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

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.