Stone arches.
All stone fireplaces are effectively a stone surround with a big hole in it that surrounds the spot where you have your grate or stove. The shape of that ‘hole’ can take many forms: You could just have three stone beams. (the top one resting on two at the side) and that I suppose is what the first stone fireplaces were like.
As time went on though, ‘design’ starting making an appearance, and some bright spark must have thought that he’d make his stone fireplace into an attractive feature, rather than just a construction to stop his house falling into his heat source.
The first thing was probably a bit of carving or smoothing of the stone blocks, but rapidly developing into full-on shaping of the stone into an architecural ‘feature’
Perhaps there was a bit of competetiveness involved: Perhaps some one visited the neighbours’ house/hovel and saw that they’d shaped their fireplace into an elegant shape? Who knows, but soon the fireplace had become an entity in its own right, and the rest is history!