These stones, which date from between 1.8 million and 70,000 years ago, -- previously thought by some to be used as tools -- served instead as weapons for defence and hunting, according to a study.
These stones puzzled archaeologists since they were unearthed at the Cave of Hearths in South Africa's Makapan Valley nearly 30 years ago.
Previous research by archaeologists suggested that spherical stones were used as percussive tools for shaping or grinding other materials.
"Our study suggests that the throwing of stones played a key role in the evolution of hunting," said one of the study authors Geoffrey Bingham, Professor in the Indiana University Bloomington College of Arts and Sciences in the US.
"We don't think that throwing is the sole, or even primary, function of these spheroids, but these results show that this function is an option that warrants reconsidering as a potential use for this long-lived, multipurpose tool," Bingham noted.
The research, which combines knowledge about how modern humans perceive an object's "throwing affordance" with mathematical analysis and evaluation of these stones as projectiles for throwing, appeared in the journal Scientific Reports.
Using these methods, the researchers used computational models to analyse 55 ball-shaped stone objects from the South African site, finding that 81 per cent of the stones were the optimal size, weight and shape for hitting a target at a 25-metre distance. The stones are about the size of tennis balls but much heavier.
The team also simulated the projectile motions the spheroids would undergo if thrown by an expert, as well as estimated the probability of these projectiles causing damage to a medium-sized prey such as an impala.
Research on biomechanics and perception, particularly vision, shows 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 metres, Bingham said.
The stones, which predate thrown spears, likely served as projectile weapons for hunting and defence since they were found to perform best as hunting weapons when thrown overhand, he added.
