Scientists developed an experiment that reveals whether our long-dead evolutionary cousins Neanderthals are right or left handed - just from dental records. They did it by relying on evidence from stone tools, and humankind's innate klutziness.
Most of us have looked skeptically TV shows where scientists gesture at six fragments of bone and say that the bones told them about an ancient human's diet, social habits, tool making prowess, winning smile, and sense of humor. How can anyone tell all that from a few bones? It's true scientists can't tell much about a person's sense of humor unless they were buried next to a fossilized rubber chicken, but it's amazing what information can be gleaned with a little data and a lot of ingenuity. For example, scientists have constructed a test to show that they can very likely interpret handedness from scratches on prehistoric teeth.
Neanderthals were hunters, who used animal skins for a variety of reasons. To make a skin useable it has to be cut, and stretched, and stripped of meat and sinew. In order to do that, Neanderthals would have to stretch it out evenly and hack or scrape at at the taut skin with stone knives. Since they wouldn't have stretching tools at the ready, a good way to do this would be to stretch the skin by clamping down on one end with their teeth and pulling at the other with one hand. They would then use their other hand for the precise work of scraping or cutting with a knife.
Most people today have had to, at some point, execute a similar maneuver, using their mouth as another clamping hand in order to free up their dominant hand for delicate work. It's clumsy, and occasionally, and I am not saying this happened to me, a person can accidentally overshoot with one hand and wack themselves in the face. Modern scientists believe that Neanderthals did just that. They smacked themselves in the teeth with their knife-wielding dominant hand. Scientists had modern people try to do the same thing, wearing mouthguards, and instructed them to deliberately smack themselves in the teeth. Right handed people had scratches on the mouthguard that started high on the left and slanted down and to the right. Left-handed people produced scratches that sloped down and to the left.
An analysis of Neanderthal teeth showed consistent scratches down and to the right on 25 out of 27 marked teeth. This indicates that lefties haven't gained - or lost - ground in half a million years. Whatever influences handedness was built into the brain before homo sapiens ever existed.