Difference between 'probably' and 'possibly'

[Raf]

Hello everybody,
my guestion regards a test sentence, which goes:

"What could Harper and his companions ........ have seen?"

The gap is to be filled in with one of these: probably, likely, possibly, doubtless. It seems to me that the right choice will be probably or possibly, but as I don't see any difference between them, I can't choose. Please help:)

[Diverhank]

My vote is could possibly. These two often go together.

[Raf]

I see. Is there any other (semantic) difference between these two?