For a long time, that was also my impression. But it's wrong. Mostly.
It turns out that there isn't a single definition for the word species—there are multiple definitions that lead to different results. Reading through the Wikipedia article about species will give you a sense of the different definitions. http://en.wikipedia.org/wiki/Species
If you use the Biological/Isolation species definition, I think you could argue that humans are or were multiple species. (Please, someone correct me if I'm wrong on this.)
It turns out that there isn't a single definition for the word species—there are multiple definitions that lead to different results. Reading through the Wikipedia article about species will give you a sense of the different definitions. http://en.wikipedia.org/wiki/Species
If you use the Biological/Isolation species definition, I think you could argue that humans are or were multiple species. (Please, someone correct me if I'm wrong on this.)