Iron has played a fundamental role in life on Earth since the earliest days of single-celled organisms navigating the planet’s primitive seas.
Employed in minute amounts, metals like iron support crucial biological processes, including respiration, DNA transcription, and metabolism.
Nearly half of all enzymes - proteins that drive cellular chemical reactions - depend on metals, particularly transition metals, named for their position on the periodic table.
New research from a multidisciplinary team at the University of Michigan , California Institute of Technology , and UCLA suggests that iron was the inaugural and exclusive transition metal of early life forms.
"We make a radical proposal: Iron was life’s original and only transition metal," said Jena Johnson, assistant professor in the U-M Department of Earth and Environmental Sciences.
The researchers argue that early life relied on metals that it could interact with in the iron-rich early ocean, leaving other transition metals practically invisible.
To explore this idea further, Johnson collaborated with UCLA bioinorganic chemist Joan Valentine, whose curiosity about early life’s evolution and metal usage sparked the study.
"When these guys told me that iron wasn't a trace element, that blew my mind," said Valentine, highlighting how surprising the iron-rich ancient oceans were in the context of biochemistry.
Ted Present from Caltech contributed to the study by designing a model to estimate metal concentrations in early Earth's oceans, which revealed the stark decrease in dissolved iron levels following the Great Oxygenation Event.
Navigating oceans of iron
This radical proposition arose from the intersection of diverse research interests.
Valentine, intrigued by how early life evolved, wondered what metals were incorporated into enzymes during primitive times to support essential life functions. She repeatedly encountered evidence suggesting the oceans were full of iron for the first half of Earth's history.
Johnson, whose team studies iron formations and ancient ocean biogeochemistry, worked closely with Valentine and Present to explore these early conditions.
Geological evidence points to an iron-dense ocean during the Archean Eon from about 4 to 2.5 billion years ago when Fe(II), a soluble form of iron, was the dominant metal.
Present's model allowed the team to estimate the availability of various metals like iron, manganese, cobalt, nickel, copper, and zinc in the oceans when life first emerged.
"The thing that changed most dramatically as the Great Oxygenation Event occurred was not really the concentration of these other trace elements," Present explained.
"The thing that changed the most dramatically was a decrease in dissolved iron concentrations. The implications for what that meant for life and how it 'sees' elements in water hadn't really been wrestled with."
Life’s unseen lifeline of iron
The study revealed that early biomolecules likely interacted almost exclusively with iron and magnesium, given the high concentrations of Fe(II) in early oceans.
"We realized iron would have to do almost everything. Biomolecules could capture magnesium and iron, but zinc's not getting in - maybe nickel can get into some biomolecules in the right circumstances, but zinc's not competitive. Cobalt is invisible. Manganese is pretty invisible," Johnson noted.
This understanding challenged the prevailing view that certain metals were irreplaceable in biological processes. For example, zinc , which is crucial for life today, was not widely available in the iron-saturated ancient oceans.
"The idea of life without zinc was really hard for me to think about until we dug into this and realized that as long as you have no oxygen around to oxidize your iron from Fe(II) to Fe(III), iron is often better than zinc in these enzymes," Valentine explained.
Evolution of diversity
As oxygen levels increased following the Great Oxygenation Event , the dominant metal in the oceans became less soluble, prompting life forms to adapt by incorporating other elements into their enzymes.
"Life, in the face of orders of magnitude more iron than other metals, couldn't know to evolve toward such a sophisticated way of managing them," said Present.
"The fall of the abundance of iron forced life to manage these other metals to survive, but that also enabled new functions and the diversity of life we have today."
The study, published in the Proceedings of the National Academy of Science s , offers a new perspective on the elemental composition of early life forms and the evolution of life on Earth.
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