OK, here are some of my thoughts on the alien evolution concept.
Some well developed ones: The “Predator” works because of the care taken in developing the physiology and the society. Especially nice is the fact that they see heat signatures. The suggestion is that they evolved under a hotter sun than humans. It also suggests a reptilian background, since a number of reptiles have this capability.
Problems: They are SO humanoid. They also have a society based pretty clearly on some savage human societies. Head hunters and warrior societies. This does clearly suggest a predatory evolution for them, which is a nice touch.
Alien, from the Alien movies. An excellent one because time was spent developing this elaborate life cycle, which is not unlike that of quite a few insects. They have insect and reptilian characteristics. It’s interesting that the eggs are soft rather than hard, which really suggests early evolving reptiles on earth.
Problems: Although quite a few insects make internal toxins and even release these into the atmosphere, the acidic blood is a little too corrosive. Eating through the kind of metal used to make spaceships would be quite a feat. Also, the growth rate of the alien in the first movie is too extreme. Growth rates can be high for some insects, and even for fish and birds, but nutrients are needed. You only get out what you put in.
The Thing: John Carpenter’s version. I really like this alien. It’s a cool enemy. But I have to think it’s pretty unrealistic. Plenty of one celled organisms grow by absorbing other cells, and the “Thing” is supposed to be like a collection of cells. Mimicry is also very common in evolution and the Thing is an extreme extension of that. However, knowing how complex the human brain is, and how complex the cell structures are in a human, I can’t imagine an organism that could assimilate so perfectly in such a short period of time and be absolutely believable as the person it assimilated. It’s still cool, though.
Some bad aliens: The “Space Herpes” from Ice Pirates. Clearly this was meant to be a take-off on the Chest burster scene from alien but I didn’t think it worked. Or that it was even funny.
From Buck Rogers, the TV series. Hawk is descended from a bird-like species. His only connection is, apparently, that he has feathers on his head instead of hair. Minor “cosmetic” changes to a human are Not Good Enough. Quite a few Star Trek aliens fall into this category. The Borg are intriguing, though.
From books? I thought ERB having the human-type Barsoomians lay eggs instead of bear live young was pretty weak. I love these books but the Human-egg-thing is definitely not an evolutionary likelihood.
