someone/somebody
no one/nobody
anyone/anybody
Note that there is no significant difference in use between -one and -body, except perhaps that -one is more commonly used. Note also that no one is the only one that is written as two separate words.
I think the clue as to why they are all used with singular verbs lies with -one, meaning one person, or one thing or not one thing or not one person. Compare the following:
There's someone at the door who wants to interview you.
There are some people at the door who want to interview you.
I'm sorry, I'm busy right now. Isn't there anyone else who can do it? What about Fred?
http://www.bbc.co.uk/worldservice/learningenglish/grammar/learnit/learnitv114.shtml
Pozdrawiam,