Study suggests ancient hominins used unmodified volcanic rock spheres as tools

A new study by Dr. Margherita Mussi, published in Quaternary International, highlights how naturally occurring basalt spheres may have been used by hominin species as a type of tool for more than 1 million years.

Across Pleistocene Europe, Asia, and Africa, researchers have recognized and been attracted to intentionally manufactured globular lithic items. Over the decades, they have termed these globular lithics “stone balls,” “spheres,” “speroids,” “bolas,” “balles polyédriques,” “boules polyédriques,” and “sphéroïdes à facettes.”

Hypotheses for their use range from percussion tools to hunting implements.

However, the equally fascinating naturally occurring lithic items have been less extensively studied, usually only given a cursory mention in the literature.

At Melka Kunture, Ethiopia, various Pleistocene sites within the Upper Awash region feature naturally occurring globular lithics, termed “spheres.”

Due to the area having been surrounded by multiple eruptive centers, these spheres are exclusively made of volcanic basalt, unlike many found in the surrounding areas, which are made of limestone.

Spheres used in the study analysis were collected from eight sites, namely Gombore IB, Atebella II, Garba XII, Gombore II-1, Gombore II-2, Garba IIIE, Gotu III, and Garba I.

Gombore IB was the oldest inhabited site, dated to 1.7 mya. It contained almost 5,000 stone tools, three spheres, and two fragments of a Homo cf. ergaster humerus (upper arm bone). Spheres that had been collected from these sites were housed at the National Museum of Addis Ababa.

Meanwhile, Garba I, III, and II were the youngest sites, dated to around 0.6 mya. Together, they contained 22 spheres and more than 7,000 lithic tools.

In total, more than 30 spheres were recovered and stored at the National Museum of Addis Ababa. In addition to these spheres, hominin bone fragments were found, providing evidence about which hominins may have collected and used these spheres.

Beyond the Homo cf. ergaster bones found at the oldest site, Gombore IB, the skull fragments of a Homo heidelbergensis and a Homo sapiens were recovered from Gombore II-1 and Garba IIIE, respectively.

These spheres were carefully analyzed, noting their weight, shape, size, and evidence of flake scars.

Despite being naturally occurring spheres, Mussi notes that they must have been deliberately brought to these sites, excluding natural transportation methods such as by water.

“The metric characteristics, shape, and orientation of pebbles transported by water have been researched and are well understood, as can be seen in previous papers. Furthermore, note that some of the sites are fine-grained deposits where the relatively heavy rock spheres are at odds with the surrounding environment, and also that the rather soft lapilli ones would have been easily crushed during water transport.”

Based on her analysis, Mussi suggests that these spheres were carefully selected by early hominin species to be used as percussive tools.

“I am convinced that the hard volcanic ones were used to knap/retouch lithic tools, while the rather soft lapilli ones were used for rubbing vegetables/hides or other stuff,” she explains.

Based on the above, these spheres were not traditional tools but can be described as outils à posteriori, (recognizable as tools only because they have been marked by use).

This discovery is particularly fascinating as it provides new insights into the evolution of tool behavior in early hominins and how they might have exploited their environments.

“It is possibly the first evidence of the use of natural shapes for varied activities, and this happened repeatedly over more than 1 million years of human evolution at Melka Kunture,” says Mussi.

“While hominin species were changing from Homo erectus to H. heidelbergensis, the round rocks were selected from various sources in a changing environment. In my opinion, this is good evidence of how the hominins were carefully exploiting any new resource and cleverly using them.”

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