Iron is an essential nutrient that almost all life requires to grow and thrive. Iron's importance goes all the way back to the formation of the planet Earth, where the amount of iron in the Earth's rocky mantle was 'set' by the conditions under which the planet formed and went on to have major ramifications for how life developed. Now, scientists have uncovered the likely mechanisms by which iron influenced the development of complex life forms, which can also be used to understand how likely (or unlikely) advanced life forms might be on other planets.