One million years ago, an early species of humans — Homo erectus — roamed the lands of East Africa, gathering food, caring for their children and banding together in groups to fend off predators. As the centuries passed, their environment became increasingly dry, shifting frequently from wet to arid conditions and back again. Slowly, their home evolved from lush forests to low grasslands.

These early humans learned to adapt to the fluctuating environment and even figured out how to use stone tools, which became more and more sophisticated. They began crafting handaxes and smaller, pointed tools that were ideal for darts, arrows and other projectile weapons used for hunting.

It wasn’t just a coincidence that these advances in stone technology occurred at the same time as environmental changes, according to Daniel Deocampo, professor of geosciences at Georgia State. For two decades, Deocampo and his colleagues have studied how the evolution of the Earth may have influenced the evolution of hominins, a tribe of primates that includes modern humans and the extinct species of our lineage. The work is meant to indicate how humankind might survive massive climate change in the future.

“We’re looking at geological sources of information about how the environment changed and interpreting what that meant for changing biological systems,” he said.

Although contemporary climate change is linked to human activity, environmental shifts can also occur because of variations in the Earth’s orbit. As the Earth moves around the sun, the shape of its orbit changes from elliptical to circular, altering the amount of solar radiation the planet receives. The tilt of the Earth’s axis also changes over time, which can in turn affect the amount of incoming solar radiation. All of these changes influence how energy and water circulate around the Earth, leading to dry, arid spells at certain times and places.

Since 2013, Deocampo and an international team of scientists have been engaged in the Hominin Sites and Paleolakes Drilling Project, dredging up ancient sediments at five sites across the East African Rift in order to determine what the Earth’s climate was like at various periods in history. At the Lake Magadi basin in Kenya, the researchers collected and analyzed sediments that date back a million years.

“The sediments showed that Lake Magadi used to be freshwater and gradually has become more and more saline,” said Deocampo. “That tells us that arid conditions developed in East Africa about half a million years ago.”

On top of the prolonged shift to a drier climate, there were also shorter fluctuations.

“You might have some wet centuries and some dry centuries, so it’s getting wetter and drier and then wetter and drier in high-frequency cycles,” said Deocampo. “That’s when we see the Middle Stone Age tools being developed by early human ancestors.”

According to Deocampo, these frequent climate fluctuations may have contributed to evolutionary advances by favoring a gene pool that is highly adaptable. During dry conditions, for example, early hominins probably had to travel much farther to get fresh drinking water or find animals to hunt.

“Those changes may create selective pressure, as evolutionary biologists would call it,” said Deocampo. “Before the Middle Stone Age, most stone tools were fashioned from rocks that could be found nearby. But starting about 400,000 years ago, we begin to see stone tools that came from geological sources many, many kilometers away.”

The sediments also revealed details about the natural pace of climate change.

“We can see that, historically, cycles of wetter and drier episodes are happening over thousands of years,” Deocampo said. “That’s much slower than the climate change we’re witnessing today, which is happening over the span of a human lifetime.”

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