How Important is Iron to the Origin of Life in the Universe?

Exoplanet with Iron

 

New research has shown that iron (Fe) can catalyze metabolic reactions without enzymes. Findings suggest that the abundant metal might have played a key role in early biochemistry before enzymes evolved. Researchers at the University of Oxford uncovered the importance of iron for the development of complex life on Earth – which also may hint at the likelihood of complex life on other planets.

Iron is an Essential Nutrient that Almost All Life Requires

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, report University of Oxford researchers in PNAS. Now, scientists at the University of Oxford 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. The work was published today in PNAS.

“Not Too Little, Not Too Much”

‘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.’

“Earth’s metallic core generates our protective magnetic field which is crucial in making the Earth’s surface a (mostly!) pleasant and habitable place for life,” says Wade, “But core formation also determines the elemental make-up of Earth’s rocky mantle, and determines the abundance and availability of a number of elements used across life, perhaps most notably iron (Fe). The terrestrial planets – Mercury, Venus, Earth and Mars – all have compositionally similar rocky mantles, but vary primarily in their iron content, which in turn reflects the different environments of planetary accretion. But what role does mantle iron content play in keeping water on the surface of Earth for timescales that allow the evolution of complex (and occasionally intelligent) life?”

“From the perspective of planets, the elemental composition of the mantle largely depends on whether the planet has undergone metallic core formation or not,” Wade told The Daily Galaxy. “A planet with no core at all implies it is either very small,  has undergone accretion under very wet circumstances such that all the iron is oxidized, or that it was missing the heat producing elements that result in early differentiation. A ‘no core’ planet is something we haven’t considered, partly because it would be pretty exotic!”

 

 

“In evolution, things always start small,” says Greg Fournier, associate professor of geobiology in MIT’s Department of Earth, Atmospheric and Planetary Sciences referring to the Great Oxidation Event, the evolutionary moment that made it possible for oxygen to eventually accumulate in the atmosphere and oceans, setting off a domino effect of diversification and shaping the uniquely habitable planet we know today.

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. 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.

Alien Earth

Early Earth on the left, had seas infused with life-enhancing iron, whereas Earth today, seen on the right, does not. (Mark A. Garlick / markgarlick.com)

Rise of Oxygen –The Evolutionary Moment that Shaped Our Habitable Planet

Iron is a Driver of Evolution and Complex Life

‘Life had to find new ways to obtain the iron it needs,’ says co-author Hal Drakesmith, Professor of Iron Biology at the MRC Weatherall Institute of Molecular Medicine, University of Oxford. ‘For example, infection, symbiosis and multicellularity are behaviors that enable life to more efficiently capture and utilize this scarce but vital nutrient. Adopting such characteristics would have propelled early life forms to become ever more complex, on the way to evolving into what we see around us today.’

The need for iron as a driver for evolution, and consequent development of a complex organism capable of acquiring poorly available iron, may be rare or random occurrences. This has implications for how likely complex life forms might be on other planets.

“It is not known how common intelligent life is in the Universe’ says 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 at planetary scale may be rare, or random, meaning that the likelihood of intelligent life may also be 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.

Can Life Exist on a Planet Without Iron?

When asked about the possibility of an Fe substitute in the origin of life beyond Earth, Jon Wade replied in an email to The Daily Galaxy: “If a core is present, which is likely given our knowledge of the terrestrial bodies,  and the planet had formed under reducing conditions (like Mercury), the rocky mantle will be depleted in not just Fe but also the other transition elements, all of which will be sequestered into the core during formation. This is something that isn’t maybe widely appreciated – no Fe, less  Ni, Co, Cu and even less Mo and W.  Fe is used by life because it is the one major rock forming element that undergoes valence changes on the surface of the Earth, and is cosmochemically abundant, unlike the elements post Fe in the periodic table.  Could anything else be used?  Maybe sulfur, as it is cosmochemically abundant is present in appreciable amounts in the rocky parts of reduced bodies, does have lots of available valence states and is commonly found associated with Fe in biology.  But absent of Fe, life may have a hard job finding elements to utilize with sulphur.

“The possibility of an Fe substitute in the origin of life beyond Earth is a very difficult question,” Alexander Drakesmith wrote in an email to The Daily Galaxy. “It is the polyvalency of iron under Earth conditions that is likely crucial,” he explained, “as well as its abundance. 

On Earth, the only two known non-iron-requiring organisms are i) Lactobacilli, and ii) a causative agent of Lyme disease, Borrelia burgdorferi, both of which appear to rely mostly on manganese instead of iron for at least some of their redox activities. This is important because it does show that metabolism does not absolutely require iron; there are potential alternatives. 

“Across the Kingdoms of life, molybdenum is also important in some enzymes (notably nitrogenases), as is vanadium (also polyvalent),” Drakesmith notes. “However, because of the relatively huge abundance of iron (due to its nuclear stability), it is hard to imagine a planetary environment with vanadium, manganese and molybdenum but not with Fe. Perhaps, if there was limited availability of Fe but relatively high amounts of a range of other metals that had useful properties in terms of biochemistry, the combination of the other metals could substitute for Fe – rather than one metal in particular.” 

A.H.C. Bonsor, a Royal Society Fellow at the Institute of Astronomy, University of Cambridge, told The Daily Galaxy: “I’m not clear whether Fe is required for life… with life postulated to occur in aerial biospheres etc, the options broaden.”

Avi Shporer, Research Scientist, with the MIT Kavli Institute for Astrophysics and Space Research via Hal Drakesmith, Jon WadeUniversity of Oxford and PNAS

Image credit top of page: Shutterstock license.

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