Alien's Design

I have just received some excellent alien models from spawn.com (IIRC),
and this has prompted some questions about the design of the alien
throughout the movies. Some of these will have been answered before, but
I'm looking for aliens-universe explanations, not cop-outs like "because
the director made it so". And also it will spark a discussion :-)

Why did the aliens in Aliens have ribbed heads? The aliens in Alien and
Aliens all came from humans, so I would expect them to be identicalish.
However, if the alien is supposed to evolve with each generation,
improving upon the previous, why then do we go back to all-smooth heads in
Alien 4?

Why are there no horns on the back of the dog alien in Alien 3? Dogs and
humans don't have horns, so what's so special that they appear in one and
not the other? (I know this is in a FAQ somewhere, but: what purpose do
the horns serve anyway? and why can the dog do without them?)

The worker aliens in Alien 4 all came from humans, so why are their legs
suddenly double-jointed? They are human-like in Aliens. Is this the alien
evolving through some form of secret genetic memory, as in the first
question?

PS. I checked and the Predator has no sexual organs - teehee!

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