Yes, all living organisms come from the same cellular lineage, and there is evidence for this in the comparative DNA. Another piece of evidence in this regard is that all living things have DNA. There are a number of chemical systems that can do what DNA does in cells i.e. DNA has no intrinsic importance to living organisms. The fact that all organisms do use DNA substantially reduces the probability that they emerged independently though it is certainly possible.
Did plant life come into being spontaneously or did a mutant blade of grass grow into a tree?
Plant life evolved from chloroplast containing microorganisms I think. But I am not a biologist, so I can't say for sure. My interest in the thread is mostly to correct really bad mathematics.
What drove that cell evolve in the first place? What possible pressures could have been put on that organism to change since by it's very uniqueness it had no natural enemies or threat to it's existence? Natural selection requires an outside threat or need to force an adaptation or evolution true or false?
Evolution doesn't happen to a cell, it shows up by comparing mutations in successive generations. Even in the absence of predators, there are tons of selection pressures. For example, many simple organisms can only live in specific temperature bands which vary depending on what proteins they use. It would take a relatively minor mutation (caused by stray alpha radiation for example) for a microorganism to have a survival preference for a different temperature band than its parents. This could mean that at some temperatures, only one of the two strains could survive. What happens is that you have the case where variations of the simple organism live at different locations in a temperature gradient with slightly different environmental differences as a side-effect of location. Wash-rinse-repeat, and you have an exponential take off in variation. Note that this exact same effect can be observed in organic chemistry as well, where a temperature gradient will generate complex variation and separation along the gradient (hence why temperature gradients are not desirable for reactions where we want only one end product) just by the nature of the types of reactions that are occurring. There also quite a number of chemicals (some proteins for example) that are quite capable of self-replicating in a rich environment. As the complexity of the chemistry increases, the chemical processes that are occurring also tend to increase in complexity, but unfortunately for us the computational complexity becomes intractable relatively quickly for molecules that can bootstrap themselves and we literally lack the computing power right now to figure out precisely what is going on beyond a certain point. All we can do then is observe the properties of the results.
So to answer a question, evolution will occur in any system that has both variation/mutation and selection; we don't even have to be talking about living organisms for this to be true. Mutations happen in single celled organisms regardless of whether or not there is a selection pressure because of things like radiation which are always in the environment. You don't have evolution unless you also have a selection mechanism, which can be any parameter that can kill the organism (changes in temp, radiation, chemistry, etc). So in practice, mutation is always occurring but selection happens intermittently. Note that sexual reproduction is a form of mutation as well, as the resulting DNA is always unique. Of course, once living organisms started to flourish they created yet another selection pressure when they started competing with themselves, but this was merely another selection pressure in a long line of selection pressures.
That's fundamentally incorrect and logically flawed.
We have variation, mutation, and selection in automobiles, but we know that the cars aren't self-evolving (rather, it is the Designers of cars who make the changes to the cars via Intelligent Intervention).
In fact, we don't have natural, unaided, unintelligent Evolution occur in many systems wherein Intelligent Intervention is present. For example, computer programs don't self-evolve (unless designers tell them to), even though the designers outside said systems might.