Scientists have long known that metals play an important role in life.
Tiny amounts of metals help living organisms perform essential tasks like breathing, copying DNA, and turning food into energy.
Now, a team of researchers has found evidence that iron was the first and most important metal used by early life forms on Earth.
The study, conducted by scientists from the University of Michigan, Caltech, and UCLA, argues that life billions of years ago relied almost entirely on iron.
The findings, published in Proceedings of the National Academy of Sciences, suggest that other metals used by living organisms today, like zinc and copper, were not available in the same way in Earth’s early oceans.
Around 4 billion years ago, during a time called the Archean Eon, Earth’s oceans were rich in iron.
A type of iron called Fe(II) dissolved easily in water and was the most common metal available. Other metals, such as zinc, manganese, cobalt, and copper, were much rarer or harder for early life to use.
At the end of the Archean Eon, about 2.5 billion years ago, the Great Oxygenation Event transformed Earth.
Life evolved the ability to produce oxygen through photosynthesis, which led to major changes in the planet’s environment.
Oxygen turned Fe(II) into Fe(III), a form of iron that doesn’t dissolve well in water. As a result, the abundance of iron in the oceans dropped dramatically, forcing life to adapt by using other metals.
The researchers used models to estimate how much of each metal was available in Earth’s early oceans.
They found that the high concentration of iron meant it played a key role in the enzymes (special proteins) that carried out life’s chemical reactions.
In fact, they believe early life forms relied on iron for nearly all of their metal-based biological functions.
“During this time, iron was so abundant that it did almost everything for early life,” said Jena Johnson, one of the study’s authors. Metals like zinc, manganese, and cobalt were far less available, so they weren’t widely used in early biology.
When oxygen became more common in the atmosphere and oceans, iron’s availability plummeted. Organisms had to adapt by using other metals to survive.
For example, many enzymes today depend on zinc, which is essential for life now. However, the study found that before oxygen was present, iron often worked better than zinc in the same enzymes.
This shift in available metals pushed life to evolve new ways to use different elements. According to the researchers, this transition helped enable the diversity of life we see today.
The study reveals how much of an impact Earth’s changing environment had on life’s evolution. Early life forms were shaped by the abundance of iron, and when oxygen levels rose, they had to adapt to survive. This insight helps scientists understand how life evolved and adapted on our planet billions of years ago.
