We humans cannot live, unaided, without the trillions of bacteria and archaea (and some protists too?) which inhabit our bodies.
And the first eukaryotes may have been the result of some sort of symbiosis.
My reading tells me that we think we have some idea of when at least some of the key symbiotic relationships began, if not much about the how; we certainly have no idea of how many such have vanished ("failed experiments").
Given that, what can we say about the likelihood of complex life forming being dependent on lucky symbiotic flukes?