I suppose it would have been rather amazing if a stool or chair had been left in the Grotte Chauvet, so we could imagine a painter sitting on it 20,000 years ago, to paint a bear. And bands of nomadic hunters can't have much stuff: it's hard to carry chairs on their backs- much easier to imagine a chair if there's a house to keep it in. But still, surviving chairs are quite early. Egyptians were using pretty modern-looking chairs by 1000 BCE. Depictions of chairs are even earlier. You can see stools in Sumerian cylinder seals, circa 2500BCE , and though there's not much detail, it's possible to see they were made of individual pieces. So, this was not an unsophisticated piece of furniture- not a lump of rock or a chunk of a tree.

It's worth pointing out that chairs are considered a rather hard thing for a cabinetmaker to create. They have to be strong, able to take abuse, from different directions. They must look good on all sides, fit a variety of human bodies and be light enough to easily get moved around to where they're useful ( the fact that they also get ordered in sets so the maker has to produce a number of them that look the same is also a problem, but not important here). It looks as though the Mesopotamian cultures had gotten to something workable by the time they were putting them on seals, so it's not unreasonable to assume that stools had been made for a good while before that.

It's another question, an intriguing one, as to how they'd be used. Chairs have often been used to indicate place, precedence. Even in surviving 17th c. farm furniture you can see status differences between the decorated armchairs and the plain joint stools. At Louis XIV's Versailles, priority of place was often expressed in who got to sit on a chair, who had to settle for a stool and who had to stand. Maybe this started very early. The gods sitting on chairs being approached by worshipers, on those cylinder seals, may be showing something about chair-status throughout the society.