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Ancient Teeth Show Early Human Favored Right Hand
Scratch that: Marks on Homo habilis teeth suggest which hand the early human used to cut its meat.
Credit: Ronald J. Clarke

Nearly 2 million years ago, an early human used a stone tool to carve hunks of meat held in its mouth, an activity that left behind wear marks in its teeth.

And the direction of the grooves suggest that this individual had a dominant right hand.

Right-handedness is significantly more common than left-handedness in modern humans, and the trait emerged early in the lineage's evolution, researchers have said. This discovery, which presents the oldest evidence of right-handedness, could prompt a deeper look into the fossil record, to determine when human ancestors first demonstrated right-handed dominance. [The 10 Biggest Mysteries of the First Humans]

The fossil, a mostly intact upper jaw, was found in an archaeological dig site in northern Tanzania, in a location with stone tools and large mammal bones nearby. The jaw belonged to Homo habilis, a human ancestor that lived 2.4 million to 1.4 million years ago and is the oldest known ancestor in the Homo lineage.

Homo habilis means "handy man," and though the newly discovered specimen's hands were nowhere to be found, its jaw and teeth provided researchers with unexpected evidence of whether it was a so-called "righty" or "lefty."

The teeth were very well-preserved, with enamel still covering most of their surfaces. Close inspection of two central incisors in the jaw revealed concentrations of scratch marks; in one tooth in particular, they were primarily angled to the right side of the body.

According to the scientists, these marks were created over time, as the "handy man" was cutting up his meat. He would have gripped a hunk of flesh in his mouth, steadied the meat by pulling on it with one of his hands and used the other hand — which was probably the dominant one — to saw pieces of meat with a stone tool, which would occasionally scrape against his teeth.

a. A right-handed Homo habilis would have pulled at the meat with the left hand and cut with the right. b. A stereomicroscopic composite of the cast of one of the teeth used in the analysis.
a. A right-handed Homo habilis would have pulled at the meat with the left hand and cut with the right. b. A stereomicroscopic composite of the cast of one of the teeth used in the analysis.
Credit: David W. Frayer

The right-leaning direction of these scrapes — which have also been detected in fossil teeth belonging to Neanderthals and other human relatives — told the scientists that a tool held in the right hand had made the marks.

Humans aren't the only mammal species that favors one hand over the other; this trait appears in animals "from kangaroos to chimpanzees," the study authors wrote. However, having a dominant hand that appears overwhelmingly across an entire species is uniquely human; 90 percent of people today are right-handed, the researchers estimated, compared with 50 percent of individuals in humans' primate relatives. 

But when did humans first develop a preference for using the right hand over the left? Fossil arm bones might hold clues, but researchers would need both arms from a single individual to tell which hand the individual used more often in life, and that's proven hard to find in the fossil record.

However, this new discovery suggests that scientists could find the evidence preserved in fossil teeth, which are more plentiful than matched pairs of arm bones, the researchers said. While this is as yet the only example of dominant hand use in humans' early lineage, other fossils may provide the clues researchers need to trace the origins of "righty" vs. "lefty," the scientists said.

The findings were published online in the November issue of the Journal of Evolution.

Original article on Live Science.