Welcome, Explorer.
Good question. I don't think anyone really knows the answer.
Darwin himself, when he proposed his theory of common descent, wasn't sure how many times life originated and how many distinct ancestries there were. Modern genetics and molecular biology indicates that all known life has evolved from a single ancestral population. That doesn't mean that a totally new and different species won't be found in some obscure environment that will be found to be descended from a completely different lineage, but among all that we know of now, we all have a common ancestor.
But we don't know how many times life originated on earth. Life appears in the geologic record immediately after the end of the Late Bombardment, the early part of the earths history when it was still being smacked by god-awfully big meteors. During the Late Bombardment, life would be pretty much impossible -- these meteors were big enough to sterilize the planet! But life appeared almost "immediately" when it was possible, suggesting that by whatever processes life originates, it originates pretty quickly (quickly, of course, meaning perhaps as long a few million years). So it is possible that life originated several times between large impacts, only to be wiped out, until the last large impact occurred.
It could have been that there were several different kinds of life on the early earth, but for whatever reason only one survived to the present day. Perhaps our kind out competed the rest and drove them all to extinction early on; perhaps it was the result of a statistical fluke -- just like some dynasties end up dying out because all the branches leave no heirs for no discernable reason, so maybe the other different kinds of life simple left fewer and fewer species.
Another possibility is that we may be the result of several different "starts". There is a theory that during the very early part of life lateral gene transfer was important -- that is, instead of inheritance occurring strictly between generations, maybe living organisms swapped genetic material with each other. In that case, there may be no real way to distinguish different "starts" of life early on, since they all eventually came to share their genetic material and so homogenize. This is why I refer to this hypothetical ancestor as the ancestral population rather than the ancestral species.
Finally, once life began, there probably was no chance for another "biogenesis" event. Any organic material that could conceivably become the start of new life would be eaten up by the now ubiquitous life that is already present.
Hope this helps.
If I had a million dollars, I'd buy you a monkey.
Haven't you always wanted a monkey?
-- The Barenaked Ladies
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