sentient, adjective \ˈsen(t)-sh(ē-)ənt, ˈsen-tē-ənt\
Able to feel, see, hear, smell, or taste. Responsive to or conscious of sense impressions. Aware.
When it comes to science fiction, it's a given that there will be non-human sentient species.
Most science fiction writers limit the number of sentient species that evolved on any given world to one or two. While this seems to make sense at first, current genetic studies on the evolution of humans suggest that there may have been four or five different species of human to begin with, of which only one species survived to the present day.
This makes me wonder - is it better to intentionally limit the number of sentient species native to a particular world for science fiction settings? Or do you think it is more fun to play around with a handful of different species?
From all the reading I've been doing, science fiction writers tend to limit themselves to one sentient species per planet nowadays. But I am really curious to see what people think about this issue. Especially if there is a point where the number of species begins to undermined the willful suspension of disbelief for the reader.