All living beings require small amounts of metals to perform various biological tasks such as breathing, DNA transcription, and converting food into energy, among other vital life functions.
Metals have played this role since the time of the first single-celled organisms in Earth’s primordial oceans. Nearly 50% of enzymes—proteins responsible for facilitating chemical reactions in cells—depend on metals, many of which belong to a category known as transition metals, identified by their position in the periodic table.
A group of researchers from the University of Michigan, California Institute of Technology, and the University of California, Los Angeles, propose that iron was the very first and only transition metal utilized by life. Their findings are shared in the Proceedings of the National Academy of Sciences.
“We present a bold hypothesis: Iron was the first and exclusive transition metal for life,” stated Jena Johnson, an assistant professor at the U-M Department of Earth and Environmental Sciences. “We contend that life relied only on the metals it could interact with, and the iron-rich early oceans obscured the presence of other transition metals.”
To investigate this theory, Johnson collaborated with UCLA professor Joan Valentine and Caltech researcher Ted Present.
Valentine, a bioinorganic chemist, focused on understanding how primitive life evolved from microscopic forms to the complex organisms we have today. She was particularly curious about the metals present in enzymes during early life that enabled essential biological processes. Frequently, she encountered claims from other scientists that Earth’s oceans were teeming with iron during the planet’s formative years.
“It’s important to know that in biochemistry and bioinorganic chemistry, iron is considered a trace element, found only in minimal quantities,” Valentine explained. “So when colleagues told me that iron was not merely a trace element, it was mind-blowing.”
Johnson, who investigates iron formations and early ocean biogeochemistry, along with Present, were aware of geological evidence indicating that early oceans were abundant in iron, particularly in the form of an ion called Fe(II). Fe(II) easily dissolves in water and would have dominated the oceans during the Archean Eon, a geological time frame that lasted from about 4 billion to 2.5 billion years ago.
The conclusion of the Archean Eon was characterized by the Great Oxygenation Event, which marked the evolution of organisms capable of oxygen-producing photosynthesis. Over the next billion years, Earth’s oceans transitioned from an iron-laden, anoxic environment to the oxygen-rich waters we see today, as reported by the researchers. This transformation also converted Fe(II) into Fe(III), making it insoluble.
While Johnson and Present recognized that geologists were aware of iron’s widespread presence during this era, it was only after discussions with Valentine that they grasped the potential influence iron might have had on the earliest forms of life.
To investigate the implications, Present devised a model to update the predictions regarding concentrations of various metals, including iron, manganese, cobalt, nickel, copper, and zinc, that were likely present in Earth’s oceans at the time life originated. The team estimated the maximum accessible concentrations of these elements for primitive life.
“The most dramatic change during the Great Oxygenation Event wasn’t necessarily the concentration of these other trace elements,” Present noted. “Rather, it was the significant reduction in dissolved iron concentrations. This shift in how life ‘perceives’ elements in water had not been thoroughly examined.”
After determining the metals available in early oceans, the team investigated which metals simple biomolecules would be able to bind with in iron-rich solutions.
“We recognized that iron would have to fulfill numerous roles,” Johnson stated. “Biomolecules could capture both magnesium and iron, but zinc wouldn’t fit—perhaps nickel could occasionally bind in specific situations, but zinc would struggle to compete. Cobalt would be nearly undetectable, and manganese would also be largely imperceptible. The vast difference in iron concentration in the oceans significantly affected the binding and awareness of biomolecules regarding elements in their environment.”
To assess whether iron could function in metalloenzymes that currently depend on different metals, Valentine and Johnson explored existing scientific literature to identify how contemporary life utilizes certain metals. They frequently found that iron or magnesium could serve as substitutes. Just because a metalloenzyme typically uses a specific metal, like zinc, doesn’t mean that other metals wouldn’t work as well.
“The comparison of zinc and iron is particularly striking because zinc is undeniably vital for life today,” Valentine said. “The concept of life lacking zinc was initially hard for me to grasp until we investigated and found that, as long as iron remains in its soluble form (Fe(II)), it often works more effectively than zinc in these enzymes.”
Present mentioned that following the oxidation of iron, making it less biologically available post-Great Oxygenation Event, life had to adapt by incorporating other metals into its enzymes.
“Life, with massive amounts more iron than other metals, couldn’t predict to evolve towards a sophisticated management of these elements,” Present commented. “As the abundance of iron diminished, life was compelled to adapt and utilize other metals for survival, allowing for new functions and ultimately leading to the rich diversity of life we observe today.”
