[Ronin]

There has been a lot of thought and discussion on that point.

One of the conclusions that people like to point out is that the human version of symmetry, two eyes for binocular depth perception, two arms for manipulation, the legs for ambulation, a single sense organ cluster located extremely close to the brain... all of these factors simply add up to the best design possible.

That being the case, evolution on other worlds would probably follow the same rules and come up with a dominant life form that would look pretty much similar to what we developed into.

For years, science fiction writers have been trying to come up with believable aliens that look “alien”. Different senses, different ways of getting around, etc. They haven’t had much luck.

[mysterio]

That being the case, evolution on other worlds would probably follow the same rules and come up with a dominant life form that would look pretty much similar to what we developed into.

If their native planet was identical to Earth. What if it's bigger/much greater gravity? What if they don't breathe oxygen?

These aliens are never seen wearing space suits, so I assume they breathe oxygen. They have limbs like ours, so I have to assume their gravity is similar. So, we are left to believe somewhere out there beyond the stars there's a planet just like earth that produces space aliens who look nearly identical to us, yet cannot be photographed and leave no evidence of themselves.

Not likely.