"I wouldn't expect populations of bacteria to give rise, without endosymbiosis, to complex morphology and the kind of intelligence that we have, elsewhere. I think that it would require (I'm going out on a limb here)... an endosymbiosis for the reasons I've been saying, and... that endosymbiosis is a) rare and b) likely to go wrong. So I can't put a number on how improbable it is. It's just that I would say that it's a factor that a lot of people would rather not think about. If you have an agenda where you'd like to find complex life out there, the SETI people for example... probably don't want to hear this kind of stuff. It says that it's less likely... it's not an inevitable outcome of physics."