1.A GREAT COLLISION AS THE ORIGIN OF OUR PLANET
About 4.568 billion years ago, in a corner of a galaxy,
a cloud of gas and dust created a new star surrounded by a protoplanetary disc,
whose material began to clump together in ever-increasing masses.
In just a few million years, the Solar System looked much like it does today, but between Venus and Mars there was not one planet, but two; according to the most widely accepted hypothesis,
the subsequent collision between the Proto-Earth and Theia,
another Mars-sized object, gave rise to our Earth and its Moon some 4.51 billion years ago.
By pure chance, planet, still incandescent, was within that band that scientists call the Goldilocks zone, like the “just right” bowl of porridge in the fairy tale: far enough away from the Sun that it would not freeze when it cooled, such that liquid water could exist.
2.WATER, THE ORIGIN OF TERRESTRIAL LIFE
But for all this to progress, the presence of water is essential.
It has traditionally been proposed that the fiery early Earth was a dry ball and that water arrived on board the meteorites and comets that fell on the planet in its early days.
However, a recent study suggests that,
despite the contribution of these possible impacts, much of the Earth’s water may have actually formed from the hydrogen and oxygen present in very building blocks of the planet itself.
Thus, about 4.4 billion years ago we had a nearly habitable Earth, covered by a global ocean. But according to the traditional idea, 3.9 billion years ago an intense asteroid bombardment —the so-called Late Heavy Bombardment—would have sterilised all early attempts at life. Although this hypothesis has recently been challenged, the evidence indicates that the cooling of the Earth was rapid; by 3.4 billion years ago, the waters had reached a comfortable temperature of about 40°C. Geological evidence indicates that terrestrial microbes were already present some 3.8 billion years ago, but perhaps the earliest living things can be traced back even further, almost to the very origin of the oceans, before the great bombardment.
3.OXYGEN: THE POISON BEFORE THE ANTIDOTE
The emergence and evolution of early life is a field in which science will have to make further progress, as the clues are still incomplete.
The first single-celled organisms lived in an unbreathable atmosphere, composed of gases such as methane and ammonia. Around 2.4 billion years ago, the so-called Great Oxidation Event took place, when the atmosphere began to be populated by oxygen in its breathable molecular form, which is attributed to the emergence of photosynthetic cyanobacteria.
However, a comparative molecular analysis concludes that these appeared after the Great Oxidation, leaving the emergence of breathable oxygen to other, even more primitive microbes.
However, what made the emergence of complex life possible was at the same time the cause of the first great terrestrial extinction, as oxygen was a poison to other early living things.
In turn, the reaction of oxygen with methane consumed this potent greenhouse gas and produced water and carbon dioxide, leading to a brutal change in climate that enveloped the Earth in ice for 300 million years.
Oxygen was thus the first major cause of extinction on Earth,