SALUTE
I have barely watched Breaking Bad
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Let's have another good week everyone 
been feeling like complete and utterly self isolating self destructive shit, so trying to channel it into writing. for years I have wanted to write a space opera, and I finally have started to do some deeper world building on the speculative evolution of different species. I have a weird autistic frustration every time I watch sci fi, even tho I love the genre, that most series do not have major species in their galaxy be anything other than 'weird humans', it's just stastically improbbably impossible that we'd have THIS much covergent evolution towards the same bodyplan (we ain't crabs buddy, it's not that solid a body plan). you're telling me, across these thousands of species, all of them somehow evolved the same symetry we did? and all of them have 2 eyes, 2 nostrils, a mouth on the face, 2 arms and legs? but they got like a weird head crst or something? na fam, that's just boring. where are their adaptations to their environment! life on earth is so diverse and fucking weird, let alone with thousands of different environmental factors on other planets
ok now do I understand on a level above as a consumer of the text that it's to get people to empathise and that's easier with characters that are humanoid, it makes them far more playable by human actors especially in low budget shit like star trek, and most people just want a weird blue guy rather than a colonial organism of 4 different symbiotic species? yes, I do. I know why the trope exists. but I do not have to like it
I mean it's also that these aliens aren't meant to be alien but rather representatives of different human things. Star Trek especially, most of its aliens are explorations of historical human cultures and ideologies if they had advanced further technologically and been pushed to certain extremes or so on. When Star Trek does want to do the truly alien it does things like the original series episode about the sentient rock slug thing. Another example would be War of the Worlds, where the martians are meant to be colonial western powers but even more advanced and further evolved by a few hundred years. So they're humans but with bigger brains and hands to emphasise how they've primarily been interfacing with tools, with the explicit theory in text that we'd evolve in the same way over time. And the things they do is just colonialism and it's not trying to be subtle. Keeping the aliens somewhat human is what allows the political commentary to have some bite to it, if they were truly fully alien it would suggest that these things aren't human traits and beliefs, because the truly alien is defined in its distinction from humanity, but by keeping them somewhat human it suggests that there is some similarity between us and that we ought to consider it. Obviously the empathy and budget constraints are there too, the point of human aliens is ultimately that they are better suited to whatever themes the work is trying to explore. The truly alien in fiction has a different role to it and is used as such, and you can explore these themes with it to an extent, but the way you would do it, and with how much depth, would be wildly different.
Usually they have an in universe explanation that humanoids are all derived from some progenitor species that was humanoid in shape - but for TV shows and movies it also makes it way cheaper and easier to film historically.
In scifi novels they'll get weird with it. Sometimes you make contact with living EM radiation that eat stars bit by bit or a pool of living water or life that evolved on Jovian planets that float or sink or Silicon based life that respiratory out blocks of sand - they can have totally different conceptions of morality or what is sentience. And this is going way back even back to the Golden Age. The Lensmen Series had some really alien aliens but you kind of forget I guess as universal communication makes it easier to relate. The 3 Body Problem series has some really weird out there biology that, mind the spoilers I guess, imply and later outright state the universe and laws of physics even the number of spatial dimensions were all products of living sentient creatures and societies.
It can also be hard to do the world building of hundreds of planets with hundreds of ecosystems and mostly independent biological histories - easier to handwave and not really get into the nitty gritty if you're doing stuff on an epic galactic or universal scale. But, some people like the world building part! You can check out Snaiad or Barlowe's Darwin IV in Expedition for some interesting speculative biology. Veldhuizen's The Cluster has some interesting speculative biology on plenty of planets.
In terms of pure biology, bilateral symmetry seems fairly successful but that could certainly be a quirk of our planet. Ravens, Humans, Pigs, Parrots, Octopuses, Bonobos, all have bilateral symmetry and are at least moderately intelligent - which leads to one mouth for the digestive tract and two eyes that help resolve depth, two ears that do the same for sound, etc. This is getting in the weeds, but personally I wouldn't say humanity's particular version of tool using/technological society forming intelligence is any guaranteefor evolution - naturally we tend to think so, but if you consider numbers of genomes or amount of carbon used in biological processes (both rough estimates), evolution by natural selection seems to settle on bacteria and plants as it's most successful preferred form of life. In terms of Earth's actual history, life formed pretty much as soon as water was on the surface, anything we'd recognize as complex animal life came much later and humanity is barely a wink in evolutions history on Earth. I wouldn't be surprised if we are it in our entire galaxy even if life is particularly abundant (and probably mostly slime balls with algae and plants lol).