We have spent all the articles up until this point establishing how the Universe began, where are the materials in the present Universe came from, and how the Earth and Moon formed. Nothing up until this point in our history is particularly remarkable. As far as we can tell, the formation of stars and even planets is very common in the Universe and our solar system was no exception. However all of that changed a few million years after the Earth formed when something emerged that is so rare, it has never been found anywhere else in the Solar System or beyond: Life.
Before continuing, it is worth asking the question: what exactly is life? This seems like it should be an easy question but it is surprisingly difficult. Life is highly diverse and tenacious. We find it everywhere we go on Earth regardless of how hot, how cold, how dark, or how toxic the environment might seem. For all the difficulty we may have defining life, one thing we can say for sure about life is that it will go to extreme lengths to live.
To understand how life arose on Earth, let's imagine ourselves exploring the oceans of the ancient Earth. One difference we would quickly notice is that we can't breathe the air since there's no oxygen yet so we'll need to bring some bottled air along. We'll also need some pretty hefty sunscreen because the early Earth had no ozone layer to block out the sun's UV rays. UV rays have enough energy to break chemical bonds and that includes the bonds that hold together the DNA in your skin cells. Once that DNA has been damaged beyond repair, the skin cells die and flake off over the course of a few days causing a painful and itchy burn.
But the bonds in your DNA aren't the only thing being damaged by the UV rays on the ancient Earth, all the other chemicals and compounds near the surface are being broken down and forming new compounds. Frequent lightning strikes from the young Earth's turbulent atmosphere would have helped this process along as well. Now let's imagine for the sake of argument that amid all this destruction and recombination of chemicals, a molecule was formed that was able to induce an identical molecule to form from its surroundings. As it floats around, its presence will cause other identical molecules to form, who will in turn induce yet more identical molecules to form.
It is easy to see that this new self-replicating molecule would quickly spread through the oceans.
It is also likely that in some cases, this molecule would cause imperfect copies of itself to be created. For example maybe it is swept away by a current before being able to induce its surroundings to form a complete copy or maybe one chemical is replaced by a slightly different one. Most of these variations will not be able to self-replicate but eventually by sheer chance, a different molecule will be created by accident that can. It's possible that this new self-replicator will not be as good at inducing copies of itself as the original, but it is also possible that it will do it even better. If the new replicator can induce copies even faster, it will spread through the oceans even quicker than the original allowing it to reach sections where the original molecule has not yet used up any of the required materials.
Let's go one step further and imagine that a new self-replicator emerges through a copying error (just like the previous one) that can not only induce a copy of itself to form from the chemicals available in the ocean, but can also use the other replicators to create a copy of itself. This new molecule would be able to reproduce regardless of whether other replicators had already used up all the materials in the area or not.
Continuing along, maybe another replicator emerges which can create copies from chemicals in the ocean or other replicators and it able to prevent other replicators from disassembling it.
We could go on and on adding layers of complexity but let's stop and have a look at what we have seen. It's clear that something that can copy itself will spread very fast as the copies make their own copies etc. It is also clear that the process of copying can produce errors and although most of these errors will produce something that is unable to copy itself, in rare cases it will have this ability. In even rarer cases, it will be able to copy itself better than the original. As a result, this new and improved copier will quickly become numerous as its copies successfully copy themselves and outcompete the original for the resources needed to make copies. The process essentially "rewards" the better copiers since they automatically become the most numerous. Any improvement will quickly spread and any inferior copiers will not make it.
This process is referred to as "natural selection" or "Darwinian evolution" and given millions or billions of years, will produce very complex copiers which are constantly improving. In our short example, we went from a simple copier to something that could "eat" other copiers and protect itself from being "eaten". From there it isn't hard to imagine these copiers becoming larger and more complex as the bar is raised with each random improvement. As copiers spread, they will encounter different environments where different things will produce an advantage. The result is an increasingly diverse set of replicators that are constantly developing ways to inhabit new environments as a result of random mutations in the copying process.
If the idea of a self-replicating molecule sounds far-fetched, remember that you have trillions of copies of one in your body right now: your DNA.