Justin explores the Precambrian period: a kind of dark ages, spanning most of our planet's history, but about which we have very few fossil records. What we do know is that it contained two of the most important developments in evolution. One gave us a breathable atmosphere. The other made possible all the animals that now breathe it. The Natural History Museum's Imran Rahman introduces Justin to this strange bacterial world, while Aubrey Zerkle of the University of St Andrews explains why cyanobacteria may have been the greatest mass murderers in history.