[Elsemeravin]

I feel it is easy to explain the existing feature and find out how very practical there are and then conclude that it must be the optimum configuration and so the only one.

Yet I'm not so sure living being born in the mist of a Gaz Giant would have a head. They could be ball-like, with the "brain" located in the middle of it (better protected that our easy-to-shout heads), yet close to everything, with some kind of wings, or even only floaters whose weight can be chemically altered to navigate.

I mean that some living environment can be so different from ours.

Then of course if you find a planet similar to the earth, then maybe (and only maybe as so many parameters, like sun light, matter, gravity, atmosphere, ...) some aliens could be "similar" (I'm just thinking than on a oxygen-full planet one being could move by burning down oxygen while being of course fire proof.

Just my idea...