We know that, around 2.1 million years ago, Homo erectus, the first humans, migrated out of Africa. But how did they make this epic journey across territory that is covered in expansive desert?
For a long time, researchers have puzzled over how H. erectus managed to cross through northeastern Africa and the Middle East, to make their way into Europe. The desert landscape in this region is merciless today, and food and water would have been scarce.
Now researchers from Aarhus University, Denmark, suggest the desert may not have been a problem for H. erectus as it may not have existed at the time.
“We know that there are recurring periods when the climate in the Sahara changes. We call the phenomenon ‘Green Sahara’ or ‘African Humid Periods’,” Rachel Lupien, one of the authors of the study explained in a statement.
“During a green period, the desert shrinks significantly and is transformed into a landscape that resembles the savannas we know from eastern Africa today.”
According to Lupien and her team’s work, the Sahara may have been much greener at precisely the time when H. erectus first migrated from Africa. In fact, the Sahara may have been greener than any other time in the 4.5 million year period they studied.
H. erectus was, Lupien said, “most likely able to walk through a green corridor out of Africa.”
The seafloor tells all
Today, the Sahara is experiencing one of its dry spells. How long these periods last for tends to vary, but it appears the region experiences a full cycle – between dry and wet periods – every 20,000 years. The rainy periods have been dubbed “African Humid Periods” by Lupien and colleagues.
“How wet the humid green periods become, varies. There are indeed two other cycles that also come into play. One lasts 100,000 years and the other 400,000 years. Over the course of 100,000 years, the wet periods will thus vary and become wetter or drier than usual. The same applies in intervals of 400,000 years,” Lupien explained.
But how do we know how the Sahara looked several hundred thousand years ago? Well, the seafloor can tell us.
“Using core samples from the Mediterranean, we can see what the climate was like millions of years back in time. Layers of sediment are formed on the seafloor, and small molecules in these layers can tell us quite a bit about what the climate was like in the past.”
The seabed is basically a kind of logbook to past climates. When material from the land is blown into the sea, it drifts to the seafloor where it gradually builds up into new layers of “stuff”. Each layer has its own suite of biomarkers that store information about the climate at that time. One such marker is the wax plants use to protect their leaves.
“Wax gives leaves on trees, bushes and grasses the coating that makes them shine. When the plants die, most plant parts decompose quite quickly, while wax molecules can survive for a long time. That’s why we often find such molecules in sediments that are millions of years old,” Lupien added.
Interestingly, the hydrogen molecules in these waxes can be used to determine the levels of precipitation at the time.
“Water contains hydrogen, so we can use hydrogen to follow the water cycle. The water on Earth contains both regular hydrogen and heavy hydrogen (deuterium). When it rains a lot, the plants are able to absorb relatively less heavy hydrogen, while when it is dry they absorb more,” Lupien added.
But while the amount of heavy hydrogen in the wax may tell us when it rained in the distant past, it does not tell us anything about the plants that thrived at the time. However, the carbon atoms contained in the leaf wax can fill that gap.
“Broadly speaking, there are two types of plants. We also call them C3 and C4 plants,” Lupien explained. “About 90 percent of all plants are C3 plants. They thrive in most parts of the globe except in areas that are bone dry or very hot. C4 plants, on the other hand, are specialized to survive in areas where rain rarely falls and the temperature is high.”
C3 and C4 plants produce leaf wax with different amounts of heavy carbon, allowing us to distinguish between them in a sample. As such, they can tell us which plants were most dominant at a given time, and it seems C3 plants were most abundant at the time when H. erectus migrated. They are also more abundant at this time than at another other humid period in the last 4.5 million years.
The Sahara, it seems, was actually a grassland right at the time when our ancestors set off towards the Middle East and on to Europe.
“The climate has therefore most likely facilitated this migration,” Lupien concludes.
The study is published in the journal Communications Earth & Environment.