In reference to evolution theory and human antiquity, polygenism was (and in cases is still) the belief that Homo sapiens is in fact composed of multiple taxonomical classifications deriving from multiple ancestral pairs in accordance to the more modern term of "race" (a term which too has its origins in polygenic thought). This word's application to humans originated in Victorian or pre-Victorian Britain as the study of geology and paleontology gradually turned into anthropology and the scores of theories associated therewith.

As one might guess, this theory has its ties to eugenics, and is somewhat rooted in white supremacy, something that Victorian science aimed without end to establish formally and objectively. Some would find it ironic that fundamentalist Christian philosophy would actually negate the possibility of polygeny, as all humans would necessarily have evolved from a single pair. In fact, for this reason and others more humanitarian, Christianity and eugenics were necessarily at odds for much of the term of popularity of the latter.

Not all theories of polygenism are necessarily eugenic or racially biased, though these are the systems that characterize the concept foremost. There is, as with every taxonomic category of interest, dispute as to what defines a separate distinction and what does not; regionality can sometimes have an impact upon these decisions, though it does not in the official taxonomy of humans.

I'd like to point out that species is not the most specific practical taxonomical division, even though it's the most basic officially--frequently further distinction seems to be made on other bases. For instance, the domestic dog is "Canis lupis familaris", while the wolf is simply "Canis lupis". Similarly, some animals are classified according to geographical origin due to physical characteristics that differ significantly. It would not be unreasonable from a purely scientific viewpoint, to the very best of my understanding (which is not professional), to classify humans in a similar manner. Taxonomy is a messy, arbitrary science.

The social, moral, and psychological ramifications of that distinction are another matter entirely. However, polygenism is not all about taxonomy--it is about ancestry. That matter, too, isn't exactly sealed at this point. While all humans alive today may have shared one ancestor, there is no way they could've shared both parts of a couple, or the gene pool would be atrociously corrupted, as per the founder effect. Likely H. sapiens would not exist, if that were true. In line with the widely accepted belief that evolution is gradual, it is unlikely that one female monkey suddenly was born 'more human' and gave birth to all mankind.

If anyone has better information on taxonomy or the technical description of polygenism as it might relate to this subject, it would be appreciated. While I think I've contributed something factual, some of this is looking unclear.