About 90% of humans are right-handed, and this is one of the traits that distinguishes us from primates, among which there are about the same number of left- and right-handers (and there are species that have no leading hand at all).
Right-handedness has played an important role in
human evolution, and recently it has been revealed how, when and why this feature emerged. Interestingly, the clue was given not by hands, but...by the teeth of ancient humans.
We have long known that the human brain consists of two roughly equal halves: the left hemisphere is responsible for speech and motor skills, and the right hemisphere is responsible for visual-spatial attention.
Could right-handedness have played a role in interhemispheric asymmetry? Ancient stone tools made and used by our distant ancestors provide some clues.
Use of tools
The earliest stone tools date back 3.3 million years ago and were found in what is now Kenya
in Africa. Making the first stone tools required a high level of dexterity. We know from experiments replicating the tool-making process that the left hemisphere of the brain, responsible for planning and execution, is actively involved in this process.
At the same time, a larger proportion of people are right-handed when it comes to making tools compared to other species. This is most likely due to the fact that the left and right hemispheres control motor activity on opposite sides of the body.
While this relationship is not so simple, in most cases, right-handedness and brain lateralization seem to go hand in hand (pun intended).
So why use teeth to study right- and left-handedness? The answer lies in the lack of matching left- and right-handed bones in the paleontological record, especially in our earliest ancestors.
Without comparing left and right hands, differences in size and shape cannot be identified to determine which hand a person preferred to perform tasks requiring manual labor.
Teeth, on the other hand, tend to be relatively well preserved in the fossil record and may retain scratches, or "grooves," that indicate which hand a person wrote with.
In an earlier study, scientists noted grooves on the front of the teeth of European
Neanderthals. They hypothesized that these marks were left when the material was held in one hand, clamped between the front teeth, and worked with the other hand using a stone tool that sometimes struck those teeth.
Photo:stena.ee
These actions were replicated during experiments in which participants wore mouth guards. The results showed that pulling the material with the left hand and hitting it with the right hand produced right-skewed grooves on the teeth. Thus, right-skewed grooves are good evidence that a person is right-handed.
The subject of the new study, an ancient upper jaw, is the earliest evidence of right-handedness known in our genus Homo.
The jaw belonged to one of our earliest ancestors,
Homo habilis (literally "skillful man"), who roamed Tanzania in Africa about 1.8 million years ago. The jaw was found in Olduvai Gorge on the Serengeti Plain, which has yielded some of the earliest archaeological finds in the world.
Tooth marks
The authors of the study noted the presence of a series of grooves on the front side of the teeth. They used high-powered microscopes and digital cameras to study these grooves and their direction.
Interestingly, almost half of all furrows were directed to the right. The furrows directed to the right were especially common on the four anterior teeth (left and right central incisors, right second incisor and right canine).
This led the authors to conclude that most of the incisions were made by the right hand. They also suggested that the four anterior teeth with a large number of right-handed grooves were the focus of the processing.
The Homo habilis jaw is important because it is the oldest evidence of right-handedness in the fossil record, but it is also important because it suggests that humans already had a high level of brain organization 1.8 million years ago.
This brain development allowed us to learn essential skills, such as making stone tools, and may have paved the way for the development of speech. Thus, right-handedness means much more to us than just a preference for using the right hand.