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A new geochemical scenario for conditions conducive to the appearance of life on Earth

A European study involving a CNRS researcher at the Institut de Physique du Globe de Paris, published at the end of December 2020, proposes a new interpretation of oceanic and atmospheric conditions on our planet 4 billion years ago. By suggesting a methane-rich atmosphere and a basic, silica-rich ocean, the scientists believe that structures favorable to the emergence of life could have been set up on a large scale on the surface of the primitive Earth.

A new geochemical scenario for conditions conducive to the appearance of life on Earth

Publication date: 08/01/2021

Press, Research

Related themes : Origins

There is virtually no trace left of the conditions that prevailed on our planet 4 billion years ago. More recent periods in the Earth’s history can be studied by analysing the composition of the rocks and minerals formed during these episodes, making it possible to trace back their oceanic and magmatic conditions. However, very little information exists about the first billion years of the Earth’s evolution, as tectonic and magmatic processes have erased most of the evidence of the planet’s early geological history. Yet it was during this period that life first appeared on Earth, and knowing the conditions that prevailed at the time is crucial to understanding the physico-chemical processes that may have given rise to the earliest form of life.

In a wide-ranging study published in the December issue of the journal Physics of Life Reviews, three researchers from the University of Granada, the Institut de Physique du Globe de Paris (University of Paris, IPGP, CNRS) and the University of Bremen propose a new scenario for the conditions prevailing on the surface of the young planet, conditions that were favourable for the emergence of life.

It is generally accepted that the young Earth was covered by an ocean of magma when its core formed, around 4.5 billion years ago. The name of this geological period, the Hadean (named after the Greek god of the underworld), aptly reflects the perceived image of the planet’s beginnings: an inhospitable world with extreme conditions, bombarded by intense ultraviolet radiation, high temperatures, no liquid water, glowing volcanoes and oceans of magma. However, analysis of zircon crystals 4.4 to 4.2 billion years old shows that oceans may have existed as early as the Hadean. But the precise nature of these early oceans remains controversial. By extrapolating the analysis of rocks formed during the Archean (a more recent era in the planet’s history), previous studies suggest that these primitive oceans were mainly composed of carbonic acid. The authors of this new study now claim that the Earth’s first oceans may instead have been alkaline.

Indeed, the Hadean rocks were probably ultramafic rocks, linked to the intense magmatism of that period. However, chemical reactions between these rocks and the surrounding water tend to make the water alkaline, producing reduced hydrogen and carbon compounds (such as methane). What’s more, samples from the planet’s oldest rock archives are very rich in silica. One of the most plausible hypotheses to explain these silica deposits on the ancient seabed is the existence of alkaline oceans.

The researchers involved in this study explain that if these conditions (methane-rich atmosphere and silica-rich alkaline oceans) existed during the Hadean, then they were ideal for the formation of silica-induced mineral self-organised structures (or MISOS). These structures (inorganic analogues of the first building blocks of life), composed of silica, carbonaceous elements and metal oxides, are capable of self-assembling into complex forms.
These complex forms could favour the emergence of life, on the one hand by acting as a catalyst for prebiotic chemical reactions, and on the other hand by creating a compartmentalisation that allows these chemical reactions not to be diluted in the surrounding fluids. Although these very specific mineral-chemical conditions are now only found in certain exceptional environments, the authors maintain that during periods of the Hadean, these conditions dominated the Earth’s surface environments. They therefore propose that silica played a key role in the development of life on Earth during the first five hundred million years of our planet, transforming the Earth into a large-scale factory of simple and complex carbon compounds.

Silica and carbon biomorph (© IPGP)

Ref: García-Ruiz JM, et al. Mineral self-organization on a lifeless planet. Phys Life Rev (2020). https://doi.org/10.1016/j.plrev.2020.01.001

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