I read above that your cycles are quite short but that’s not the case for everyone. Mine tend to be many months depressed with shorter bursts of hypomania. It would fit well, I could be a sloth all winter and then run around super efficient in spring.

In your example they’d give you different things to do, it’s not worthy of being abandoned at birth. There’s archeological evidence of mature injured people being cared for. Slightly wacky, unstable behaviour wouldn’t rule one of contributing to the gene pool. It would just mean every generation was used to having unstable people. It was probably just accepted as part of the range of human behaviour. It’s not like they had to sit in traffic or wear a suit all day sitting still in boring meetings - things that can be unbearable for me when hypomanic. And depressed I could still be useful sitting in a cave by a fire snuggling babies or preparing food. Modern life has the benefit of medication but also a lot of complicated rules and expectations that make getting by with bipolar really hard.

Plus cave people didn’t have to worry about ruining and wasting their lives. Life was short! And they merrily passed their genes down.