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How environmental changes can help make ancient humans more adaptable

An unforgiving environmental turn deserves at least some credit for the flexibility of behavior that has characterized the human species since our African origins about 300,000 years ago, a new study suggests.

For hundreds of thousands of years in parts of East Africa, food and water supplies have remained fairly stable. But new evidence shows that from about 400,000 years ago, hominids and other ancient animals in the region faced a harsh environmental calculation, says a team led by paleoanthropologist Rick Potts of the Smithsonian Institution in Washington, D.C.

The weather began to fluctuate drastically. Faults caused by volcanic eruptions fractured the landscape and reduced the size of the lakes. Large animals disappeared and were replaced by smaller creatures with more diverse diets. These changes heralded a series of booms and busts in the resources needed to survive hominids, Potts and colleagues reported Oct. 21 in Science Advances.

At that time, hominids in a place called Olorgesailie in what is now Kenya transformed their culture. Scientists argue that this change, between about 500,000 and 320,000 years ago, was probably influenced by increasingly unpredictable periods of water and food shortages.

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Stone hand axes and other cutting tools made with local stone had dominated African tool kits for 700,000 years before the transition. After that, Middle Stone Age tools, such as spearheads made from rocks imported from distant sources, gained popularity, as the Potts team has already found (SN: 15/03/18). The tools of the Middle Ages of the Stone were smaller tools and made with more care. Very scattered hominid groups began trading with each other to obtain rock and other suitable resources for tool making.

Potts has long argued that Olorgesailie hominids evolved genetically and behaviorally to handle frequent climate change, a process called variability selection (SN: 7/12/97). But the new study indicates that ancient humans have adapted to various environmental forces, not just climate fluctuations, he says.

“A cascade of ancient ecological changes has led to alternating periods of abundance and scarcity of resources, probably helping us become the most adaptable (hominid) species that has ever existed,” says Potts.

Erosion in Olorgesailie has destroyed layers of sediment dating back to the transition to the Middle Stone Age. Thus, investigators hired a Kenyan company to drill as deep as possible in the Koora Basin, located about 24 kilometers south of Olorgesailie. Dating the extracted 139-meter-long core has found that sediments spanned much of the last million years, making it the best environmental record from that time period anywhere in Africa, Potts says.

photographs of sediment nucleiA segment of an East African sediment core (bottom) includes volcanic eruption signatures, dry and rainy seasons, and changes in soil cover (all show close-ups).Human Origins Program / Smithsonian, central image courtesy of LacCore / Univ. of Minnesota

Chemical and microscopic studies of the core have revealed signs of volcanic eruptions that have created faults that have fragmented the Olorgesailie landscape for about 400,000 years. Small ponds and lakes then replaced the larger lake basins at a time when rainfall was becoming inconsistent. Intermittent and increasingly frequent dry periods caused severe water shortages.

Vegetation changes followed. Round-trip changes from grassy plains to forests have denied large animals, such as elephants, regular access to ancient grazing areas. Landscape failures have also reduced the size of any grazing area available. As the Potts team found earlier, smaller animals with diverse diets, including antelopes and pigs, became prominent in Olorgesailie during the Middle Stone Age. Researchers claim that the stone tools of that time could have been adapted to hunt and process smaller prey.

Potts states that the peaks and failures in resource availability during the Middle Stone Age generally lasted a few thousand years, based on evidence from the Koora sediment core. That time resolution is a great improvement over previous studies that used global climate data to reconstruct ancient African environmental changes that have occurred over tens of thousands of years, says archaeologist and paleoanthropologist Manuel Will of the University of Tübingen in Germany. did not participate in the new research.

The findings of Pott and colleagues "provide even better evidence of a link between environmental changes in East Africa and the spread of Middle Stone Age technology and increased mobility throughout the landscape," says paleoanthropologist Chris Stringer of the History Museum Born in London. Although it is not yet clear where in Africa, as well as when and by whom, the tools of the Middle Stone Age were invented, early humans would find such invaluable tools to adapt to environmental disturbances, Stringer says.

Olorgesailie's Middle Stone Age boom and bust scene may not apply to other parts of Africa where spearheads and related instruments did not appear until later, warns archaeologist Lyn Wadley of the University of the Witwatersrand in Johannesburg. . In those scenarios, the tools of the Middle Stone Age proved to be useful even for groups that enjoyed relatively stable sources of water and food.

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