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You probably know that the human body needs iron to keep going. However, if Earth wasn’t fortified with iron (kind of like your cereal), life here might have never existed.
When the solar system was still forming, the amount of iron that is now in Earth’s mantle and core was decided by whatever rocks smashed together to form the protoplanet that eventually accreted enough material to become a decently sized rocky planet. It also ended up with just the right amount of iron in its mantle to to keep water on the surface — too much, and water would have been absorbed by it. This could be one of the reasons life somehow spawned.
Earth already has an outer core of liquid iron that powers our magnetic field to block out potentially deadly space radiation, but it isn’t accessible to organisms on the surface. The need and search for iron drove organisms to evolve. Researchers Alexander Drakesmith and John Wade of Oxford University coauthored a study, recently published in PNAS, looking into the emergence of life on this planet and how iron was one of the things that helped it find a way.
“Mars contains significantly more iron in its mantle — over twice that of Earth, but the high iron content of Martian rocks may be responsible for the rapid loss of water from the surface of Mars,” Wade told SYFY WIRE. "Hence, Mars, the red planet, went rusty very early on.”
Rocks on rocky planets usually contain Fe2+, which is iron in its chemically reduced iron oxide form. That is the one that is thought to have given life an assist when Earth was just yawning into being. It can also explain part of the reason Mars is so dry and desolate, besides its lack of a magnetic field, which means constant bombardment with killer radiation. The Red Planet is so red because iron in much of its rock has morphed into the more oxidized Fe3+. Higher levels of iron in rock mean there is a greater tendency for the iron to take this form and absorb water.
It wasn’t just on land that iron helped life get ahead. Before the Great Oxygenation Event, there was not much oxygen in Earth’s atmosphere. While we may not have been able to breathe it in, the lack of atmospheric oxygen meant that iron could dissolve at the shores of ancient lakes and seas. Early aquatic organisms would have been able to benefit from that with their regular water intake. When the atmosphere started to overflow with oxygen, that iron oxidized into rust. It was no longer soluble and sank to the depths where almost nothing could reach it.
“Because iron is crucial for supporting the basic functions of life — making energy, creating DNA, growing—early simple life forms could get all the iron they needed with little effort,” Drakesmith said. “When it became more difficult for life to obtain, life adapted.”
Parasitism and infection were the more nefarious methods of organisms figuring out a way to get the iron they need. Finding different ways to feed off of other organisms gave parasites and pathogens the iron they needed to stay alive, whether or not the victim survived. Some forms of life found ways to cooperate instead of stealing from one another. Symbiosis came out of both wanting to benefit. When our cells get a hold of iron, they give it to the mitochondria (the powerhouse of the cell) which then uses it to create energy that goes right back to the cell.
As organisms grew more complex and many went from unicellular to multicellular, they also became able to recycle their own iron. There are always red blood cells dying in our bodies. Human red blood cells that die are broken down, and the iron in them is reused in the creation of new cells. The rise of more complex life led to more consumption of iron that led to the evolution of further adaptations. So at least we have an idea of how iron both supported early life and caused it to evolve on Earth, but what does that mean for other planets out there?
“Given how the biology of life is so focused around the acquisition and defense of iron, I personally hope we don’t bump into any complex alien life forms,” Wade said. “After all, there is a reason nature is red in tooth and claw!”