Well, as has been pointed out before, just because life appears to stem from one thing, doesn't mean there wasn't a "Second Genesis" (or multitudes of them, even happening today). However, those other lifeforms have to compete for the same resources as better adapted ones (per natural selection). And then there are things that are "arguably life" that seem awfully close to life and awfully orthogonal to the existing tree, like viruses.
But besides that, what amazes me is that we are not only the most intelligent life on earth (for some values of intelligence), but as far as we can tell, the most intelligent life to have ever developed on earth.
This seems odd, given that there are so many other intelligent life (but nowhere near our level) like cetaceans, some birds (which are descended from Dinos, which had a longer time to evolve in interesting forms- not like early, ratlike mammals, to boot), other primates, some species of octopuses, and I am sure I can think of more examples.
Maybe the trick is having a big brain and a body plan that is flexible enough to do many things, starting with a high metabolism. I would think that a therapod with human intelligence levels would have an awfully hard time building spaceships.
And the brain consumes a lot of energy, which is why human muscles are relatively weak and energy efficient (also for endurance).
In that sense, it is my opinion that life is probably common, but intelligent life is rare, maybe even extraordinary, and probably not inevitable for a planet or system.