For example, Star Trek operates under assumption that all (or nearly all) life forms in Galaxy evolved from common source (see The Original Series episode Return to Tomorrow, and The Next Generation’s The Chase). I'm drawn in that regards to V'ger as well, don't know why.
That being said, it is not unreasonable assumption that evolution will follow similar, but not necessarily identical paths. I can deep a bit deeper into theory involving DNA, proteins, amino acids and sugars, which shows just how incredibly complicated our life is...
That's why most of races of Star Trek are humanoid in apparition and most of them can (however shouldn't be able to) crossbreed and gain sustenance from extraterrestial foods.
But it all depends on assumptions od initial conditions put on by universe creator.
My favourite writer - John Ringo - usually populates his universes with very diverse life forms, so it all depends on who you read. His universe of Citadel series contains humans, porcupine-pig-like Glatun, velociraptor-like Rangora, squid-like Horvath, as well as giant caterpillars, very large crabs and others... Some smarter, some dumber, some... different.
Stargate Universe is based on notion that galaxy is human-compatible for specific reasons, but there are several species completely different as well. With dumber ones also.
SO it all depends on who where what... And since licentia poetica is in force, each sf is the way as the writer came up with.
One more thing. There are very few movies or TV shows which can be described as even soft Sci-fi... This is because science is not flashy most of the times. So it is greatly dumbed-down for audience (pun intended). However, once we cross to hard science (for example: Looking Glass series by Ringo & Taylor), the language becomes... Well, geeky is putting it mildly. And usual redneck will not cope with passage like that:
“There’s not supposed to be build-up on the covalent shearers,” Miriam said. “The only way that you’d get that is if molecules with polar bonds were getting through. The covalent shearers can’t break polar bonds. Check the polar corpuscle. It’s probably detuned. Check the point and dwell settings. As to repairing the covalent shearer and the carbon cracker, you can’t repair them perfectly. But you can take them and cut them up and run it through the fabber on a recycler setting. The parts will come out clean. Use a melder to join them and you’ll get about ninety percent efficiency. See if that works.”
"There was one more thing.” Miriam said. “I think that the first excited state of the flavor neutral must have the required rest mass of three zero nine six point nine million electron volts in oscillating flux density but the half life of the up-type pair must be longer, frame relative, than the rest frame seven point two times ten to the minus twenty-one seconds. The modulation and control of the flux density and pair half life can increase or decrease the flat space metric within the motivation metric to accommodate potential well suitability. But it’s just a guess.”
So, since scriptwriters cannot parse above, thus we are left with duct-taping Tribble to phaser in order to attempt to regenerate di-lithium crystals...
EDIT 2: I'm eating the words above... Just revisited one series and there is really funny Sci-fi moment. Shows how geeky one sometime is.
Stargate SG-1, S07E21 - The lost City pt. 1
O'NEILL: (Continues shaving, he's almost finished) No, no, no, wait. Don't hang up. I need a seven letter word.
JACKSON: I told Sam I wouldn't help you.
O'NEILL: Well, then this will be the one thing she doesn't know. Up, down, charmed, blank.
JACKSON: (With NO hesitation to think about it) Strange.
O'NEILL: (Not getting that Daniel gave him the answer ... I think he thinks Daniel called him strange!) Yeah. Well thanks anyway.
JACKSON: (Thru the phone as Jack takes it away from his ear) No the word you're looking for... (Jack hangs up)