Otherwise known as "Middle Stone Age", the Mesolithic period covered a brief span of around 2,000 years. While it served as an important bridge between the Upper Paleolithic and Neolithic ages, the art of this period was, well, sort of boring. From this distance, it's not nearly as fascinating as the discovery of (and innovations in) the art of the preceding era. And the art of the subsequent Neolithic era is exponentially diverse, besides being more well-preserved and offering us thousands of examples of itself, instead of a "handful". Still, let's briefly cover the artistic events of the Mesolithic because, after all, it's a distinct era from any other.
What was going on in the world?
Most of the glacial ice in the Northern Hemisphere had retreated, leaving behind geography and climates familiar to us in the present day. Along with the glaciers, certain foods disappeared (the wooly mammoth comes to mind) and the migration patterns of others (reindeer) changed as well. People gradually adapted, assisted by the facts that more temperate weather and diverse edible plants were there to aid in survival.
Since humans didn't have to live in caves or follow herds any longer, this era saw the beginnings of both settled communities and farming. Apparently people also had a few spare minutes on their hands, because the Mesolithic period saw the invention of the bow and arrow, pottery for food storage and the domestication of a few animals - either for food or, in the case of dogs, for help in the hunting of food.
What kinds of art were created during this time?
There was pottery, though it was mostly utilitarian in design. In other words, a pot just needed to hold water or grain, not necessarily exist as a feast for the eyes. The artistic designs were mainly left up to later peoples to create.
The portable statuary of the Upper Paleolithic was largely absent during the Mesolithic era. This is likely a result of people settling down and no longer requiring art that could travel. Since the invention of the arrow had occurred, much of this period's "carving" time seems to have been spent knapping flint, obsidian and other minerals which lent themselves to sharp, pointy tips.
The most interesting Mesolithic art that we know of consists of rock paintings. Similar in nature to the Paleolithic cave paintings, these moved out of doors to vertical cliffs or "walls" of natural rock, often semi-protected by outcroppings or overhangs. Though these rock paintings have been found in locations ranging from the far north in Europe to southern Africa, as well as elsewhere around the globe, the largest concentration of them exists in eastern Spain's Levant.
While no one can say with certainty, the theory exists that the paintings' locations weren't chosen at random. The spots may have held sacred, magical or religious significance. Very often, a rock painting exists within close proximity to a different, more suitable spot upon which to paint.
What are the key characteristics of Mesolithic art?
(We'll just stick to rock paintings for this part.) Between Upper Paleolithic and Mesolithic eras, the biggest shift in painting occurred in subject matter. Where cave paintings overwhelmingly depicted animals, rock paintings were usually of human groupings. The painted humans typically seem to be engaged in either hunting or rituals whose purposes have been lost to time.
Far from being realistic, the humans shown in rock painting are highly stylized, rather like glorified stick-figures. These humans look more like pictographs than pictures, and some historians feel they represent the primitive beginnings of writing (i.e.: hieroglyphs). Very often the groupings of figures are painted in repetitive patterns, which results in a nice sense of rhythm (even if we're not sure what they're meant to be doing, exactly).