‘The initial amount of iron in Earth's rocks is ‘set’ by the conditions of planetary accretion, during which the Earth's metallic core segregated from its rocky mantle,’ says co-author Jon Wade, Associate Professor of Planetary Materials at the Department of Earth Sciences, University of Oxford. ‘Too little iron in the rocky portion of the planet, like the planet Mercury, and life is unlikely. Too much, like Mars, and water may be difficult to keep on the surface for times relevant to the evolution of complex life.’

 

Initially, iron conditions on Earth would have been optimal to ensure surface retention of water. Iron would have also been soluble in sea water, making it easily available to give simple life forms a jumpstart in development. However, oxygen levels on Earth began to rise approximately 2.4 billion years ago (referred to as the ‘Great Oxygenation Event’). An increase in oxygen created a reaction with iron, which led to it becoming insoluble. Gigatons of iron dropped out of sea water, where it was much less available to developing life forms.