It all started in a cloud of gas and dust 4.6 billion years ago. There a star was incubating surrounded by what astronomers call a protoplanetary disk . The dust grains collided with each other, aggregating and becoming larger: it took 1,000 to 100,000 years to form objects a kilometer in diameter. They were the planetesimals , which orbited the Sun in giant rings, much like those of Saturn, surrounded by a disk of gas, essentially hydrogen and helium. Originally there were a few hundred million irregular objects a few kilometers across; the nucleus of Halley’s Comet is a good example of this. After all, the Kuiper Belt, an area between Neptune and Pluto, contains some of these ancient objects and comets .
Planetesimals were very abundant and there were countless collisions: if they collided at low speeds they merged and formed larger objects. Little by little, as matter was accreted, gravity began to play its role : the more massive planetesimals attracted the smaller ones, triggering the accumulation effect; on the other hand, those small objects that managed to avoid the collision were deflected into more elliptical orbits. This produced a split that led to two different populations: while the more massive ones increased in size forming planets, the smaller ones remained as such, forming comets and asteroids.
This is how our planet, Earth, was formed.
collision course
4.5 billion years ago, when the Earth was still forming, a protoplanet the size of Mars and which has been baptized with the name of Theia or Tea (in honor of the mother of the Greek lunar goddess Selene) starred in the largest collision suffered by our planet throughout its history . The impact almost shattered the Earth, but thankfully, it only ripped off a large part of it. Behind him, a dense cloud of rock and dust remained orbiting around the Earth that, little by little, ended up giving shape to what we know today as the Moon . Although we must point out that despite this being the dominant theory to explain the origin of our satellite -some scientists even think that two moons were formed that ended up merging into one-, there are several questions that are still unanswered. Among them, the most important: we do not have a consistent model of how our satellite could have been formed from the debris of the collision .
At that time the Earth was spinning rapidly: the day lasted only 6 hours and the crust was beginning to form. Then, 4.4 billion years ago, the oldest mineral we know of was formed, zirconium , of which we can find tiny crystals in the sand on our beaches.
During all that time the surface of the Earth was a sea of lava, and then the most significant event of the first moments of life of our planet took place: the formation of the Earth’s core. This implies that the magnetic field that surrounds the Earth appeared, which has played a fundamental role in the evolution of life. At the same time, another singular phenomenon, unique on our planet, began to settle: plate tectonics. The Earth was taking shape.
Comet and asteroid shower
While this is happening, from space our young planet is subjected to a very intense shower of asteroids and comets that spans from 4.1 billion years ago) to 3.9 billion years. We find the evidence of that rain of ice and fire, not looking at the ground, but at the sky: most of the craters on the Moon and Mercury are from that distant era. It is estimated that around 20,000 craters with a diameter of more than 20 km and almost a hundred with a diameter of more than 1,000 km must have been formed on Earth , greater than the distance from A Coruña to Cádiz.
It is obvious that prebiotic chemistry had little to do with such a storm: the intense bombardment swept away any attempt at the appearance of complex organic molecules . However, the fall of comets, those balls of dirty snow, brought us an essential compound for life: water. It is estimated that during the Great Bombardment , comets provided between 30 and 50% of the water that exists on Earth today. Certainly, it is extraordinary that almost half of the water with which we shower every morning is of extraterrestrial origin.
But let us not believe that they formed oceans and seas; Earth was too hot for that. It was 3.9 billion years ago when the temperature dropped below 100°C and rain made its appearance for the first time in history . And it did not stop raining for centuries! The clouds, formed thanks to both the water provided by comets and the “degassing” of the planet’s interior, gave way to the oceans, which would never leave us. Well, except for the fall of huge meteorites: the energy released during the impact was such that it vaporized the primitive oceans.
and life appeared
But all storms end and the Late Heavy Bombardment came to an end. We still don’t know very well how it happened, but at the beginning of March the first molecules capable of copying themselves appeared. There are various hypotheses that scientists use to explain its appearance, but unfortunately we will never be sure of what happened. Of course, we have evidence that life already existed on our planet 3.7 billion years ago. They were found in Isua (Greenland), a place where the oldest known sedimentary rocks are also found. That fossil evidence consists of small 1- to 4-cm bumps that look a lot like modern-day stromatolites that we find in, say, Shark Bay in Western Australia.
If the oldest known living thing is to be singled out, that’s the stromatolites . Strictly speaking, they are not really living beings, but the product of the metabolic activity of unicellular beings called cyanobacteria. They group together forming communities that are dedicated to filtering seawater in search of food while expelling the fine sedimentary dust that is suspended in the waters, together with a sticky mucus. The stromatolite is formed in layers, from the inside out , when the grains of sediment join the calcium carbonate of the water, to end up forming a mushroom-shaped structure with a height of 30 cm and a width of about 20. The stromatolites they do not stop growing (about half a millimeter a year) as long as the cyanobacteria survive; they constitute his particular monument to the first forms of known life. And what a monument! Happened to human dimensions, it would be as if we built some mushrooms of 105 by 75 km. The oldest recognized stromatolites were found in the Pilbara, Western Australia, in a place jokingly known as “the North Pole”, because the temperature there rarely falls below 40ºC.
Reference
Delsemme, A. (2001) Our Cosmic Origins: From the Big Bang to the Emergence of Life and Intelligence, Cambridge University Press
