We don't actually know that at all. It could have happened many times and one line won out, it could have been more of a diffuse process than a single event (picture how microbes share genetic material ~freely but even less structured), or there could be a ton of life out there on Earth that's from a completely different tree. We really have very little idea what's living around us.
Hm, I don't think it does. The problem is vastly different. Here, on Earth the problem is: sift through all of life for some that's different than the rest. A _hard_ problem with how little of microscopic life we've cataloged completely and with how much of the volume of Earth we can't see.
The problem looking for life in the stars is more: find evidence of _any_ life, so radio signals or chemicals that can't reasonably come from anything else but biology. Those are hard as hell, but fundamentally different.
> Also, if life had multiple false starts here on Earth, that does also suggest that it is very difficult to take hold even on the original Goldilocks planet.
That would be interesting. I kind of guess it's less likely than some kind of winner-take-all outcompeting thing, but who knows. Life that we see is just very good at spreading, escaping and holding on tight.