Despite the vast distances separating their homeworlds, many humanoid species have been found to share a remarkable commonality in form and genetic coding. These similarities were believed to be evidence of a common ancestry, an ancient humanoid species, who lived in our galaxy’s distant past some four billion years ago.
To preserve their heritage, this species apparently seeded the primordial oceans of many potentially hospitable planets with encoded DNA fragments. The genetic information incorporated into the earliest lifeforms on those planets and through preprogrammed mutations caused by a genetic template, directed evolution toward a physical development similar to their own. Because of this controlled mutation mechanism, most habitable planets in the galaxy evolved with many physically similar species (e.g. fish, trees, dogs, insects), and on many of those worlds with at least one sentient species with a humanoid configuration. Most of these humanoids are even interfertile with each other.
This is from the star-trek wiki which gives an explanation as to why many of the aliens resemble each other—http://memory-alpha.org/en/wiki/Humanoid
Quote -
Despite the vast distances separating their homeworlds, many humanoid species have been found to share a remarkable commonality in form and genetic coding. These similarities were believed to be evidence of a common ancestry, an ancient humanoid species, who lived in our galaxy’s distant past some four billion years ago.
To preserve their heritage, this species apparently seeded the primordial oceans of many potentially hospitable planets with encoded DNA fragments. The genetic information incorporated into the earliest lifeforms on those planets and through preprogrammed mutations caused by a genetic template, directed evolution toward a physical development similar to their own. Because of this controlled mutation mechanism, most habitable planets in the galaxy evolved with many physically similar species (e.g. fish, trees, dogs, insects), and on many of those worlds with at least one sentient species with a humanoid configuration. Most of these humanoids are even interfertile with each other.