[Evgeny]

A point to consider is the probability that microbes ever evolve into something more complex (multicellular organisms). After all, there are huge amounts of bacteria on Earth these days, but I'm not aware of any cases of them evolving into more complex organisms in the last billion year or so.

A book called "Power, Sex, Suicide: Mitochondria and the Meaning of Life" provides some insights. The author argues that the only reason multicellular organisms evolved is that one type of cells ("host") engulfed (or engaged into symbiotic relationship with) another, that became mitochondria and was used as a "power plant". This probably happened only once in history and under very specific conditions, which are very unlikely to be repeated.

[joeyo]

  > This probably happened only once in history and under
  > very specific conditions, which are very unlikely to be
  > repeated.

Anything not forbidden is compulsory. -- Murray Gell-Mann

[ajju]

"Anything not forbidden is compulsory" only if you assume an infinitely long time horizon

Without having more data points, it is hard to say whether the average life span of a planet in the Universe is long enough for multicellular life to inevitably develop.

So far, all evidence points to no. It is also true, however, that so far we have negligible data.

[carbocation]

Wolbachia has a remarkable relationship, with some species dependent upon it for fertility. I'd say that endosymbiosis has occurred independently a few times.