Modern humans evolved as East Africa’s environment transformed

Just before the beginning of the Middle Stone Age around 300,000 years ago, major technological and ecological changes followed an abrupt environmental shift in East Africa.

The results of three new studies suggest that, around the time that modern humans were evolving, environmental factors drove changes in human behavior toward more widespread dispersal, more trading, and better tool technology.

A team led by Rick Potts analyzed well-preserved sediments in the Olorgesailie Basin in Kenya. The experts found that the region began transforming around 800,000 years ago. The sediment samples provided evidence that the Olorgesailie Basin was mostly floodplains up until this time, when it started fluctuating between moist and arid conditions.

Carbon isotopes of the soil indicated that the region transitioned into a vast grassland. At this time, large mammals such as certain horse and elephant species were driven to extinction and smaller-bodied mammals emerged in their place. The researchers explained that this is another sign of climate variability.

When the climate became unpredictable, so did the food availability, and hunter-gatherers were forced to become more mobile and innovative. Archaeological evidence from the basin revealed that East African residents began using new tool-making techniques.

Almost all of the rock used to manufacture tools was once from a small area in the Olorgesailie Basin. After the dramatic climate change, tools were steadily replaced with obsidian from far away regions, suggesting that the people of this time had started traveling and trading for survival. The study authors said that this represents a significant revision in African hominin behavior around the time Homeo sapiens began to evolve.

In a separate study, a team led by Alison Brooks took a closer look at the human-made artifacts recovered from the Olorgesailie Basin. The research was focused on five sites that dated back between 500,000 and 298,000 years ago.

The ancient artifacts shed light on early technology and trade. Tools from the older site were bulkier weapons such as hand axes, and were made from localized volcanic rock. The younger site contained much smaller and more refined weapons, and around 42 percent of the more recent tools were crafted from obsidian, which was not found locally.

Using argon and uranium dating techniques, a third team led by Alan Deino set out to determine the timeline of sites within the basin. The research confirmed that older sites contained larger tools and that, beginning around 320,000 years ago, sites no longer had these types of tools. The study authors pointed out that these artifacts represent the oldest of their kind from the Middle Stone Age that have ever been identified in East Africa.

The research is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer