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 speakers of English because it makes the term sound plural. That’s why so many writers treat Homo sapiens as a plural noun—for example:
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 writers use Homo sapiens correctly—as a singular noun:
That’s 10 times longer than homo sapiens has existed [Anchorage Daily News]
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 best avoided in all contexts as even many nonscientists would recognize it as wrong. Homo sapiens is appropriate when referring to an individual human. For example, these writers use the term well:
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]
