It is an established fact. Life appeared on Earth very early in its history. But what incredible chain of events could have occurred to lead to the emergence of the very first living organisms? To understand it, we must go back in time and explore this young, rather inhospitable planet that was Earth four billion years ago.
Imagine… An ocean as far as the eye can see… Here and there, a few islands whose numerous volcanoes belch dark smoke… The sky is laden with heavy clouds, and the faint glow of the young SunSun struggles to penetrate this leaden cape. But that’s okay. Because beneath this shell, it’s already stiflingly hot, excessive. The immense ocean of this Dantesque planet even reaches a temperature of 70°C.
We know this strange and somewhat disturbing world well. It is ours. But to recognize it, we must go back in time four billion years. Only five hundred million years have passed since the formation of the Earth. That is not much, if we consider the drastic changes that have taken place during this period of time: formation of the Moon, constitution of an atmosphere, crystallization of the ocean of magma… In short, only five hundred million years to go from the state of a ball of magma to an ocean world.
From the formation of the Earth to the first continents
Of course, the landscape is still very different from the one we know. But many elements specific to our Planet are already there: liquid water, an atmosphere composed mainly of nitrogen and carbon dioxide, and plate tectonics in its infancy.
So many ingredients that will play an essential role in the emergence of life and its development. Earth owes these characteristics to its position within the young Solar System. The planet is in fact in just the right place. Its orbit is neither too close nor too far from the Sun, which allows it to maintain a vast ocean of liquid water on its surface. Its size is also optimal, which prevents its atmosphere from escaping.
Its internal heat, accumulated during its formation by the collision of planetoids, will allow thermal instabilities to be initiated at the bottom of the mantlemantle and to start a mantle convectionconvection. These upwellings of hot material will weaken the solid and immobile crust formed following the crystallization of the primitive magma ocean, and tear it apart. Thus, plate tectonics would have been started, approximately 4.3 billion years ago, via the formation of the first subduction zonessubduction zones.
By plunging into the depths of the Earth, this primitive crust will modify the chemistry of the mantle and cause its partial fusion. Magma forms, rises and erupts on the bottom of this primitive ocean. Underwater volcanoes are formed. They will grow, until they form emerged islands. These are the cores of the very first continents.
Constitution of the basic bricks of life: the role of alteration
Listen: everything is calm in this desert landscape, certainly very monochrome… On the shore of these islets composed of volcanic rocks with sharp edges, only the backwash of the waveswaves comes to disturb the heavy silence, punctuated sometimes by the thunderclaps that resonate on the sharp foothills of the volcanic mountains. A fine and acidic rain begins to fall on the bare and totally virgin ground of these first emerged lands. Difficult to realize it, but, in this apparently anecdotal scene, an essential process in the evolution of the Planet is playing out.
As it flows down the steep slopes, this freshwater will slowly alter the igneous rocksigneous rocks, gnaw at them, peel them, taking with it certain elements that will play a decisive role in the rest of the story. These are phosphorusphosphorus and potassiumpotassium. Two “building blocks” of life. As the streams and rivers form, these nutrientsnutrients, which today constitute the skeleton of our DNADNA and RNARNA, will end up in the ocean.
Although the alteration of the first emerged lands thus allows the ocean to be progressively enriched with elements essential to the development of life, the conditions are still far from being favorable. This primitive ocean is in fact acidic, and highly enriched in halogenshalogens (chlorinechlorine and fluorinefluorine in particular), sulfursulfur, nitrogennitrogen and various metalsmetals. A chemical composition not very conducive to the emergence of life.
Four billion years ago, the Earth appears, at first glance, as a dead world. But only on the surface. Because at the bottom of the ocean, something is happening.
See you in the next episode to find out the rest of this great story.
