Ball-shaped stones unearthed at an archaeological site in South Africa may have been used as weapons by early humans rather than as tools, scientists have said. Considering the damage they can make if thrown, these spheroids could have been used for hunting or defence against an enemy.

The stones, which are between 1.8 million and 70,000 years old were found about three decades ago at the Cave of Hearths in South Africa's Makapan Valley.

They are considered to be one of the oldest known "technologies"' in the world – they would either have been fabricated by human hands or naturally shaped in this way, but transported to places of use.

Since their discovery, the purpose of these stones had however remained a mystery for archaeologists. Most hypothesised that they were used as tools for shaping or grinding other materials.

The recent study, published in the journal Scientific Reports, suggests that throwing stones at a target has been a unique property of humans for millions of years.

With their spherical shapes, heavy weights and relatively big sizes – about the size of a tennis ball – the stones could have been very useful as projectile weapons.

Perfect for throwing at distance

The team, which was constituted of psychologists, kinesiologists and archaeologists at Indiana University, used computational models to analyse stone objects collected at the site. They evaluated the stones "throwing affordance" – assessing which ones were the optimal size, weight and shape for hitting targets at a 25-metre distance.

Stones throwing
The archaeological site near South Africa's Cave of Hearths is where the round stones were found Judy Maguire

"Past research on biomechanics and perception, reveals that the human shoulder joint and perceptual abilities are uniquely specialised for throwing objects aimed at a particular target at a distance of 20 to 30 meters," says lead author Geoffrey Bingham.

With his colleagues, he found out that 81% of the stones would have been well suited to hit preys or other targets at this distance. Improving hunting techniques is such a way would have been a key development, allowing humans to get food more easily.

"The stones, which predate thrown spears, could therefore have served as projectile weapons for hunting and defence since they were found to perform best as hunting weapons when thrown overhand.The ability to throw great distances was not a small thing. It was how we got lunch," Bingham concludes.