Drier, Less Predictable Environment May Have Spurred Human Evolution

Lake Magadi, pictured during the wet season, periodically dries and floods in response to seasonal rains that cover the lakebed with up to 3 to 6 feet of water. (Image: Richard Owen)
Lake Magadi, pictured during the wet season, periodically dries and floods in response to seasonal rains that cover the lakebed with up to 3 to 6 feet of water. (Image: Richard Owen)

Evidence of a variable but progressively drying climate coincides with a major shift in stone-tool-making abilities and the appearance of modern Homo sapiens. A progressively drying climate punctuated by variable wetter episodes may have precipitated the transition from our hominin ancestors to anatomically modern humans, according to research published in the online edition of the Proceedings of the National Academy of Science (PNAS).

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 study, 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. The cores were sampled from Lake Magadi as part of the Hominin Sites and Paleolakes Drilling Project, or HSPDP, which is directed by Andrew Cohen, University Distinguished Professor in the UA Department of Geosciences.

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. (Photo: Robin Renaut)

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

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 authors suggest that 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 in the paper “Progressive aridification in East Africa over the last half million years and implications for human evolution.”

According to the study, a trend toward intense aridification in the area began 575,000 years ago. The change, 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. The study’s lead author, Richard Owen, of Hong Kong Baptist University, said:

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. Cohen, who holds a joint appointment in UA Department of Ecology and Evolutionary Biology, said:

At the same time that 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 much speculation whether the two are connected, Cohen said, adding:

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

From left to right: Study co-authors Veronica Muiruri, Anthony Mbuthia and Andrew Cohen label a freshly sealed sediment core sample from Lake Magadi, Kenya. (Photo: Anne Billingsley)

From left to right: Study co-authors Veronica Muiruri, Anthony Mbuthia, and Andrew Cohen label a freshly sealed sediment core sample from Lake Magadi, Kenya. (Image: 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 said. Under that scenario, the larger brains of anatomically modern humans would have allowed our ancestors to adapt quickly to an increasingly less predictable world. Owen added:

Drilling at other nearby sites by HSPDP has been completed as researchers gather more of the region’s climate data to continue studying the importance of environmental variability in the course of human evolution.

Provided by: Daniel Stolte, University of Arizona [Note: Materials may be edited for content and length.]

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