Do you think simple life is common in the Universe?

 

The vast, vast majority of the history of life on Earth was comprised of prokaryotic cells accomplishing very little other than releasing free oxygen into the atmosphere.

 

Perhaps life is common in the Universe, but most life cannot jump to the eukaryotic or multicellular stage. Or, maybe these fragile lifeforms are continuously wiped out by simple changes in the climate or chemical make-up of the oceans.

 

The tiny minority reaching a more complex stage would have to deal with the occasional extinction event. Possibly asteroid strikes, GRBs, are major volcanic activity are more common on other developing worlds, and we were lucky to get away with only 5 or so.

 

Finally, there is the possibility that viruses evolved faster than the immune systems of multicellular organisms evolved (or possibly sooner than multicellular organisms themselves, so that any time MC organisms evolved, they were destroyed immediately).

I think you're probably right. If nothing else, complex life has to start out simple, and have a long time frame in probably much more selective conditions - it's much easier to imagine microbes surviving in a much larger variety of conditions than anything else, but of course that is completely speculative, as we only have the one individual, Earthlife, to try to extrapolate from.

 

It's also a little bit tricky because towards the "simple" end, the line between life and nonlife is pretty much arbitrary, especially with ETs when presumably it's going to be fundamentally different even from "low-end" life here on Earth.