion” — the geological waiting room for the modern era of the Phanerozoic leading to the appearance of intelligent life on Earth. But what was the pacemaker determining the exit from the Boring Billion? Were we unlucky in the duration of the wait? Were we just lucky, and could it have been the Boring Two Billion? That would have in fact precluded the emergence of complex life on Earth, or any other planet orbiting a star like the Sun, since the gradual brightening of a Sunlike star over time throws an Earthlike planet into a runaway greenhouse state after about 4.5 billion years (roughly a half billion years from now), whereafter the planet loses its oceans and turns into an uninhabitable Venus-like world. Thus, the nature of the Boring Billion, and the factors that terminated it, have a very great bearing on whether we are alone in the universe. Dim red dwarf stars, which age more slowly than Sunlike stars, are known to have planets and perhaps offer more chances for complex life to emerge, but have their own challenges, which will also be discussed in this lecture.