Analysis casts doubt on ancient drying of northern Africa’s climate, raising new questions about early human evolution

A study led by researchers from Brown University reveals that the precipitation patterns across North Africa remained largely stable between 3.5 and 2.5 million years – a pivotal period in the history of the earth climate when the northern hemisphere has cooled and places like Greenland became freezed.

New conclusions, published in Scientific advancesDriving long -term interpretations of the history of the climate of North Africa, which had suggested that the region has dried considerably during this period. Timing coincides with the appearance of the first known member of the genus Homo in the fossil file, leading to speculation that this drying can have played an important evolutionary role near the dawn of the human line.

But compared to previous studies, this new study analyzed a more direct indicator of precipitation – leaf waits produced by land vegetation – and has reached a new conclusion.

“Plants produce these waxes during the summer growth season, so they provide a direct signal from summer precipitation over time,” said Bryce Mitsunaga, who has conducted research while finishing his doctorate. In the Brown department of Earth, the environment and the planetary sciences and is now a postdoctoral researcher at Harvard. “We found that the precipitation cycles did not change much even if all these more important changes in temperature and glaciation occur.”

The previous evidence of drying across North Africa comes from dust deposits found in the nuclei of oceanic sediments removed from the West African coast. Oceanic sediments preserve fossil microorganisms, plant material and other markers who help scientists follow the climate at the time.

The researchers discovered that the amount of continental dust found in the samples increased considerably in the samples dated between 3.5 and 2.5 million years, a period known as the Pliocene-Pléistocene transition. This increase in dust was interpreted as an expansion of the Sahara desert caused by a decrease in summer monsoons.

For this new study, the researchers meticulously analyzed the waxes of the leaves in the same nuclei in which the evidence of dust was found. The leaf waxes preserve the isotopic signature of water plants absorb as they develop, and this signature varies with the amount of rain that falls.

Rainwater generally contains two forms of hydrogen: light hydrogen without neutrons and a heavier shape with a neutron. Rainwater with heavier hydrogen falls first. Leaf waits with a higher ratio of light hydrogen therefore indicate more sustained rains.

The analysis of the leaf wax has not revealed any significant drying tendency bordering on the plicene-péistocene. The summer precipitation models remained largely stable on each side of the border, which indicates that the African precipitation patterns were not affected by changes in the world climate – describing the temperature in the increase in the glaciation of the northern hemisphere – which occurred at the time.

Research suggests that dust found in previous studies is attributable to something other than precipitation – perhaps changes in wind models or intensity.

The results have a range of implications to understand the past and future climate, according to the researchers.

It is believed that carbon dioxide levels at the Pliocene-Pléistocene border are similar to their situation today, although heading in opposite directions (increasing today and decreasing).

“If we can see how the global climate has influenced what the water cycle is doing at this stage of history, it could shed light on the predictions of future precipitation in this region already stressed by water,” said Mitsunaga.

Jim Russell, professor of the Brown department of the Earth, the Environment and the Planetary Sciences and the main author of the study, said that the results raise new questions about the history of the climate of North Africa and its implications for human evolution.

The time of the assumed African drying event coincides with the appearance in the fossil file of the first hominid ancestors, in particular Homo Habilis and Paranthropus, leading to speculations according to which the darkest conditions may have led to adaptations for a vertical march in a new environment of food research. But the absence of a drying trend on the Pliocene-Pléistocene border complicates this story.

“This calls for new research to determine when and why the climate and African environments have gone to a drier state and new theories to understand our ancestry,” said Russell.