Changing climate conditions have been known to foster human evolution throughout history, especially in Africa, where our story first unfolded. However, new research has flipped the script on a common interpretation of an ancient African drying period between 3.5 million years and 2.5 million years ago, a time building up to the dawn of humans.
Scientists previously believed that Northern Africa experienced a dry spell between 3.5 million years and 2.5 million years ago, during a period known as the Plio-Pleistocene transition. This was thought to have somehow been instrumental in the emergence of the Homo genus, yet a study recently published in Science Advances has brought forth an opposing view; the transition period, it suggests, saw not continued drying, but stable rainfall in Northern Africa.
Early Humans Move to a Drier Climate
The Plio-Pleistocene transition marked the passage from the wetter and more humid Pliocene epoch (5.3 million years to 2.6 million years ago) to the colder and drier Pleistocene epoch (2.6 million years to 11,700 years ago). For most of the world, increased aridification led to dense forests being replaced with open grasslands. The advent of new habitats has been argued as a major factor in hominin evolution, particularly inviting the idea that drier conditions drove our ancestors’ adaptation to rely less on climbing trees and to start walking upright in foraging environments.
Previous studies have leaned on evidence of dust deposits in ocean sediment cores off the West African coast to explain the supposed drying period of Northern Africa during the transition. Increased levels of continental dust were found in samples dated to between 3.5 million and 2.5 million years ago, which scientists had originally interpreted as an expansion of the Sahara Desert resulting from decreased summer monsoons.
Read More: This Ancient North American City May Have Collapsed Due to Climate Change
Records of Ancient Rainfall
The new study took a different approach to studying ancient rainfall levels in Northern Africa. Instead of focusing on dust deposits, researchers examined leaf waxes produced by terrestrial vegetation.
“Plants produce these waxes during the summer growing season, so they provide a direct signal of summer rainfall over time,” said author Bryce Mitsunaga, who led the research while completing his Ph.D. at Brown University and is now a postdoctoral researcher at Harvard, in a statement. “We found that precipitation cycles didn’t change much even as all these larger changes in temperature and glaciation were happening.”
The researchers analyzed leaf waxes in the same cores where the dust evidence was found. Leaf waxes essentially hold past records of rainfall, preserving the isotopic signature of water that ancient plants absorbed. The presence of two forms of hydrogen typically indicates how past rainfall would have functioned. While a heavier form of rainwater with one neutron falls first, a lighter form with no neutrons falls after. A higher ratio of lighter hydrogen in leaf waxes, therefore, correlates with more sustained rains.
Rethinking African Aridification
Based on their analysis of leaf waxes, the researchers observed no significant drying trends in Africa during the Plio-Pleistocene boundary. Summer rains appear to have been stable on either side of the transition, meaning Africa may not have experienced the same global climate trends at the time.
The researchers say that the dust featured in previous studies may be related to something other than changes in rainfall, such as changes in wind pattern or intensity.
These findings may even inform future climate change, since carbon dioxide levels at the Plio-Pleistocene transition are thought to be similar to today's levels, according to the researchers. However, CO2 levels were decreasing during the transition, whereas they're increasing now.
“If we can see how global climate influenced what the water cycle is doing at that point in history, it could inform predictions of the future rainfall in this already water-stressed region,” Mitsunaga said.
This research also somewhat scrambles the story of ancient humans, clashing with the former idea that a drying period during the Plio-Pleistocene transition had a hand in the evolution of hominins like Homo habilis. Now that the mystery grows deeper, answers are needed to determine when exactly Africa shifted to drier conditions and what — if not a lack of rainfall — caused our ancient humans to evolve their most important characteristics.
Read More: What We Know About Homo Habilis
Article Sources
Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:
Geophysical Research Letters. Directly Dating Plio-Pleistocene Climate Change in the Terrestrial Record
Science Advances. Fundamentally unchanged northwestern African rainfall regimes across the Plio-Pleistocene transition
Jack Knudson is an assistant editor at Discover with a strong interest in environmental science and history. Before joining Discover in 2023, he studied journalism at the Scripps College of Communication at Ohio University and previously interned at Recycling Today magazine.
