You have a framing issue, so go ahead and invite a few experienced framing contractors/general contractors to look and estimate. They will know what to look for, you don't have to tell them what to look at.
The causes are unknown at this point, but once the joists are rebuilt, replaced or beefed up, you should have a nice flat floor.
Is there any possibility of saving the original floor boards or is it just standard that they will need to be replaced? Maybe it's something that is decided based on the amount of bowing?