A Tale Of The Earth

The Solar System formed 4.6 billion years ago from the gravitational collapse of a region within a large interstellar cloud which was big enough to probably birth several stars. This formation of the Sun and the eight planets orbiting it took its time. For a start, 50 million years just for having, in the core of the protostar -an "embryo" of what would become the Sun-, a pressure and density of hydrogen high enough to begin the thermonuclear fusion -the conversion of hydrogen into helium, which is the reaction that releases the Sun's energy-.

Eventually, the Sun was born and then the 8 planets around it were born too. And no one would have ever said that in the third planet, at an average distance of 150 million km (147-152), a series of miracles would lead to the appearance of life. So life as we know it on Earth is nothing but an accident, or even most amazingly, a chain of accidents. Isn't it incredible?

Many things have happened on Earth for such an achievement. From one end, it seems that the planet is at a correct distance from the Sun: not too close and at the same time not too far, in order to have a suitable light and heat from the star, as well as a huge amount of water on liquid state (over 70% of the planet is covered by water, which contains about half of the planet's species). But this alone would not be enough, and on our planet things were not always so easy for the development of life. The geological history of the planet is full of challenges that life forms, and their very self-survival, had to deal with.

Earth's atmosphere and oceans formed by volcanic activity and outgassing that included water vapour. The origin of the world’s oceans was condensation augmented by water and ice delivered by asteroids, proto-planets and comets. Atmospheric “greenhouse gases” kept the oceans from freezing while the newly forming Sun was only at 70% luminosity. By 3.5 billion years ago, the Earth’s magnetic field was established, which helped prevent the atmosphere from being stripped away by the solar wind.

A crust formed when the molten outer layer of the planet Earth cooled to form a solid as the accumulated water vapour began to act in the atmosphere. Continents formed by plate tectonics, a process ultimately driven by the continuous loss of heat from the Earth's interior. On time scales lasting hundreds of millions of years, the supercontinents have formed and broken up three times.

The ice ages began about 40 million years ago and then intensified during the Pleistocene about 3 million years ago. High-latitude regions have since undergone repeated cycles of glaciation and thaw, repeating every 40–100,000 years. The last continental glaciation ended 10,000 years ago. There is a very curious (and important) correlation between the ice ages and the composition of the Earth's atmosphere, and also with some milestones of the evolution of life on Earth.

There was a moment when the atmosphere contained a lot of methane (about a 1000 times as much as there is now), which is a very strong greenhouse gas (it gives a greenhouse effect higher than that of carbon dioxide). This methane was generated by the volcanic activity and contributed to a quite warm temperature in the planet. As life evolved in the oceans and organisms able to do the photosynthesis appeared, oxygen was a harmful by-product which was released into the water and then it bubbled until it reached the atmosphere. 

What happens when oxygen meets methane? Methane burns, reacting with oxygen to produce carbon dioxide and water. So after a time, oxygen almost depleted the atmosphere from methane, and the temperature decreased very significantly. That was a hard period for life survival, but life did not disappear from the planet.

Another difficult period was the "Snowball Earth", in which the temperatures went very low once again, and almost the whole planet was covered by ice. If snow falls for a long time and layer on layer, it forms ice by compression. And the white colour reflects the light, so it makes the temperature go yet colder. The "Snowball Earth" lasted quite a long time, and still, life resisted. 

Eventually, the volcanic activity generated enough carbon dioxide which increased again the global temperature and finally melted the "snowball". After this melting, many multicellular life forms appeared on the planet, and this was accompanied by a new increase in the oxygen content of the atmosphere. It even got to a point with an atmosphere having 30% of oxygen for a time in which animals grew very big in size!

But what I find most amazing is, how did life actually begin?

Earth formed, like the other seven planets of the Solar System, by colliding masses that melted and got stuck together, and in the early times many more asteroids and comets came to meet Earth on their way. Blame it on the gravity. At a point, the different layers were established (nucleus, mantle and crust), and the volcanic activity began. The water vapour condensed (blame it, again, on the gravity) and rain fell to form the oceans. And once the oceans were on the surface of the planet, their water allowed many interesting chemical reactions to take place.

It is generally accepted that small molecules dissolved in the water formed the first amino acids (the building blocks of the proteins), and this was the starting point of it all. It is surprising that this happened to such a great extent, but to add a note of curiosity, Friedrich Wöhler synthesized, in 1828, urea (an organic compound) from ammonium cyanate (an inorganic substance), and in 1845 Hermann Kolbe did something similar by synthesizing acetic acid from carbon disulfide. So why would not react ammonia salts and other small inorganic molecules to form the amino acids (organic compounds) that are part of the proteins? Then, the amino acids would combine between themselves to form proteins, and these proteins would have particular three-dimensional structures which would allow specific functions. 

What would be the next step towards life? The definition of living organism includes “the capacity to self-replicate” (more extensively, “organisms are capable of responding to stimuli, reproduction, growth and development, and maintenance of homeostasis as a stable whole”). So here is one of the great mysteries of life: how is it possible to achieve a self-replicating complex form like a cell from just proteins?

In another post I will explain the relationship between the DNA and the proteins, but for the moment let’s say it kind of went in a "reverse mode". From the proteins, somehow the biological information was finally stored in another molecule, the DNA, which can be copied when the self-replication comes. The DNA has a constant structure, the double helix, as opposed to the many different structures that proteins can have, and it can be coiled and packed to store a lot of information in a much smaller space (the 3D structure of proteins make them quite big, and they need to keep that big 3D structure to be "operative"). So DNA has a great advantage in front of the proteins when it comes to storing information.

Then the complexity increased progressively. The different “biomolecules” (molecules present in the life forms) self-assembled first into cells to form unicellular organisms. The first unicellular organisms, called prokaryotes, were smaller cells and without a defined cell nucleus (the DNA was not in a separate compartment from the rest of components of the cell). Bacteriae are microorganisms that belong to the prokaryotes.

Later, some prokaryotes specialised and started living in symbiosis with larger cells, so the next milestone of the evolution was achieved: the appearance of eukaryotes. Eukaryotes are larger than prokaryotes, and have a nucleus which contains the DNA, as well as other different compartments for different functions.

And finally, as some eukaryotic cells within colonies (groups of cells) became increasingly specialised, the first multicellular organisms made their appearance. We belong to this last evolved group of organisms, but this does not mean that we are “superior organisms”...