Brown UniversityA study led by researchers found that rainfall patterns in North Africa remained largely consistent between 3.5 million and 2.5 million years ago. This period marked a major shift in Earth's climate, with temperatures in the northern hemisphere falling and areas such as Greenland permanently covered in ice and snow.
The findings, published in the journal Science Advances, challenge the long-held view that North Africa was significantly dry during this period. This idea is linked to the appearance of the first known species of Homo in the fossil record, further dispelling the theory that drought may have affected early human evolution.
Compared to earlier studies, this study used a more direct indicator of rainfall and reached different conclusions by analyzing leaf wax produced by land plants.
"Plants produce these waxes during the summer growing season, so they can serve as a direct signal of summer rainfall," says Bryce Mitsunaga. He led the research while a doctoral student in the Department of Earth, Environmental and Planetary Sciences at Brown University and is now a postdoctoral fellow at Harvard University. "We found that even during these large changes in temperature and glaciation, the precipitation cycle did not change much."
Early evidence of drought in North Africa comes from dust deposits found in marine sediment cores collected off the coast of West Africa. These sediments contain fossilized microorganisms, plant material and other markers that help scientists reconstruct past climate conditions. The researchers observed a dramatic increase in continental dust in samples from the Pliocene-Pleistocene transition between 3.5 and 2.5 million years ago. The increase in dust has been interpreted as a sign of desert expansion, most likely due to a weakening of the summer monsoon.
In the new study, the researchers scrutinized leaf wax preserved in sediment cores previously used to track dust. Leaf wax carries the isotopic signature of water absorbed by the plant, which reflects rainfall. Rainwater contains both light hydrogen (which has no neutrons) and heavy hydrogen (which has one neutron). Heavier hydrogen tends to fall first during rainfall. Therefore, leaf wax with a higher proportion of light hydrogen predicts longer or more frequent rainfall.
Leaf wax analysis shows no clear drying trend at the Pliocene-Pleistocene boundary. Summer rainfall patterns remained largely stable on both sides of the border, suggesting that rainfall patterns in Africa were largely unaffected by global climate changes at the time (increased glaciation and falling temperatures in the Northern Hemisphere).
The research suggests that the dust found in previous studies may be caused by factors other than changes in rainfall - possibly changes in wind direction or strength.
The findings have a range of implications for understanding past and future climates, the researchers said.
Carbon dioxide levels at the Pliocene-Pleistocene boundary are thought to be similar to today, albeit in the opposite direction (increasing today, decreasing then).
"If we can understand how global climate affected the water cycle at that time in history, then we can predict future rainfall in this already water-scarce region," Mitsunaga said.
The findings raise new questions about North Africa's climate history and its impact on human evolution, said Jim Russell, a professor in the Department of Earth, Environmental and Planetary Sciences at Brown University and the study's senior author. The timing of the so-called African Drought Event coincides with the emergence of the fossil record of early great ape ancestors, including Homo habilis and Paranthropus, leading to speculation that drier conditions may have prompted humans to adapt to walking upright in new foraging environments. However, this speculation is complicated by the lack of aridity trends at the Pliocene-Pleistocene boundary.
"This will require new research to determine when and why Africa's climate and environment transitioned to a drier state, and to develop new theories to understand our ancestors," Russell said.
Compiled from /scitechdaily