One thing that was much more common in the fifties then it is today, at least in books, is what I call human aliens. These are aliens that look and act human to a degree that they could easily pass as one.
In TV and movies this is done for practical reasons, even Babylon 5 did it. Any number of good stories would not work if every alien was so different that it took years to understand them, and years longer to have a real conversation.
To that end if you do use it there really are only two ways to have it make any sense.
First, convergent evolution. This means that if you have two planets with the same conditions, and both develop intelligent life, they would look and act the same.
From a current science point of view, this just does not work. It would require EVERYTHING about us to be forced that way by evolution. There are just too many things that could have gone any number of ways. For example, do we really need five toes? Why not six? If aliens on a very earthlike planet also had five that would mean there was a strong evolutionary reason for exactly five.
That said the storyteller in me does like the idea, it means you can find aliens that might as well be human on the other side of the galaxy.
I would note that you should not make any two races EXACTLY the same, have at least some difference between them, even if it’s just skin or hair color.
Second is a common origin.
This one has less use by far, and is generally the center of a book, rather then being background info inside it.
This could mean that humans did not come from earth or that humans left earth before and we lost all of our technologies and had to start over.
To be honestly, neither of the above holds much water scientifically. It would take a VERY long time for the signs of a space faring society to be removed from a planet, like well past what it would take to make those technologies again.
If we did not come from earth you would almost have to have earth’s whole biosphere (maybe even including fossils) be transplanted too.
A much better way, at least in keeping with what we know scientifically, would be to have a third party involved. Meaning someone came to many planets and nudged life in the same direction, making it look like evolution. Or maybe they took humans from earth and planted them elsewhere, if it happened before written history then we would not know.