Did ancient hunters throw these stone balls?

One of the 55 ball-shaped stone objects found at the South African site. (Credit: Judy Maguire)

Scientists once thought spherical stones found in South Africa were used as tools, but now there’s evidence they were actually weapons for defense and hunting.

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, appears in the journal Scientific Reports.

“The ability to throw great distances was not a small thing. It was how we got lunch.”

“Our study suggests that the throwing of stones played a key role in the evolution of hunting,” says Bingham, an author of the study and a professor in the psychological and brain sciences department at the Indiana University Bloomington. “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.”

The use of these stones, which date from between 1.8 million and 70,000 years ago, has puzzled archaeologists since they were unearthed at the Cave of Hearths in South Africa’s Makapan Valley nearly 30 years ago.

site near South Africa's Cave of Hearths
The site near South Africa’s Cave of Hearths where the round stones were found. (Credit: Judy Maguire)

The study’s lead author, Andrew Wilson of Beckett Leeds University in England, and coauthor Qin Zhu of the University of Wyoming, were both PhD students in Bingham’s lab. The other researchers are archaeologists Lawrence Barham and Ian Stanistreet, both of the University of Liverpool in England.

The team’s conclusions are based upon a theoretical framework and computational tools developed in the Perception/Action Lab, which is directed by Bingham, who investigates human coordinated action and perceptual capabilities. This 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.

Humans are really good at throwing stuff

Employing these methods, the researchers used computational models to analyze 55 ball-shaped stone objects from the South African site, finding that 81 percent of the stones were the optimal size, weight, and shape for hitting such a target at a 25-meter (82-foot) 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 specialized for throwing objects aimed at a particular target at a distance of 20 to 30 meters, Bingham says.

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The stones, which predate thrown spears, likely served as projectile weapons for hunting and defense since they were found to perform best as hunting weapons when thrown overhand, he adds.

“Humans are the only animals—the only primates even—with that talent,” Bingham says. “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.”

Previous research by archaeologists suggests that spherical stones were used as percussive tools for shaping or grinding other materials. Most of the objects analyzed in this study had weights that produce optimal levels of damage from throwing, however, rather than simply being as heavy as possible.

“Imagine a human, searching for an object to throw so as to cause the most damage possible to potential prey or a competitor,” Wilson says. “This is a perceptual task: the person needs to perceive throwing-relevant properties of objects and be able to discriminate between objects that vary in those properties.”

“The ability to throw great distances was not a small thing,” Bingham adds. “It was how we got lunch.”

Source: Indiana University