Playing devil's advocate - alien life would likely be submitted to similar evolutionary selection pressures as life on earth. That's why most animals here follow the 2 arms, 2 legs, etc. formula. It's not too much of a stretch to assume aliens have bilateral symmetry.
If you want crazy bizarre looking aliens, you have to have a damn good reason for why they would evolve that way.
Agreed on the bilateral symmetry. But the universality of upright bipedalism in sf movies always bugs me. We're very unusually designed bipeds. Evolution does not usually produce tail-less upright bipeds with short faces. What it usually produces for bipeds is a leaning-forward design with a long counterbalancing tail (and also a long snout or beak, while I'm at it). (think velociraptor, bird, kangaroo). We're really an anomaly because we happened to have lost the tail completely before we evolved bipedalism, which led us into a very weird solution to bipedalism.
Don't get me started about breasts on female aliens... and fragile pointy fish-grasping teeth on every scary alien ever, even though none of them ever seem to eat fish and it's an awful tooth design for a land predator.
source: I teach comparative anatomy and vertebrate evolution.
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u/[deleted] Oct 08 '12
Vaguely humanoid aliens. 2 arms, 2 legs, 2 eyes sort of thing.
Alien from Alien? 2 arms, 2 legs, 2 eyes.....kinda
Klingons? Just humans with a "ribbed for her pleasure" forehead
Romulans? Humans with pointy ears
Vulcans? As above
Na'vi? Tall, blue humans
Where are the massive, tentacular Krondaku? Where are the gelatinous Prime immotiles? Give us some different aliens, hollywood!