Paleoenvironments Opens Window on Human Evolution

Aerial view of red-brown Oltulelei Formation channel deposits superimposed on white lacustrine deposits of the Olorgesailie Formation. Base level shifts in the southern Kenya rift valley transformed the Olorgesailie Basin from lake to land multiple times over the past 500,000 years, requiring early humans and other inhabitants to adapt to rapidly changing physical landscapes. 

Behrensmeyer used traditional section measuring and mapping to document the strata across three different sub-basins, then analyzed and correlated hundreds of section logs with the help of new computer-based methods. This research also involved intensive laboratory work using 40Ar/39Ar absolute dating (by coauthor Alan Deino) of the volcanic tephras to pin down the ages of the strata and the archeological sites. The team, including co-author (and Olorgesailie project leader) Richard Potts, then worked together to integrate the geology, the ages, and the archeological sites.

Behrensmeyer notes, “The synthesized result is a picture of geological change in which both tectonics and climate altered the East African landscape inhabited by early populations of our genus, Homo.”

The Olorgesailie rift basin was relatively stable for about 700,000 years, but then was divided into sub-basins and subject to rapid change over the last 500,000 years. This shift correlated in time with a transition from the Acheulean (“hand-axe”) technology to Middle Stone Age technology, along with other changes indicating that hominins were adapting to greater uncertainty in their environments.

This study also provides new, well-calibrated information on sedimentation in active rift settings as well as a three-dimensional, basin-scale geological history that can be integrated with emerging drill-core paleoclimate records from southern Kenya. The stratigraphic record preserved in the Oltulelei Formation advances understanding and poses new questions about how tectonics and climate interact to create different types of rift valley deposits.

Three related reports were published in Science on 15 March and are now online at the Science website at http://science.sciencemag.org. The reports are: “Environmental dynamics during the onset of the Middle Stone Age in eastern Africa” by Richard Potts et al.; “Chronology of the Acheulean to Middle Stone Age Transition in Eastern Africa” by Alan Deino et al.; and “Long-distance stone transport and pigment use in the earliest Middle Stone Age” by Alison Brooks et al.
 

 Contacts and sources:
 Geological Society of America, The (GSA)

Citation: The Oltulelei Formation of the southern Kenyan Rift Valley: A chronicle of rapid landscape transformation over the last 500 k.y.
A.K. Behrensmeyer, R. Potts, and A. Deino.
Geological Society of America Bulletin