Homo sapiens, Latin for wise man or knowing man, is a singular phrasal noun. Like all Latin taxonomic names, Homo sapiens is italicized. The genus name (Homo) is capitalized, and the species name (sapiens) is not. After the first mention, it is often abbreviated H. sapiens.
The s at the end of sapiens is deceptive to English speakers because it makes the term sound plural, even though it’s not. That’s why so many writers give it plural verbs, as these do:
Homo sapiens are an awfully vain species. [Time]
Bingham said Homo sapiens have been so successful as a species because of three factors. [U.S. News & World Report]
These are exceptions though. For the most part, homo sapiens is correctly treated as a singular noun in edited writing, as in these cases:
Homo sapiens is entering uncharted territory. [io9]
But poor, fumbling Homo sapiens seizes on logos as a way of creating order in a confusing world. [The Economist]
If we were following Latin rules, the plural of Homo sapiens would be Homines sapientes, but few scientists actually pluralize it this way. Most turn Homo sapiens into a phrasal adjective and employ phrases such as “the two Homo sapiens males,” or “the group of Homo sapiens individuals.”
Still, writers can usually get away with using Homo sapiens as a plural, especially in nonscientific contexts. Homo sapien, on the other hand, is simply a nonstandard backformation, and it is extremely rare in edited writing. Homo sapiens is appropriate in reference to an individual human, as it is used in these cases:
The shape of LB1’s skull is just too different for it to be a Homo sapiens. [BBC News]
A human bone, taken to Switzerland for anthropological analysis, was of a homo sapiens, or modern man. [Reuters]