Or what if a drop of water were trickling down the trunk of the tree?The water in motion would be alive, but the motionless tree trunk would be.Sometimes they look rather less like humans, in which case (if friendly) they often resemble large teddy bears.Not only do Aliens with Forehead Ridges mostly look like Earth Humans, they tend to act like Earth Humans as well, or at least one particular (real or speculative) Earth Human culture.We can not only communicate, Trade, and fight, but form joint business ventures, cheat each other at cards, and even fall in love.Indeed, Aliens with Forehead Ridges raise a profound question in evolutionary biology.Etymologically, and in mainstream English the word means "feeling" but is rare and now archaic.[From Poul Anderson's `Polesotechnic League' stories, going back at least to 1963]An evolved biological intelligence.Implies human-level cognitive and linguistic ability but not necessarily tool use.
Nevertheless, what we call "life" is sufficiently important to warrant an attempt at a definition.
Except for the Energy Beings, most seem to be hydrocarbon life forms, but methane breathers who thrive at -200 C will sometimes turn up.
What they all have in common is that they are Really Alien.
They have the same secondary sex traits - as species-specific as you can get.
Only their males have much facial hair, and their females often have bodacious figures. This leads to some speculation that they may be of Earth Human descent.