Around 3.5bn years ago the first forms of life emerged: bacteria and archaea. These so-called prokaryotes had the Earth to themselves for a very, very long time. Then, for some mysterious reason, another new microbial kingdom formed. Eukaryotic cells came into being and complex life began. But how and why did this happen? Hannah Devlin dives into the 12-year scientific odyssey that gives us an important piece of the puzzle
