No ET! Ever?

[Anomalocaris]

Until we know how life began on this planet, speculating on whether it might occur on other planets seems premature.  Most abiogenesis theories look for easily repeatable conditions to lead to life.  If life is very improbable given the background conditions, this approach may never bear fruit.  Still, it's awfully early to bet against life being sufficiently probable for such a research strategy to bear fruit.

I think the origin of life reaseatch community does not on the whole hold that abiogenesis is relatively improbable. The rise of prokaryotic cells seems at the present to be reasonably probable in relatively common environments over geologically brief timescales. So the equivalent of prokaryotic cells on earth may be expected to be ubiquitous.  

Instead some seem to hold that complex organisms capable of evolution into sophisticated behavior and intelligence require cellular complexity and metabolic efficiency equivalent to eukaryotic cells. On earth no prokaryotic cell bridged the gulf to the complexity and efficiency of true eukaryotic cells even though some prokaryotic cells seems to have independently evolved features analogous to some parts of the repertoire of eukaryotic cells. As a result some Researchers have argued the development of eukaryotic cell does not easily follow from normal development of prokaryotic cells, but instead require an particular set of interplays between specific prokaryotic cells that is of much lower probability than abiogenesis itself. As a result the rise of the equivalent of eukaryotic cells, and hence complex life able to evolve intelligence, may be far more unlikely than abiogenesis,  and may be comparatively quite rare.