> How would their ancestors have been surviving for all those thousands of years before Ritalin had been invented?

Different demands. I was diagnosed with ADHD and started medication. At that time, I was an engineer doing the problematic, bleeding-edge R&D-ish stuff pretty much by myself with close support of a small team and needed to focus, something that never came easy to me. The company was acquired, I moved on and accepted a technical management position at a much smaller place. Being constantly interrupted annoyed me and I stopped the medication (my psychiatrist agreeing). Without it, I felt again more at ease spread over more, different tasks.

So, to answer your question, people survived by doing jobs that didn't demand the kind of focus that's difficult for people with ADHD.

Such jobs, sadly, are not abundant anymore or evenly distributed geographically.