As our earliest ancestors migrated from Africa to the Sahara, a green corridor emerged across the Sahara. New research from Aarhus University shows this. About 6 million years ago, something spectacular happened in the deep mountains and old forests of eastern Africa. Chimpanzees, our closest relatives in the animal kingdom, evolved in one direction, while our earliest ancestors continued in another.
Over the next few million years, the differences between early humans and chimpanzees grew. Our ancestors climbed down from the trees and began walking upright on two legs, freeing their hands to use tools.
This was the beginning of human development, which would eventually lead to the conquest of much of the globe.
About 2.1 million years ago, the first humans - Homo erectus - migrated from Africa. The migration took them through northeastern Africa and the Middle East - an area today largely covered by desert - before reaching Europe and Asia.
Researchers have long speculated how Homo erectus made it across the dry, unforgiving desert, which had neither food, water, nor shade.
New research from Aarhus University suggests that Homo erectus may not have crossed the desert when it left Africa, explains Rachel Lupien, one of the researchers on the new results.
"We know that the climate of the Sahara changes frequently. We call this phenomenon the 'Green Sahara' or the 'African Wet Period.' During the Green Period, the desert shrinks significantly, becoming a landscape similar to what we know today as the eastern African savanna. Our results show that the Sahara was greener during the migration of the first Homo erectus than at any other time in the 4.5 million years we studied. So it is very likely that they were able to travel out of Africa through a green corridor."
The species that conquered the world
The earliest Homo erectus appeared in eastern Africa more than 2 million years ago.
Homo erectus was the first human to learn to carve an ax out of stone. These axes were likely used as weapons for knocking down prey and cutting flesh from bones. They may also have been the first people to learn how to control fire.
Homo erectus was slightly shorter and more muscular than modern humans. Their hips are wider and their skulls are longer. Additionally, their brains are much smaller, about half the size of ours.
Undersea environments reveal past climates
The Sahara Desert as we know it today is in the midst of one of its dry spells. Dry periods vary in duration, but about every 20,000 years, the Sahara goes through a complete cycle of both rainy and dry periods. These rainy periods are what Rachel Lupien calls the "African Humid Period."
The degree of moisture in the "wet green phase" varies. In fact, there are two other cycles at work. One lasts 100,000 years, the other 400,000 years. So, over the course of 100,000 years, wet periods will change, becoming wetter or drier than usual. The same is true for the 400,000-year interval.
But how do we know what the climate was like in Africa hundreds of thousands of years ago? She explains that the seafloor can tell us, and in fact, we already know a lot about past climates for this very reason.
"Using core samples from the Mediterranean Sea, we can see what the climate was like millions of years ago. Sedimentary layers form on the seafloor, and the small molecules in these sedimentary layers can tell us a lot about past climates."
Help from substances that make leaves glow
Over time, material blown in from northern Africa formed new layers of sediment on the sea floor, which slowly descended above the sea. The buried seabed therefore acts like a logbook, telling us what climate was like in the past.
Within these layers are a suite of biomarkers that store information about past climate. One of the markers is a series of molecules that plants use to protect their leaves. They're also called leaf waxes, explains Rachel-Lupien.
"Leaf wax provides a protective coating to the leaves of trees, shrubs and grasses, making them sparkle. While most plant parts decay quickly after a plant dies, the wax molecule can survive for a long time. This is why we often find this molecule in sediments dating back millions of years."
The chemical composition of the wax molecules can tell us about the climate conditions when the layer formed. For example, hydrogen molecules in wax can tell how much precipitation there was.
"Water contains hydrogen, so we can use hydrogen to trace the water cycle. Water on Earth contains ordinary hydrogen and heavy hydrogen (deuterium). When it rains a lot, plants absorb relatively less of the heavy hydrogen, and when it's dry, plants absorb more of the heavy hydrogen," she said.
Carbon holds important knowledge
Rachel Lupien and her colleagues were able to tell when it rained a lot and when it was dry from the deuterium content in the leaf wax. However, hydrogen doesn't tell us which plants thrive in humid climates.
But the carbon atoms in the leaf wax are telling, she explains. "Broadly speaking, there are two types of plants. We also call them C3 and C4 plants, and about 90% of all plants are C3 plants. They thrive in most areas of the world, except in very dry or very hot areas. C4 plants, on the other hand, specialize in areas with less rain and higher temperatures. Because the leaf wax produced by C3 and C4 plants contains With the same amount of heavy carbon, the researchers can "read" which plants were most dominant at the time. We found more C3 plants in the samples when Homo erectus migrated from Africa than at any other wet period in the past 4.5 million years, suggesting that a wetter climate transformed parts of the region from desert to grassland."
Three types of photosynthesis
In the plant kingdom, there are roughly three different ways of photosynthesis. There are C3 and C4 plants, and there is a third variant, the CAM plant.
90% of the plants are C3 plants, 6% are CAM plants, and only 3% to 4% are C4 plants. But that's not the case in Africa, where savannahs have a much larger proportion of C4 plants.
The differences between plants are due to their different strategies for coping when moisture in the air and soil is limited.
When the weather is too dry, C3 plants close the small pores in their leaves that absorb carbon dioxide. After the stomata close, the plant cannot photosynthesize and begins to deplete its carbon reserves while exhaling water and carbon dioxide. If this goes on for too long, the plant will die.
C4 plants, on the other hand, can photosynthesize even when dry. Even though the stomata are closed, they continue to convert carbon dioxide into energy. They perform photosynthesis with the help of molecules with four carbon atoms, hence the plant's name. CAM plants use a third method that allows them to survive in drier areas.
The greenest period 2.1 million years ago
Africa's Green Age, like the Ice Ages in northern latitudes, was caused by small changes in the Earth's orbit around the Sun. Geologists call these changes Milankovitch cycles.
Rachel-Lupien explains that two of these changes in particular play an important role when precipitation increases in the Sahara. Another cause of fluctuations is the Earth's pi rate as it moves around the Sun. During some periods, the Earth's orbit was more elliptical, and during other periods it was more circular. This creates fluctuations between about 100,000 and 400,000 years.
About 2.1 million years ago, the Sahara Desert was at its greenest. Here, several cycles are likely to occur simultaneously, creating such an environment. This coincides with the migration of Homo erectus. Therefore, climate likely facilitated this migration, she concluded.
Compiled source: ScitechDaily