This article was originally published at The conversation. The publication drove the article to Space.com’s. at Expert voices: Op-Ed & Insights.
Hal Drakesmith, Professor of Iron Biology, Oxford University
Jon Wade, Associate Professor of Planetary Materials, Oxford University
Our red blood is full of iron. We need iron for growth and for immunity. It’s even added to foods like grains to make sure there is enough of this nutrient in the diet to help prevent iron deficiency.
On a very different scale, however, iron deficiency may have stimulated evolution during the development of life on planet Earth over billions of years. According to our new research published in the Proceedings of the National Academy of Sciences (PNAS) rising and falling iron levels on our planet may have enabled complex organisms to evolve from simpler ancestors.
The terrestrial planets of our solar system – Mercury, Venus, Earth and Mars – have different amounts of iron in their rock mantle, the layer under the outermost planetary crust. The mantle of Mercury contains the least amount of iron, that of Mars the most. This variation is due to differences in distance from the sun. It is also due to the different conditions under which the planets originally formed their metallic, iron-rich cores.
Related: These 10 super extreme exoplanets are out of this world
The amount of iron in the mantle regulates several planetary processes, including the retention of surface water. And without water, life as we know it cannot exist. Astronomical observations of other solar systems can Enable estimates of the mantle iron of a planet and helps to narrow down the search for planets that can harbor life.
Iron not only contributes to the habitability of the planet, but is also for that Biochemistry that makes life possible. Iron has a unique combination of properties, including the ability to form chemical bonds in multiple orientations and to gain or lose an electron with relative ease. As a result, iron mediates many biochemical processes in cells, particularly by enabling catalysis – a process that speeds up chemical reactions. Vital metabolic processes such as DNA synthesis and cellular energy production are dependent on iron.
In our work we have calculated the amount of iron in the oceans of the earth over billions of years. Then we looked at the evolutionary impact of enormous amounts of iron falling from the seas.
Iron through the ages
The first formative events in geochemistry, which evolved into biochemistry, into life, took place more than 4 billion years ago. And there is agreement that Iron was a key element for this operation. Early Earth conditions were very different from what they are today. In particular, there was almost no oxygen in the atmosphere, so that iron as “iron iron” (Fe2 +) was readily soluble in water. The abundance of nutritious iron in the Earth’s early seas helped life evolve. However, this is “Iron paradiseâShouldn’t last.
the Big oxygenation event led to the appearance of oxygen in the earth’s atmosphere. It happened about 2.43 billion years ago. This changed the surface of the earth and caused one severe loss of soluble iron from the upper ocean and the planet’s surface waters. A second, more recent “oxygenation event”, the Neoproterozoic, occurred between 800 and 500 million years ago. This increased the oxygen concentrations even higher. As a result of these two events, oxygen in combination with iron and gigatons of oxidized, insoluble “iron (III)” (Fe3 +) fell from the ocean water and became inaccessible to most life forms.
Life had developed an inescapable dependence on iron – and maintains it. The loss of access to soluble iron had serious consequences for the evolution of life on earth. Behavior that optimized the acquisition and use of iron would have had a clear selection advantage. We can still see this in genetic analyzes of infections today: bacterial variants that are able to efficiently remove iron from their hosts perform better than less capable competitors over a few short generations.
A key weapon in this battle for iron was the “Siderophore“- a small molecule produced by many bacteria that traps oxidized iron (Fe3 +). Siderophores became spectacularly more useful after oxygenation and allowed organisms to take up iron from minerals that contain oxidized iron. However, siderophores also helped remove iron from stealing other organisms, including bacteria.This shift in focus, from obtaining iron from the environment to stealing other life forms, has a new dynamic of competitive interaction between pathogens and their hosts. Thanks to this process, both parties continued to develop attack and defend their iron resources. Over millions of years, this strong competitive drive led to increasingly complex behavior, which led to more advanced organisms.
However, in addition to theft, other strategies can help manage dependence on a sparse nutrient. One such example is symbiotic, cooperative relationships that share resources. Mitochondria are iron-rich, energy-producing machines that were originally bacteria, but are now live in our cells. Multiple cells clumping together into complex organisms enable rare nutrients to be used more efficiently than single-celled organisms such as bacteria. For example people Recycle 25 times as much iron per day as we ingest it through our food. From an iron-distorted point of view, infection, symbiosis and multicellularity offered different but elegant means for living beings to counteract the limitation of iron. The need for iron may have shaped evolution – including life as we know it today.
The earth shows the importance of being ironic. The combination of both an early earth with bioavailable iron and the subsequent removal of iron during surface oxidation has created unique environmental pressures that facilitate the evolution of complex life from simpler precursors.
These specific conditions and changes over such long periods of time may be unusual on other planets. The likelihood that other advanced life forms will be found in our cosmic neighborhood may therefore be small. However, looking at the iron abundance on other worlds could also help us find such rare worlds.
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