The Acheulean Factory Floor
The Olorgesailie basin in Kenya's Great Rift Valley holds one of the world's largest collections of Acheulean handaxes. For 700,000 years, early humans—likely Homo erectus—crafted these large, teardrop-shaped tools with remarkable consistency. Excavations, first started by Mary and Louis Leakey in 1943, have revealed dense accumulations of these tools, suggesting campsites or butchery locations. One site has over 2,300 stone artifacts surrounding the fossilized bones of an extinct elephant, Elephas recki, with cut marks on its ribs.
The toolmakers were selective about their materials. They primarily used up to 14 different types of local volcanic rock, quarrying large flakes from outcrops within a few kilometers of the main sites. The standard handaxe was about 14 centimeters tall, flaked on two sides to create a sharp edge for butchering animals and processing plants. This technology remained stable for an immense period, from about 1.2 million to 500,000 years ago, in an environment that was also relatively stable—a area of grassy plains and a persistent large lake.
An Abrupt Technological and Environmental Shift
Around 320,000 years ago, the archaeological record at Olorgesailie shows a dramatic change. The large, all-purpose Acheulean handaxes disappear completely. In their place, a new toolkit emerges, characteristic of the Middle Stone Age (MSA). These new tools were smaller and more precisely crafted. They included pointed tips designed to be hafted onto shafts as projectile weapons, as well as specialized scrapers and awls.
This technological revolution coincided with a period of intense environmental instability. Geological evidence shows that frequent earthquakes fractured the landscape, and the climate began to fluctuate wildly between wet and dry periods. The large, shallow lake that had defined the basin for millennia was replaced by smaller, more ephemeral ponds. This environmental upheaval led to a massive faunal turnover; specialized grazing animals like extinct species of zebra and a giant baboon (Theropithecus oswaldi) were replaced by smaller, more adaptable herbivores. The hominins at the site, likely early Homo sapiens, also changed their behavior. They began using non-local materials like obsidian and chert, sourced from up to 90 kilometers away, which indicates planning and possible trade networks. Researchers also found processed manganese and ochre, indicating the use of pigments for symbolic communication.