Depending on the source, the earliest known use of it is somewhere between 12,000 and 20,000 years ago.
My own personal opinion is that it is as old a man.
All depends on your abilities. I've yet to see a person that was more capable than the firearm. In the few cases where it appeared so, better quality ammo solved the accuracy problem.
I too have a stool to sit on in my reloading room. When loading on my SDB I find it is better to stand than sit. I do need to get a padded floor mat to stand on though.
For most other tasks I sit. However I have learned to limit my sessions to about an hour. Then I take a little break before starting again.