As generally understood by the United Federation of Planets, a humanoid was an intelligent, bilaterally symmetrical, bipedal lifeform. Such a lifeform would be characterized by an upright posture, two arms, two legs, hands, feet, one thorax, a neck, and a head containing the brain. They were generally mammalian and originated on class M planets. It was common for them to have two to four sexes. Among the many humanoid species known throughout the Milky Way Galaxy were Andorians, Bajorans, Betazoids, Bolians, Cardassians, Hirogen, Humans, Klingons, Romulans, and Vulcans.
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 pre-programmed 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 (for example fish, trees, dogs, insects), and on many of those worlds with at least one sentient species with a humanoid configuration. Most of these humanoids were even interfertile with each other.
Anatomically, a significant percentage of those lifeforms were outwardly indistinguishable from Humans, right down to such morphological minutiae as the same racial phenotypes, hair distribution patterns, fingernails, and cartilaginous folds of the nose and ears. The reason they were so astonishingly similar to the Human race was because the "Human form" represents the basic way that evolution most likely takes for the dominant mammalian race on an average class M planet under the influence of the genetic codes and the other humanoid races (for example Klingons, Cardassians, Vulcans) developed under very specific and usually unique environmental conditions.
In one of the most remarkable scientific detective stories in history, archaeologist Galen of Earth uncovered the similarities between certain DNA sequences in lifeforms from widely separated planets. He discovered that these DNA sequences were a puzzle deliberately left behind by these ancient progenitors. The DNA sequences, when assembled by protein-link compatibilities, formed an ingenious computer program, a message of peace and goodwill to their progeny. This message, assembled in 2369 in an unprecedented example of interstellar cooperation, was a confirmation that many humanoid species in this galaxy are indeed members of the same family, despite their significant differences. (TNG: "The Chase")
The humanoid form was further distributed by species such as the Preservers, a super-race which passed through the galaxy rescuing primitive cultures in danger of extinction, and seeding them on other worlds where they could be allowed to grow and thrive. In addition, when the non-corporeal lifeform Sargon was encountered by the crew of the USS Enterprise in 2268, he implied that some humanoid races, such as the Vulcans, were the product of ancient colonization by his race before they lost their physical forms. (TOS: "The Paradise Syndrome", "Return to Tomorrow")
Taxonomy of humanoids
The Xindi did not apply the term in the same way as many of the other social groups in the Milky Way Galaxy. For them, "humanoid" was a word that only applied to the Xindi-Primates, despite the fact that the Arboreals and the Reptilians also generally met the criteria. (ENT: "Proving Ground", "The Council")
The TNG Season 1 Writers'/Directors' Guide states that "Humanoid in our series means a life form which is similar to humans. (Two legs, arms, head, etc.) In the past, we have met humanoid aliens, such as Klingons, Vulcans and Romulans."