*Let me preface this post by saying I'm a firm believer in evolution, I'm not trying to "disprove" evolution or anything of that nature*
Okay, so usually when evolution is presented alongside cladograms and web-like visual representations of evolution over time, there seems to be an implied consensus that, at some point, there had to have been some original one-celled species from which all subsequent life evolved.
However, is it implausible that, perhaps, life arose independently at different geographic locations on the planet? What I'm saying is that say a species of one-celled organism arose in the primordial ooze somewhere in the southern hemisphere. Now what if another species of one-celled organism arose (completely independent of the first) in the northern hemisphere. In this hypothetical situation, there would then be two evolutionary lineages rather than one.
Has it been proven that there really is a single evolutionary lineage (I'm genuinely curious)? Is it even possible to know? Have scientists studied this? Are there any scientific ramifications of there being multiple evolutionary lineages, or is it just interesting food for thought?