As anyone familiar with Thor's hammer Mjolnir knows, sometimes the line between tool and weapon can be mighty thin. So when researchers unearthed a collection of smooth round stones at the Cave of Hearths in South Africa's Makapan Valley nearly 30 years ago, they were a bit puzzled. Were they tools or weapons? The prevailing theory has been the former, but new research provides evidence that the rocks were more likely projectiles than pounders.

To come to their conclusion, a group of kinesiologists, archaeologists and psychologists from Indiana University Bloomington (IU) and the University of Liverpool worked together to analyze 55 of the stones, which are about the size of tennis balls and estimated to be between 1.8 million and 70,000 years old.

To those stones, they applied a theoretical model developed by the Perception/Action Lab at IU, directed by Geoffrey Bingham. Part of that model includes "judging an object's throwing affordance, which is the selection of the best object in terms of size, weight and shape for throwing at maximum distance, speed and damage."

Their results showed that 81 percent of the rocks were ideal for hitting a target at a 25-m (82-ft) distance and inflicting the most amount of damage.

The archeological site in South Africa's Makapan Valley(Credit: Judy Maguire)

The team also simulated how the rocks would fly if thrown by an expert, and what kind of damage they could inflict on an impala. The researchers chose that animal because it was found in abundance in the Cave of Hearths excavation site.

"None of the spheroids could kill an impala, but most of them would inflict significant damage if they hit, even at maximum distance," the researchers conclude in a paper detailing their work in the journal Scientific Reports. "The disabled animal could then be dispatched with a suitable weapon (eg, club, spear, stone) or run to death through blood loss and exhaustion, and of course multiple spheroids could be thrown at the animal as well."

The research analyzed such criteria as stone weight, throwing height and angle, release velocity, impact velocity, and the diameter of impact on the animal's torso.

According to a IU report on the research, Bingham says that the human shoulder joint and our ability to perceive distances makes us uniquely equipped to throw things at objects about 20-30 m (66-98 ft) away, which is just over the 60-ft (18-m) distance that exists from a pitcher's mound to home plate in a baseball stadium.

"Humans are the only animals – the only primates even – with that talent," Bingham said. "We can throw something to hit something else – like a quarterback throwing to the running back all the way down the field. That's how in large measure we survived the ice ages. The available food was largely on hoof, or it was 'mega-fauna,' such as a mammoth. You don't want to get close to them."

"The ability to throw great distances was not a small thing," he added. "It was how we got lunch."

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