Our observations are biased because anything that occurs multiple times is very easy to see but something that occurs only once could be completely missed as an essential step towards civilization because we assume it was inevitable.
Where's the bias?
1 and 3 seem obviously true. There are multiple trials separated either by geography or time, and they have enough failures / successes to make our intuitions right. Anthropic principle doesn't get involved here in any way. If agriculture was invented 5 times independently it couldn't have possibly be the limiting unlikely...
We have a sample of one modern human civilization, but there are some hints on how likely it was to happen.
Major types of hints are:
Data for:
Data against:
To me it looks like life, animals with nervous systems, Upper Paleolithic-style Homo, language, and behavioral modernity were all extremely unlikely events (notice how far ago they are - vaguely ~3.5bln, ~600mln, ~3mln, ~200k or ~600k, ~50k years ago) - except perhaps language and behavioral modernity might have been linked with each other, if language was relatively late (Homo sapiens only) and behavioral modernity more gradual (and its apparent suddenness is an artifact). Once we have behavioral modernity, modern civilization seems almost inevitable. Your interpretation might vary of course, but at least now you have a lot of data to argue for your position, in convenient format.