Study Debunks Theory of Climate-Driven Evolution in Africa

The study questions the long-held belief that northern Africa became arid around 3 million years ago, which coincides with the appearance of the earliest known hominids in the fossil record, the journal Science Advances reported.

A study led by researchers at Brown University found that rainfall patterns across northern Africa stayed mostly consistent between 3.5 and 2.5 million years ago. This period marked a major shift in Earth’s climate, when the Northern Hemisphere cooled and regions like Greenland became permanently covered in ice.

The findings challenge long-standing views that northern Africa became significantly drier during this time. That belief has been linked to the appearance of the first known species of the genus Homo in the fossil record, fueling theories that drying may have influenced early human evolution.

Compared to earlier research, this study used a more direct indicator of rainfall by analyzing leaf waxes produced by land plants and reached a different conclusion.

“Plants produce these waxes during the summer growing season, so they provide a direct signal of summer rainfall over time,” said Bryce Mitsunaga, who led the research while completing his Ph.D. in Brown’s Department of Earth, Environmental and Planetary Sciences and is now a postdoctoral researcher at Harvard. “We found that precipitation cycles didn’t change much even as all these larger changes in temperature and glaciation were happening.”

Earlier evidence for drying in northern Africa came from dust deposits found in ocean sediment cores collected off the West African coast. These sediments contain fossil microorganisms, plant material, and other markers that help scientists reconstruct past climate conditions. Researchers observed a sharp increase in continental dust in samples dated between 3.5 and 2.5 million years ago, during the Pliocene-Pleistocene transition. This rise in dust was interpreted as a sign of desert expansion, likely due to weakening summer monsoons.

In this new study, researchers closely examined leaf waxes preserved in the same sediment cores that had previously been used to track dust. Leaf waxes carry the isotopic signature of the water that plants absorb, which reflects the amount of rainfall. Rainwater contains both light hydrogen, which has no neutrons, and heavy hydrogen, which has one. The heavier form tends to fall first during rain events. Therefore, waxes with a higher proportion of light hydrogen indicate longer-lasting or more frequent rainfall.

The leaf wax analysis revealed no significant drying trend at the Pliocene-Pleistocene boundary. Patterns of summer rainfall remained largely stable on either side of the boundary, indicating that African rainfall patterns were largely unaffected by changes in global climate — decreasing temperature in increasing Northern Hemisphere glaciation — that were occurring at the time.

The research suggests that the dust found in prior studies is attributable to something other than changes in rainfall — perhaps changes in wind patterns or intensity.

The findings have a range of implications for understanding both past and future climate, the researchers say.

Carbon dioxide levels at the Pliocene-Pleistocene boundary are thought to be similar to where they are today, although heading in opposite directions (increasing today and decreasing then).

“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.

Jim Russell, a professor in Brown’s Department of Earth, Environmental and Planetary Sciences and senior author of the study, said the results raise new questions about the climate history of northern Africa and its implications for human evolution. The timing of the supposed African drying event coincides with the appearance in the fossil record of early hominid ancestors including homo habilis and Paranthropus, leading to speculation that dryer conditions may have driven adaptations for upright walking in a new foraging environment. But the lack of a drying trend at the Pliocene-Pleistocene boundary complicates that story.

“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,” Russell said.

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