At a Glance
- A recent study challenges the belief that northern Africa became drier, showing that summer rainfall patterns remained stable between 3.5 and 2.5 million years ago.
- Previous interpretations were based on a sharp increase in dust found in ocean sediments, which scientists had attributed to the expansion of the Sahara Desert and weakening monsoons.
- Researchers analyzed the chemical composition of ancient plant leaf waxes, a more direct indicator of precipitation that records the specific isotopic signature of rainwater from past eras.
- The study concludes that summer rains were governed by solar radiation cycles while stronger winter winds, intensified by global cooling, were responsible for the increased dust levels.
- These findings complicate the evolutionary narrative that drier conditions spurred the development of early human ancestors, suggesting environmental pressures on hominids were different than previously thought.
A new study published in the journal Science Advances challenges a long-held theory about the climate history of northern Africa, revealing that summer rainfall patterns remained surprisingly stable even as the world plunged into a major ice age. Research led by Brown University scientists suggests that between 3.5 and 2.5 million years ago, a critical period for both global climate and early human evolution, the region did not dry out as dramatically as previously thought, complicating our understanding of the environmental pressures that may have shaped our earliest ancestors.
For decades, the prevailing view of African climate during the Pliocene-Pleistocene transition—the period when Earth’s climate cooled and large ice sheets formed in the Northern Hemisphere—was based on dust. Scientists analyzing sediment cores from the ocean floor off the African coast found a sharp increase in wind-blown dust beginning around 2.5 million years ago. This was interpreted as evidence of an expanding Sahara Desert caused by weakening summer monsoons. However, the new study analyzed a more direct record of rainfall: the chemical composition of ancient leaf waxes preserved in the same sediment cores. This provided a more nuanced and different picture of the past.
The researchers focused on hydrogen isotopes locked within the fossilized plant waxes. Plants incorporate water into their tissues as they grow, and water contains two main forms, or isotopes, of hydrogen: a standard, lighter form, and a rare, heavier form. During extensive rain events, the heavier hydrogen tends to fall first, leaving the remaining clouds enriched with lighter hydrogen. Therefore, leaf waxes with a higher proportion of light hydrogen serve as a chemical fingerprint for periods of more sustained rainfall. The analysis showed that summer rainfall patterns did not change significantly across the Pliocene-Pleistocene boundary, indicating that the African summer monsoon was not weakening.
These findings suggest that the forces driving summer rain and winter winds in northern Africa are separate. Summer rainfall, the study concludes, is primarily driven by solar insolation—the amount of energy from the sun hitting the African continent—which follows predictable 21,000-year cycles. The increased dust, on the other hand, was likely caused by stronger winter trade winds, such as the Harmattan, which was intensified by the growing temperature difference between the cooling high latitudes and the equator. This distinction has significant implications, challenging theories that link a drying climate to the evolution of early hominids, such as Homo habilis, who appeared in the fossil record during this period. “This calls for new research to determine when and why African climate and environments transitioned to a drier state and new theories to understand our ancestry,” said Jim Russell, a professor at Brown University and senior author of the study, in a university press release.
References
- Mitsunaga, B. A., Jewell, A. M., Buchanan, S., Crocker, A. J., Wilson, P. A., Herbert, T. D., & Russell, J. M. (2025). Fundamentally unchanged northwestern African rainfall regimes across the Plio-Pleistocene transition. Science Advances, 11(25), eads3149. https://doi.org/10.1126/sciadv.ads3149
- Stacey, K. & Brown University. (2025, June 20). Analysis casts doubt on ancient drying of northern Africa’s climate, raising new questions about early human evolution. Phys.Org; Brown University. https://phys.org/news/2025-06-analysis-ancient-drying-northern-africa.html
