Life as we know it relies on small quantities of metals to perform essential biological functions like breathing, DNA transcription, and energy conversion. Since the earliest forms of life floated in Earth’s primordial oceans, metals have played a crucial role in these processes. Almost half of the enzymes—proteins that drive chemical reactions within cells—require metals to function. Many of these are transition metals, named for their position on the periodic table. However, recent research suggests that among all transition metals, iron was the first and only metal essential to the origin of life.
A team of scientists from the University of Michigan, the California Institute of Technology, and the University of California, Los Angeles, published a study in the Proceedings of the National Academy of Sciences proposing that iron was the sole transition metal in early life. We propose a radical idea: iron was the original and only transition metal of life, said Jena Johnson, an assistant professor at the University of Michigan’s Department of Earth and Environmental Sciences. She argued that early life depended solely on metals it could interact with, and the iron-rich ancient oceans made other transition metals essentially “invisible”.
To explore this hypothesis, Johnson collaborated with Joan Valentine, a bioinorganic chemist from UCLA, and Ted Present, a researcher from Caltech. Valentine was particularly interested in how early life evolved from microscopic forms to the complex organisms that exist today. She focused on which metals were incorporated into enzymes during the earliest stages of life to carry out vital processes. Over and over, Valentine heard from researchers that Earth’s early oceans were filled with iron. She explained, In my field, iron is considered a trace element, present in tiny amounts. When I learned that iron was not just a trace element but was abundant, it was a revelation.
Johnson and Present, who study iron formations and the biogeochemistry of early oceans, were already familiar with geological evidence suggesting that Earth’s early oceans were rich in iron—specifically, an ion known as Fe(II). Fe(II) dissolves easily in water and would have been the dominant metal in Earth’s oceans during the Archean Eon, a geological period from about 4 billion to 2.5 billion years ago.
The end of the Archean Eon marked the Great Oxidation Event when life evolved to perform oxygen-producing photosynthesis. This led to a transformation of Earth’s oceans from iron-rich and anoxic to the oxygenated bodies of water we see today. During this transformation, Fe(II) oxidized to Fe(III), making it insoluble and less biologically available.
To understand iron’s potential impact on early life, Present designed a model predicting the concentrations of various metals, such as iron, manganese, cobalt, nickel, copper, and zinc, that could have been available in Earth’s early oceans. They discovered that while other trace metals remained relatively unchanged, dissolved iron concentrations dropped dramatically after the Great Oxidation Event, impacting how life interacted with elements in the water.
The team then explored which metals simple biomolecules would bind to in iron-rich solutions. We realized that iron would have to do almost everything, Johnson said. Biomolecules could capture magnesium and iron, but zinc was less competitive, and metals like cobalt and manganese were nearly “invisible”. This vast difference in iron concentration influenced what biomolecules could “see” and use from their environment.
Valentine and Johnson further investigated whether iron could function in metalloenzymes that currently depend on other metals. They found numerous examples where iron or magnesium could substitute for metals like zinc, which is now considered essential for life. Valentine noted, While zinc is essential today, when there was no oxygen around to oxidize Fe(II) to Fe(III), iron was often better than zinc in these enzymes.
As iron became less available post-Great Oxidation Event, life had to adapt to incorporate other metals into its enzymes. According to Present, Life, facing vastly more iron than other metals, did not know how to evolve a sophisticated way to manage them. The decline in iron forced life to adapt, manage other metals, and ultimately led to the diversity of life forms we see today.
SOURCES
Jena E. Johnson, Theodore M. Present, Joan Selverstone Valentine, Iron: Life’s primeval transition metal. PNAS, September 9, 2024, 121 (38) e2318692121. doi.org/10.1073/pnas.2318692121
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