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A collaboration between researchers at the MRC WIMM/RDM and Department of Earth Sciences uncovers the importance of iron for the development of complex life on Earth.

Early Earth (left): 4 billion years ago, in the absence of oxygen, iron was soluble in water, and supported the initiation of simple life.
Later Earth (right): Abundant oxygen means that iron is insoluble in water and scarcely available. The need to acquire and utilise iron more efficiently may have driven evolution of complex life. © Image courtesy of Mark A. Garlick / markgarlick.com

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, a new paper co-led by Professor Hal Drakesmith, group leader of the Iron and Immunity Group in the MRC Human Immunology Unit, and a RDM researcher, uncovers 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 possible on other planets. The work was published today in PNAS.

‘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 year ago (referred to as the ‘Great Oxygenation Event’). An increase in oxygen created a reaction with iron, which led to it becoming insoluble and dropping out of sea water, where it was less available to developing life forms.

‘Life had to find other ways to obtain the iron it needs,’ says co-author Hal Drakesmith, Professor of Iron Biology at the MRC Weatherall Institute of Molecular Medicine, Radcliffe Department of Medicine, University of Oxford. ‘We propose that infection, symbiosis and multicellularity are behaviours that enable life to more efficiently capture and utilise a scarce but vital nutrient. Having to adopt these characteristics would have also been the force that propelled early life forms to become more complex, eventually evolving into what we see around us today.’

The need for iron as a driver for evolution, and that evolution into a complex organisation capable of metabolising iron was successfully achieved, is a rare or random occurrence. This has implications for how likely complex life forms might be on other planets.

‘A difficult question is how common intelligent life is in the Universe,’ says Prof Drakesmith. ‘Our concepts imply that the conditions to support the initiation of simple life-forms are not enough to also ensure subsequent evolution of complex life-forms. Further selection by severe environmental changes may be needed – for example, how life on Earth needed to find a new way to access iron. Such temporal changes may be rarer or more random, meaning that the likelihood of intelligent life may also be also low.’

However, knowing now about how important iron is in the development of life may aid in the search for suitable planets that could develop life forms. By assessing the amount of iron in the mantle of exo-planets, it may now be possible to narrow the search for exo-planets capable of supporting life.

Read the full paper in PNAS.

Listen to an audio summary of the research on the MRC WIMM YouTube page.

Read the audio transcript here.

Read more in an article in The Conversation

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