Not sure if this helps but i was taught that there are effectively two kinds of, or maybe better to say "aspects of" depression and these are reactive and endogenous. In both kinds you get chemical changes in the body so neither is the same as "feeling sad"or being upset.
The endogenous kind is what we would now describe as the depressive aspect of being bi polar. Its purely a chemical imbalance and likely one of the things that a primitive society would call being possessed. As it probably wouldn't be considered lucky or safe to mate with a possessed person, also any existing children would likely be considered tainted, its easy to understand how the genetic aspect of the condition would quickly vanish.
Reactive depression is more complicated. Almost anybody hit with enough bad stuff, for long enough can become reactively depressed. Antidepressants can work, indicating that there is a chemical imbalance issue, but other non chemical interventions can also help which suggests that the normally healthy body can self regulate. We know that activities that increase the pleasure hormone, such as exercise, even doing enjoyable activities can help as can some kinds of talking therapies.
RATTER, I'd love to know what led you to the conclusion that being bipolar was a learned behaviour?