Here is my theory on the topic. I would guess our ancestors used their muscles more and so had more back strength possibly leading to fewer back troubles. Also, I remember being a young girl, I was thin, very thin, and could sleep on the floor with no problem. All of these curves and bumps we now have need to be able to sink into the mattress to keep our spines straight. Interestingly, when I was in Japan the beds in the hotel I stayed in were rather firm, and they are a thin population for the most part.
Also, I believe sleeping on your left side in a slight fetal postition is the way to go, with a pillow under your neck and head, being careful not to have it under your shoulders. You can add a pillow between your legs and under your right arm if you want total support. I once heard that sleeping on your left is good for digestion also, not sure if that is true.
Having the room really dark helps, and quiet. Oh, and don’t drink anything two hours before bed time. And, try not to even have soup or cereal if you eat dinner late. Having to get up to pee is annoying.
And, don’t go to bed way before your bedtime. I think our ancestors probably went to bed when they were sleepy, not trying to go to sleep at a certain time, because they have an alarm the next morning that will torture them into wakefulness.