Drier climate may have sparked human evolution

Dry lake bed and salt crusts at Lake Magadi, Kenya. (Credit: U. Arizona)

A drying climate may have led to the transition from our hominin ancestors to modern humans, a new study reports.

A rich assemblage of human fossils as well as stone tools and other archeological evidence is present in the rift valley of East Africa, a region often referred to as the cradle of humanity.

Since those discoveries, scientists have attempted to piece together the complex puzzle that is the history of our human origins, including the environmental context of that history.

The new study, which researchers based on lake sediment cores, is the first to provide a continuous environmental context for the diverse archeological evidence recovered from nearby localities in the rift valley basins of southern Kenya.

Researchers sampled the cores from Lake Magadi as part of the Hominin Sites and Paleolakes Drilling Project, or HSPDP, which Andrew Cohen, a professor in the geosciences department at the University of Arizona, directs.

dry lake bed
During the dry season, evaporating water leaves behind trona crystals, which grow on the Lake Magadi lakebed. The drilling rig used in the study towers above the dry lakebed. (Credit: Robin Renaut)

Leap in abilities

Lake Magadi, a shallow, periodically dry lake, is close to the Olorgesailie basin in Kenya, one of the most productive sites for archaeological evidence of human evolution in Africa. The profound climatic changes may have been driving forces behind hominin evolution, the origins of modern Homo sapiens, and the onset of the Middle Stone Age.

While previous hypotheses have related hominin evolution to climate change, most prior studies lack regional-scale evidence for a link between environment and hominin evolution, the authors write.

magadi core
A core sample of soild chert from one of the Lake Magadi drill cores. (Credit: U. Arizona)

A trend toward intense aridification in the area began 575,000 years ago, according to the study. The change, which researchers had not previously documented in continuous continental cores from East Africa, corresponds with faunal extinctions and a major transformation in stone tool technology documented in the Olorgesailie region.

“Much evidence for human evolution has been gathered from the area, but linking those records to detailed environmental records was missing until now,” says lead author Richard Owen of Hong Kong Baptist University. “There is a big gap in the records between the last Early Stone Age tools 500,000 years ago and the appearance of Middle Stone Age tools about 320,000 years ago. Our results plugged that gap with a continuous environmental record.”

A critical transition occurred sometime during this gap, a period for which archeologists have unearthed evidence of a leap in early humans’ abilities to make, use, and trade stone tools.

The cores from Lake Magadi provide the first detailed link between climate change and events known from the region’s archeological record.

magadi drilling tools
Coring bits used to cut into the lake floor sediments of Lake Magadi stacked up at the drill site. (Credit: U. Arizona)

“We have known for a while that the climate at the time was very varied, but the key here is that the records are in proximity to the archeological evidence for this transition,” says Cohen, who holds a joint appointment in the ecology and evolutionary biology department.

“The older stone tools found at Olorgesailie did not change much between 1.2 million and a half-million years ago. And suddenly, after 500,000 and before 320,000 years ago—we don’t know exactly when, but in that timespan—there was a critical transition in archeology when tools became more sophisticated and were transported over longer distances.”

Arrival of Homo sapiens

At the same time, the lake core records point to the climate becoming drier and more variable, there is evidence elsewhere in Africa of the appearance of modern Homo sapiens, prompting speculation whether the two are connected, Cohen says.

“Whether the evolution of bigger brains goes hand in hand with new toolkits is not entirely clear,” he says. “But the earliest modern Homo sapiens fossils from Morocco date back 325,000 years, the same time we see this transition of tools. And both happened around the same time that our core record indicates severe drying very close to the archeological sites.”

The deepest core drilled at Lake Magadi reached 200 meters (650 feet), penetrating all sedimentary layers down to the volcanic bedrock of the lake. Researchers cut the core samples—each about 10 feet long and 2 1/2 inches in diameter, into manageable 5-foot segments, package them, and airfreight them to the National Lake Core Facility at the University of Minnesota for curation, analysis, and storage.

scientists in a tent
From left to right: Study coauthors Veronica Muiruri, Anthony Mbuthia, and Andrew Cohen label a freshly sealed sediment core sample from Lake Magadi, Kenya. (Credit: Anne Billingsley)

According to the hypothesis of variability selection, a rapidly changing environment creates selective pressure that forces species to adapt to rapid change, Owen says. Under that scenario, the larger brains of anatomically modern humans would have allowed our ancestors to adapt quickly to an increasingly less predictable world.

“Now we have evidence that at the same time the toolkits were changing, the mammal fauna changed and the climate became more arid,” Owen says. “So you have a series of coincidences that makes you think, ‘This could be real.’ Now we can say when the environment changed and then compare that to the archeological evidence of the region.”

Researchers have completed drilling at other sites near HSPDP, as they gather more of the region’s climate data to continue studying the importance of environmental variability in the course of human evolution.

The research appears in the Proceedings of the National Academy of Sciences.

Source: University of Arizona