A lot has to go right for a planet to support life. Some of the circumstances that allow life to bloom on any given planet stem from the planet’s initial formation. Here on Earth, circumstances meant Earth’s crust contains about 5% iron by weight.
A new paper looks at how Earth’s iron diminished over time and how that shaped the development of complex life here on Earth. Is iron necessary for complex life to develop on other worlds?
Iron played essential roles on early Earth. Without enough iron, a planet’s surface won’t retain water. And without water, as we all know, there can be no life. (Ok, there might be a tiny possibility of life without water.) But too much iron might be problematic. Too much could be a detriment to the development of complex life.
The paper is titled “Temporal variation of planetary iron as a driver of evolution,” and it’s published in the journal PNAS, Proceedings of the National Academy of Sciences of the USA. The first author is Jon Wade, Associate Professor of Planetary Materials at the Department of Earth Sciences at Oxford.
Life needs water. Cells can’t go about their business without it. But iron is also essential. Cells need iron to duplicate DNA, for instance, and the hemoglobin in our blood transports oxygen throughout the body and relies on iron to do it. Some invertebrates don’t need iron, and some fish don’t, either. But the bulk of living things do.
In this new research, the authors examined iron’s role in the development of complex life here on Earth. From there, they extended their thinking to exoplanets. The chemical composition of distant stars and the mass fraction of iron on planets orbiting those stars could be clues to the development of complex life.
We need to rewind the clock to Earth’s earlier days to understand all of this. Back to when our planet accreted out of the solar nebula.
“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,” said study co-author Jon Wade. “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.”
The mechanism behind iron and surface water retention is a complex one. As evolved iron-bearing magmas cool on the surface of a planet, they can form hydrated minerals. That process binds water into minerals and removes surface water. This may have happened on Mars, where its higher iron content relative to Earth removed water from any potential biosphere. This process may have occurred in Mars’ first billion years.
Things were different on Earth, primarily because of Earth’s lower iron content than Mars. Earth retained surface water because there wasn’t enough iron to bind water into minerals. So Earth’s seawater contained iron in bioavailable forms that life could access.
If you could press pause here, Earth would be a young world with lots of surface water. And living in that water is simple life. The atmosphere is low in oxygen, and the seas are teeming with anaerobic life.
Now wind the clock forward to the Great Oxygenation Event (GOE.) The GOE occurred between 2.4 billion and 2 billion years ago. At that time, the amount of oxygen in the atmosphere and the ocean rose dramatically. It was likely cyanobacteria that did it, and all of that oxygen changed things for Earth. It triggered an extinction among anaerobic life.
Professor Hal Drakesmith is a co-author of this study and a Professor of Iron Biology at the University of Oxford. In an email exchange with Universe Today, Prof. Drakesmith wrote, “Although we use oxygen to breathe and generate energy when oxygen first appeared on the planet, it was very highly toxic to most forms of life. As a result, the Great Oxygenation Event has also been referred to as the Oxygen Catastrophe.”
This is where iron comes back into the picture. “Not only that, but the oxygen was indirectly lethal – first, it combined with iron to make rust, which then dropped out of the seawater, meaning that iron was no longer available for life – and almost all life needs iron,” Professor Drakesmith explained.
“Gigatons of iron dropped out of seawater, where it was much less available to developing life forms,” a press release said.
The loss of iron had enormous consequences for life. What was once plentiful was now scarce. Organisms adapted, and an adaptation that made it easier for an organism to acquire and use iron was advantageous.
Siderophores were one advantage. A siderophore is a small molecule that many bacteria produce. It allows an organism to capture iron from oxidized iron in its environment. So siderophores had an advantage in the new oxygen-rich environment, where so much of the iron was oxidized.
But siderophores could also steal iron from other life. This set up a dynamic between pathogens and their hosts. As this arms race between organisms fighting over iron escalated, they continually developed new ways to attack and defend their iron. And that drove the evolution of more complex life.
But it wasn’t all a war over iron. Some organisms became symbiotic and shared iron. Others became multicellular.
“Life had to find new ways to obtain the iron it needs,” said 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 behaviours 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.”
Iron’s changing abundance and scarcity have played a role in life on Earth. First, its abundance allowed surface water to persist, making Earth habitable. Then its decline spurred on the evolution of complex life. So the conditions that allowed life to flourish in simple forms had to change for complex life to evolve. Is the same true on other worlds?
“It is not known how common intelligent life is in the Universe’ said 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 at planetary scale may be rare, or random, meaning that the likelihood of intelligent life may also be low.”
What does this mean for the search for life? We know of thousands of exoplanets, and we’ll discover many more. How does this help us understand what we’re looking for?
“Given the abundance of both water-rich planetary bodies and life’s elemental building blocks, it is likely that simple life is common in the universe. However, the Earth’s transition to complex, multicellular life required it to clear multiple environmental hurdles, not least those presented by temporal changes in elemental abundance,” the authors write.
It might come down to an exoplanet’s core mass fraction. The relative proportion of an exoplanet’s metallic core to its mantle “…may provide an observable constraint on the iron content of an exoplanet’s mantle and hence, its suitability for the emergence and evolution of complex life,” the authors write. So if we find exoplanets with similar core mass fractions as Mars and Mercury, they’re not good candidates for hosting life, especially complex life.
Instead, we can more closely examine exoplanets similar to Earth in their core mass fractions. If we can assess the amount of iron in an exoplanet’s mantle, we can refine our search for life on other worlds. There’s a kind of sweet spot there that can be a clue, although there’s a lot that scientists don’t understand yet. “Although the Earth appears to occupy a compositional and positional “sweet spot,” the sensitivity of planet habitability to mantle Fe content and how this may change with planet mass remain unclear,” the authors write.
Earth hasn’t stopped changing. Our increased CO2 emissions will lead to diminished iron. “Lastly, back on Earth, continued anthropogenic escalations in atmospheric CO2 are anticipated to increase the prevalence of iron deficiency via relatively poor iron uptake into plants that are iron sources for humans and livestock.”
So iron abundance and scarcity as evolutionary pressure is ongoing. “Therefore, looking to the future, modulation of iron availability on a planetary scale brought about, potentially rapidly, by climate change would be expected to generate selection pressures on hosts and pathogens throughout the biosphere, inevitably impacting human health,” the authors write.
Will Earth life be forced to evolve and develop new adaptations for a lower-iron biosphere once again?