Where did life on this planet originate? And perhaps more importantly, how did it originate?
The answer is that no one knows for sure. However, there are enough clues left in the fossil record that we have a pretty good idea of what must have happened.
Life requires certain features - the ability to replicate, the ability to adapt, the ability to utilize chemicals in the surroundings to achieve the first two objectives - but not a lot more.
Many chemical reactions are capable of the first. In my research, I have performed reactions that went sideways and did not generate the product that I was after. Yet, they did produce a product. These reactions were self-replicating, generating uncountable billions of billions of copies of the product. (Billions of billions is actually not a lot when you are talking about something as small as a molecule.)
It is harder to find chemical reactions that adapt but they do exist. RNA, for example, is quite capable of undergoing a form of self-editing and self-manipulation resulting in changes to its reactions. It is capable of producing a wide variety of products and more importantly, it is capable of generating a copy of itself with the changes incorporated.
This is the critical step in an evolving system. Once the changes are made, the resulting daughter is either more or less suited to the environment and will either prosper - at the expense of the parent - or fade away.
The key to evolution is that it is not directed but rather a test of a molecules fitness against a background of competing molecules or organisms. Fitter molecules and beneficial changes produce a next generation.
With respect to the utilization of chemicals from the surroundings - that is, feeding off of other compounds - the issue is not really if you can but how to do it. In the deep past, there were all sorts of local concentrations of chemical compounds capable of sustaining a self-replicating adaptable molecule but how do you access them?
The answer is that many times - many millions or billions of times - the molecules didnt. Nothing happened. Self-replicating molecules ran out of supplies before they could really take hold and spread. They died out.
But a sustainable self-replicating molecule only had to happen once for life to begin. Just once. Billions upon trillions upon quadrillions of chemical replicating systems could have arisen and fizzled out over the billions of years during the early evolution of the planet. However one time, a self-replicating molecular system was successful.
For many people, this would seem to make life improbable. Nothing could be further from the truth. It actually means that life was inevitable since there were so many experiments going on all over the surface of the planet and only one had to succeed. Statistically, life is very likely.
The interesting thing is that we know that life evolved along a singular path from a single source because much of the apparatus that supports life is the same, regardless of the type of life that one examines. We do not have silicon based bugs interfering with our computers because life originated around the element carbon. The fact that all life, from bacteria to humans, utilizes D-sugars and L-amino acids - sugars and protein pieces with a particular handedness - means it had a singular, common origin.
The latter might need a bit of explaining. There have been many mass extinctions in the past. At times, as many as 95% of the species alive on the planet have been wiped out. The most famous is the disappearance of the dinosaurs but there have been other, more dramatic extinctions. Prior to any of these, other forms of life could have existed but died off. Only our particular branch of D-sugar, L-amino acid eating organisms survived the devastation. It is these creatures that are the last universal common ancestor.
In any case, the original self-replicating molecules became more diverse and more fit for their environment. They fed off of the chemical soup that surrounded them and cannibalized each other in order to dominate. They also learned to cooperate for mutual benefit.
Maybe learned isnt the best word as it implies learning in the sense of a conscious decision and that is not what is meant. They got better at surviving - they improved their fitness - by coming to a mutually beneficial and supportive relationship with other molecules.
It took around two and a half billion years for the first complicated cells to appear. They were simple creatures with little more than a membrane and some rudimentary chemical factories but they were capable of self-replicating and adapting to their environment. And they were better at it than anyone else.
They were the beginning of the evolution of life on this planet.