The use of a tool that resembles the modern day paint brush dates back to prehistoric times. Wall and cave drawings display a form of strokes which was discovered to not be made with sticks or stones. Fossilized brushes were discovered in Egypt. They were made of split palm leaves while others, used by different ancestors to beautify their surroundings, were made from sticks, bones and even wood shavings.
We do not know the whole history of the paintbrush, but we do know that they were originally made with animal hair. The best still are. Sable brushes are for small painting, and for larger brushes they are only assembled after harsh winters in Siberia and Mongolia when a sable’s tail becomes particularly bushy. Applying the handle to the gradated hairs was largely done by hand up until the 18th century. The finer art brushes are still manufactured this way. Artificial materials such as rayon and nylon are now used and assembled by machine for a less expensive and larger brush make.
A fun fact: the reason that the end of the brush is call a bristle is because it was originally made from the bristle of hogs, pigs, and boars, all characterized with particularly rough hair.