Climate Change May Have Spurred Hominin Tech Advances
Rick Potts surveys a field of Early Stone Age handaxes found at Olorgesailie Basin, Kenya. | Human Origins Program/ Smithsonian
Environmental factors in East Africa may have spurred a turning point in hominin behavior between 800,000 and 320,000 years ago, encouraging more widespread dispersal, trade and novel tool-making. The evidence of these changes is highlighted in three studies in the March 16 issue of Science.
Kenya's Olorgesailie Basin is a land formation sculpted by a lush, ancient lake that has since dried up. Its sediments are very well-preserved, offering researchers insights into how much the lake thrived and ebbed between 1.2 million and 500,000 years ago. These environmental records, in combination with ancient tools that have been excavated from archeological sites within Olorgesailie, help tell the tale of a hominin species that could very well be the ancestor of modern humans.
The sediments suggest that the Olorgesailie was a grassland-filled basin containing a lake that fluctuated in size until roughly 500,000 years ago, when the climate began to oscillate between wet and dry states. Within the basin, this changing climate caused erosion and river sediments to accumulate, and floodplains to develop. Furthermore, carbon isotopes of soil samples suggest that grass increasingly took root in the region during this time.
"The kinds of environmental changes we detect very likely lowered the predictability of food and water available through this region of East Africa," explained Rick Potts, director of Smithsonian Institution's Human Origins Program and a co-author of the Science studies. "Times of scarce resources would have been challenging to all organisms in the area, including early human foragers."
For human hunter-gatherers, this food unpredictability may have pushed people to travel farther, gather more information and perhaps even engage in trade. Unfortunately, the geological record for this crucial time is in part missing due to erosion in the Olorgesailie Basin between 499,000 and 320,000 years ago.
Artifacts recovered from geological layers after this gap, however, reveal how much human behavior had changed.
Potts noted that older tools excavated from the Olorgesailie Basin, which were made roughly 500,000 years ago, were large, bulky handaxes — a kind of technology known as Acheulean. Yet by 320,000 years ago, Acheulean handaxes were gone, replaced by more carefully designed tools, including refined stone points that would have served as the tips of projectile weapons. These more sophisticated tools marked a new era for hominins, called the Middle Stone Age.
Importantly, the archeological evidence reveals that people were not just crafting new technology, but traveling farther to get more desirable materials for their tools.
By 320,000 years ago, hominins at Olorgesailie Basin were making sophisticated tools. | Human Origins Program/ Smithsonian
The researchers note that 500,000 years ago, 98% of rock used to manufacture tools found at the site was from a tiny localized area of the Olorgesailie Basin. spanning just five kilometers. By about 320,000 years ago, tools were replaced with obsidian from regions farther away — up to 95 kilometers away over rough terrain — an indication of travel and potentially trade. Obsidian was likely highly valued because it is the sharpest type of rock when fractured, Potts noted.
Furthermore, about 46,000 obsidian flakes were recovered from a Middle Stone Age site, indicating that obsidian was brought in as raw material and manipulated onsite, rather than imported as finished artifacts.
"We take this to provide evidence a system or network of procuring obsidian involving distant social groups, and potentially the exchange of resources. This form of social group interaction over long distances very likely offered a means of mitigating periods of environmental risk, uncertainty and scarcity," said Potts.
Along with exotic obsidian, archeologists discovered green, brown or white chert rock at Middle Stone Age sites that appears to come from faraway places. Of particular interest is a lump of ochre pigment with two perforated holes, which makes it among the oldest-known clearly worked pigments, Potts and his colleagues said. They noted that exotic bright red and black rocks may have been valued, and worth transporting, for their intense color and possibly used as symbolic communicators of identity or status.
"If we think about how people today use color, such as in clothing, flags, body decoration, among other things, it seems that color would have been procured and used for symbolic purposes," Potts said.
Taken together, this archeological evidence outlines the critical window of human evolution during which our species began to differentiate and become distinct from an earlier ancestor," said Potts.
Next, Potts and his team plan to analyze a new, 166-meter deep drill core obtained near another Middle Stone Age archeological site at Olorgesailie. "This drill core is likely to fill in the gap in our current environmental record and to provide exquisite details about the climate dynamics and the adaptive challenges during the Acheulean-to-Middle Stone Age transition," he said.