No doubt if astronauts from Earth were to journey to some other planet with civilized life, they would probably find beings who looked very different from a human. But what is the chance that somewhere out there exists extraterrestrials who very closely resemble humans?
Some astronomers who have considered this question have guessed that the chance of extraterrestrials looking like humans is basically zero. Such thinkers have pointed out that human evolution was the result of a long series of events stretching over millions of years, and many of these events were due to blind chance. As the whole series of events leading to the current human form is unlikely to be repeated again on another planet, these thinkers reason, we should not assume there are any extraterrestrials who look like us.
I'm not sure that this reasoning is very compelling, as it ignores the fact that there are multiple ways to randomly wander about and finally reach the same destination. If I leave Grand Central Station, and randomly walk around New York City, finally arriving at the Chrysler Building, it would not be right to reason that some other person leaving Grand Central Station and wandering around would not reach the Chrysler Building, on the grounds that he would have been most unlikely to have followed my random path. The person might have followed any number of other paths, and also arrived at the Chrysler Building. Similarly, we can imagine a thousand different paths of evolution which might lead eventually to the existence of some organism resembling a human.
A better argument against the likelihood of extraterrestrials looking like us is based on the sheer fact that there are so many different ways in which the body of an extraterrestrial might look. Extraterrestrials might have two legs, three legs, four legs, or five legs. They might be blobs that move around like a jellyfish, or aquatic creatures that swim. Or perhaps they could have wings. If they have two legs, they might have two gigantic legs like a Tyrannosaurus Rex, and only very small arms. Some argue that there are so many possible combinations that we should assume there would only be an extremely small chance (or perhaps no chance) that extraterrestrials would look like us.
Such arguments have a certain degree of force, although I think those who argue along these lines exaggerate their case. It is true that there are many different types of organisms that could be produced by evolution, but if an extraterrestrial planet were to have civilized life, it would have to have some life form that met certain characteristics. A civilized organism would have to have powerful appendages capable of manipulating tools. That would mean a civilized extraterrestrial presumably could not look like a horse or a limbless blob or a limbless sphere or a shark (although an appearance like a centaur could not be precluded). There are surely many combinations that one could imagine for the body of a civilized extraterrestrial, but the number of combinations is probably less than a million. I would also argue that we should expect the simplest types of body arrangements to be more commonly found across the universe. If we had ten legs, ten arms, and five heads, it might be right to say that a weird body arrangement such as ours must be very rare. But as we have only two legs, two arms, and one head, we should not assume that our type of body arrangement is all that rare across the universe.
Another factor to consider is the huge number of planets on which intelligent life could evolve. Our galaxy the Milky Way contains perhaps 400 billion stars, and scientists recently estimated that there may be as many as 60 billion planets in our galaxy which may have life. Our galaxy is only one of billions of spiral galaxies in the universe. So even if it is very unlikely that civilized beings on another randomly selected world would look like us, we should still expect that there are many planets that have civilized beings that look not very different from us. Given a sufficient number of chances, even extremely unlikely things happen not just once, but repeatedly.
But if man goes exploring the distant stars, should we expect to someday find by chance civilized creatures that look rather like us? No, we should not, not unless we were to visit thousands of stars. The chance of another planet developing civilized life that looks like humans is sufficiently low that we should not expect to find humanoid creatures on the first few hundreds of planets that we explore.
Somewhere out there probably exists creatures looking much like us, but it may be eons before we ever get in touch with such beings (unless we get help from some galactic social network that helps civilized beings find out about the existence of other civilized beings with a similar appearance). We should expect that the first few dozens or hundreds of alien species that we communicate with in future ages will look very, very different from us.
SBH (Single Blue Humanoid) seeks social interaction across the light years