How ancient ocean water shaped early life evolution

In a groundbreaking study, researchers have made significant progress in understanding the ancient ocean conditions that influenced the evolution of early life on Earth. 

The collaborative effort by scientists from the University of Cape Town (UCT) and the University of Oxford involved recreating Archean seawater to investigate the availability of essential nutrients during this critical period in Earth’s history.

Focus of the study 

The research, led by Dr. Rosalie Tostevin, focused on the preference of early microbes for certain metals like molybdenum and manganese over others like zinc and copper. This preference, according to the study, likely reflected the availability of these metals in the ancient oceans. 

The oldest life forms, evolving over three and a half billion years ago, showed distinct metal preferences compared to more recent life forms.

Formation of greenalite

To explore this further, Dr. Tostevin and her colleague Imad Ahmed recreated ancient seawater conditions within an oxygen-free chamber. They observed the formation of greenalite, a mineral prevalent in Archean rocks, and its role in altering the metal concentrations in seawater. 

“We know that greenalite was important on the early Earth because we keep finding it in old rocks, such as the iron ore in the Northern Cape, South Africa, and similar rocks in Australia. We think this may have been one of the most important minerals in the Archean. But we don’t know exactly how greenalite was forming in nature,” explained Dr. Tostevin.

“One possibility is that greenalite formed deep in the ocean at hydrothermal vents. But it could also have formed in shallow waters, wherever there was a small change in pH.” 

Critical insights

Dr. Tostevin and Ahmed decided to run their experiments under both types of conditions and found that regardless of how greenalite forms, it removes metals in a similar way.

The formation of greenalite was found to deplete metals like zinc, copper, and vanadium, while enriching the seawater in manganese, molybdenum, and cadmium.

Confirming the results

The researchers employed X-ray adsorption spectroscopy at the Diamond Light Source synchrotron to confirm the incorporation of these metals into forming minerals. This process left other metals unaffected, maintaining their high levels in seawater.

“We were very excited when we noticed that our results match predictions from biologists who use a completely different approach. It is always reassuring when specialists in other fields are making similar findings,” said Dr. Tostevin.

Seawater composition 

A crucial aspect of the study was understanding the permanence of these metal changes in seawater. By simulating natural processes like burial and crystallization, the team found that the trapped metals in minerals likely represented a permanent sink, significantly influencing early seawater composition.

This research sheds light on the vastly different composition of Archean seawater, characterized by higher dissolved iron and silica levels and virtually no oxygen, compared to modern oceans. It addresses the longstanding challenge of reconstructing ancient ocean conditions, given the altered chemistry of old sedimentary rocks. 

Ancient seawater conditions

“We can’t go back in time to sample seawater and analyze it, so reconstructing Archean conditions is quite a challenge. One approach is to look at the chemical makeup of sedimentary rocks, but the chemistry of very old rocks has sometimes been altered,” explained Dr. Tostevin.

“We instead decided to create a miniature version of ancient seawater in the laboratory, where we could directly observe what was happening.”

By recreating these ancient ocean conditions in the lab, the team has provided invaluable insights into the environmental factors that shaped the evolution of life on our planet.

The study is published in the journal Nature Geoscience.

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