Is what is evident - that all life on Earth has the common source, actually logi - (Sep/26/2018 )

Hi.

There are multiple evidence that all life on the Earth decends from the same ancestor - the same gene coding mechanism, the 4 nucleotides A, C, G and T, the same 20ish amino acids used for building proteins, the facts that all organism use L chirality for amino acids, the RNA translation and synthesis mechanism is the same,...

But is this actual state of the facts logical - does not it look that due to the number of different solutions that could be found for a single problem or multitude of choices that could be taken for the starting building blocks or the complexity of all parameters involved, make it more probable that several different independent lines of life forms arose and coexisted, at least at some moment on the Earth?

I have read an argument somewhere that states "due to the complexity of all different existing processes and obsticles that life had to surpass it is almost impossible to imagine that another line of life managed to do this all"
To me, this is very non-scientific : the fact that a system is more complex does not make it less probable that another system with similar functions and a different composition could be built. It makes it more likely actually.

The only reason I could think of for the existence of the single life line is the competitivness of life - the fiersome fight for living space occurs between much closer members of our line, so it could be expected that the life form that we belong to was more successful than others that potentially existed, so the others were eliminated by evolution.
Still, there are no fossils founds of any other life forms, if this can be determined for fossil founds at all.

What are your thoughts?
Thank you

The evidence so far points to a single origin for life as we know it, due to the conserved nature of the mechanisms and information encoded in the genome. This does not preclude other origins that might have happened and then been lost due to any number of reasons.
 
Given that the earliest fossils we can recognize as formerly living are some 4 billion years old (this is heavily debated, as to whether they are even fossils and if so, how old they are), and that we don't know what forms other life-forms would have taken, we could easily miss these independent origins. In addition, fossils only form under specific conditions, and so only capture a small snap-shot of what was present at the time. For example, to quote Bill Bryson from A Short History of Nearly Everything:

"...our understanding of human prehistory is based on the remains, often exceedingly fragmentary, of perhaps five thousand individuals. You could fit it all into the back of a pickup truck if you didn't mind how much you jumbled everything up...

...They appear randomly, often in the most tantalizing fashion. Homo erectus walked the Earth for well over a million years and inhabited territory from the Atlantic edge of Europe to the Pacific side of China, yet if you brought back to life every Homo erectus individual whose existence we can vouch for, they wouldn't fill a school bus.

...Something as short-lived as our own civilization would almost certainly not be known from the fossil record at all."

These are recent fossils from a relatively abundant wide-spread species - think about how abundant and wide-spread an organism would need to be for us to 1) be lucky enough to find it as a fossil, and 2) recognize it as using having a different origin.

Also note that the earliest DNA samples we can detect are a few hundred thousand years old - a tiny fraction of the time life has been around for, so we have no way of determining which of the many potential mechanisms most fossil organisms used to live, other than that we can sometimes detect degradation products of metabolites in matrix surrounding fossils (e.g. here). Many of the techniques used for these sorts of things are rudimentary and only possible in recent times due to advances in technology. This means that a lot of what we know is from a tiny fraction of all the samples collected (mostly recent samples for DNA analysis - this requires special conditions so as to not contaminate the sample with modern DNA). I am sure as techniques advance and more people devote more time to this sort of research we will know more.  Just think - only 30ish years ago most people thought that dinosaurs, apart from species like Archaeopteryx, didn't have feathers, now it is thought that almost all did, and that's for a macro-scale structure on a relatively abundant fossil type.