For Star Trek, there is an in-universe explanation which is essentially that Humanity and many other classic humanoid species you see (Klingons, Vulcans, Romulans, et al) were all essentially “seeded” by the same ancient race of aliens. That race (the Progenitors) evolved way earlier than everyone else and found themselves alone in the galaxy, so they got down to the seeding. (I know this has also been touched on in S5 of Discovery so please forgive me if they changed anything lol)