A lush, green Sahara Desert helped early humans migrate out of Africa

New research indicates that Homo erectus likely capitalized on a “greener” corridor through the Sahara Desert in northeastern Africa, which was wetter and more vegetated than it is today, during their migration out of Africa. Climate cycles aligned to create this green passage, facilitating their journey.

Early humans and the green Sahara Desert

Around 6 million years ago, a significant divergence occurred between humans and chimpanzees, our closest relatives in the animal kingdom. Over time, our early human ancestors developed distinct characteristics, such as the ability to walk upright and manipulate tools.

The transition from tree-dwelling to bipedal locomotion allowed our ancestors to venture out of their forest habitats and explore new environments.

Approximately 2.1 million years ago, a group of early humans known as Homo erectus embarked on a migration out of Africa. This journey took them through northeastern Africa and the Middle East, eventually leading to Europe and Asia.

For many years, researchers have pondered how Homo erectus managed to traverse the arid and unforgiving deserts that now dominate these regions. New research from Aarhus University suggests that during the time of Homo erectus migration, the Sahara Desert may have provided a green corridor through which they could travel.

Clues about past Sahara climates

Rachel Lupien, one of the researchers involved in the study, explains. She says, “We know that there are recurring periods when the climate in the Sahara changes. We call the phenomenon ‘Green Sahara’ or ‘African Humid Periods’. During a green period, the desert shrinks significantly and is transformed into a landscape that resembles the savannas we know from eastern Africa today.”

The Sahara, as we know it today, is currently experiencing a dry period. However, the region undergoes cyclic changes, with full cycles of wet and dry intervals occurring approximately every 20,000 years. These wet intervals are referred to as “African Humid Periods” by Lupien.

The extent of the wet periods during these humid intervals varies, influenced by two additional cycles. One cycle lasts 100,000 years, while the other spans 400,000 years. This means that the wet periods fluctuate within these longer cycles, becoming either wetter or drier than usual.

But how can we accurately determine what the climate was like several hundred thousand years ago in Africa?

Core samples and biomarkers

According to Lupien, valuable insights into past climates can be obtained from core samples taken from the Mediterranean. The layers of sediment on the seafloor provide a historical record of climate conditions.

Lupien elaborates, “Over time, new layers of material blow onto the seabed from northern Africa, creating sediments that serve as a logbook of past climate conditions.”

Within these layers, researchers can find biomarkers that store information about the past climate. One such biomarker is leaf wax, a series of molecules produced by plants to protect their leaves.

Lupien explains, “When plants die, most parts decompose quickly, but wax molecules can survive for a long time. This is why we often find these molecules in sediments that are millions of years old.”

Green Sahara Desert and the water cycle

The chemical composition of leaf wax, particularly the hydrogen molecules, provides insights into the amount of precipitation that occurred during the formation of a specific sediment layer.

Lupien says the role of hydrogen in tracking the water cycle, “Water contains hydrogen, and we can use hydrogen to track the water cycle. When there is abundant rainfall, plants absorb relatively less heavy hydrogen. Conversely, during dry periods, they absorb more heavy hydrogen.”

The research team can analyze the amount of heavy hydrogen in leaf wax to deduce periods of abundant rainfall and dry conditions. However, hydrogen alone does not provide information about the types of plants that thrived in the wet climate.

Shifting plant dominance

This is where the carbon atoms in leaf wax come into play. They can differentiate between two types of plants: C3 and C4 plants.

According to Lupien, “Approximately 90 percent of all plants are C3 plants, which thrive in most regions except extremely arid or hot areas. C4 plants, on the other hand, are specialized to survive in low-rainfall and high-temperature environments.”

By analyzing the proportion of heavy carbon in leaf wax, researchers can determine which type of plant dominated during a particular period.

Based on their findings, Lupien states, “During the migration of Homo erectus from Africa, we observed a higher proportion of C3 plants compared to any other humid period in the past 4.5 million years. This suggests that the wetter climate transformed parts of the region from a desert into grasslands and savannah.”

“Green Sahara” corridor paved the way

The occurrence of green periods in Africa, similar to ice ages in northern latitudes, can be attributed to small orbital variations of the Earth around the Sun, known as Milanković cycles.

Lupien explains the role of these cycles in increasing precipitation in the Sahara. She said, “The Earth wobbles slightly in its orbit around the Sun, causing climate fluctuations every 21,000 years, which lead to the ‘African Humid Period’.

“Additionally, the circularity of the Earth’s orbit varies periodically, with some periods being more elliptical and others more circular. These variations result in fluctuations occurring every 100,000 and 400,000 years,” concluded Lupien.

Approximately 2.1 million years ago, multiple cycles likely coincided, creating the greenest period in the Sahara. This timing aligns with the migration of Homo erectus, suggesting that the climate conditions likely facilitated their journey.

In summary, recent research indicates that during the time of Homo erectus migration, the Sahara Desert experienced significant changes, creating a green corridor that allowed early humans to venture out of Africa.

By analyzing sediment core samples from the Mediterranean, scientists can uncover information about past climates based on biomarkers such as leaf wax. This research sheds light on the environmental conditions our ancestors faced, and demonstrates the pivotal role climate played in shaping human history.

The full study was published in the journal Communications Earth & Environment.

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