“My own prejudice against the existence of alien humanoid intelligent lifeforms is based on the fact that there are only two creatures on Earth which are erect bipeds;”

I don’t think it is particularly telling that only one species inhabits the niche of Homo Sapiens. There have been many species of erect bipeds, but they have all gone extinct, quite possibly as a consequence of competition from modern humans. Bipedalism has obviously been an extremely successful body plan on Earth, but one species of bipeds has replaced all of the others, which may have numbered several hundred.

I think it is really difficult to speculate about the evolution of extraterrestrial bipedalism since we do not have any firm understanding of why bipedalism evolved on Earth. “Why do we walk on two legs? We don’t know.”

Bob