- Homo sapiens, the species to which humans belong, originally evolved somewhere on the African continent between about 300,000 and 200,000 years ago. Until recently, it was believed that 200,000-year-old fossils found in Ethiopia were the oldest Homo sapiens fossils, suggesting that the species evolved in Africa’s Rift Valley (image below) c. 200,000 years ago. However, 300,000-year-old fossils uncovered in Morocco in 2017 indicate that Homo sapiens evolved at least 100,000 years earlier and likely in several places across Africa.
- It was previously thought that Homo sapiens migrated out of Africa and into the Middle East, Europe and Asia c. 70,000 – 60,000 BCE. However, new evidence suggests that there were actually several distinct waves of migrations beginning around 100,000 BCE, with each subsequent one occurring about 20,000 years apart. These waves of migrations occurred from about 104,000 to 92,000 BCE, 87,000 to 71,000 BCE, 57,000 to 45,000 BCE, and 43,000 to 27,000 BCE.
- The groups that were part of the earliest migrations faded out once they reached Eurasia and some lineages went completely extinct.
- The migration that occurred c. 57,000 – 45,000 BCE likely led to the population of the rest of the world.
- It is important to note that migration between Africa and the Middle East and Eurasia was fluid rather than unidirectional, with Homo sapiens traveling in both directions.
How is this related to climate?
- During the last ice age (c. 110,000 – 10,500 BCE), the cold dry conditions of the Northern Hemisphere were interrupted by warm summers about every 20,000 years.
- These climate fluctuations led to the appearance of so-called green corridors, or regions of warm, wet climate and lush vegetation, between Africa and the eastern Mediterranean and the Arabian Peninsula (map below). The favorable conditions of the green corridors allowed Homo sapiens to move north and east out of Africa, following other animals that they hunted for food.
- Researchers have recently developed a model integrating paleoclimate data, vegetation change and human migration that shows how the climate fluctuations coincide with the migrations of Homo sapiens out of Africa.
- The model contradicts the idea of a single migration of Homo sapiens out of Africa c. 70,000 – 60,000 BCE because this coincided with one of the most extreme periods of drought and low temperatures in the last 125,000 years, making migration difficult.
- These regular climate fluctuations are caused by precession, a type of Milankovitch cycle, or a variation in Earth’s orbital movements that affects where and how much solar radiation reaches the atmosphere.
- Precession is the wobble of the Earth’s axis of rotation caused by the gravitational pull of the Sun and the Moon (below). It causes changes in the timing of the seasons and seasonal contrasts between the Northern and Southern Hemispheres, including shifts from periods of drier, colder climates to warmer, wetter climates in tropical and subtropical regions. On average precession cycles occur every 26,000 years, which generally corresponds to the timing of documented Homo sapiens migrations out of Africa.
Axial precession, which is the wobble of the Earth’s axis of rotation caused by the gravitational pull of the sun and the moon (from Buis, 2020).
- Milankovitch cycles affect Earth’s climate on the scale of c. 20,000-100,000 years. These cycles, for example, are responsible for triggering the beginning and end of ice ages, as they can cause variations of up to 25% in the amount of incoming solar radiation. Precession is the shortest of three Milankovitch cycles:
- Eccentricity is the deviation of the Earth’s orbit from a perfect circle, which operates on a 100,000-year cycle.
- Obliquity is the change in the angle Earth’s axis is tilted with respect to Earth’s orbital plane during a 41,000-year cycle.
References and additional resources
- Buis, A. “Milankovitch (Orbital) Cycles and Their Role in Earth’s Climate.” National Aeronautics and Space Administration. 2020. https://climate.nasa.gov/news/2948/milankovitch-orbital-cycles-and-their-role-in-earths-climate/.
- Krajick, K. “Seeking Humanity’s Roots.” State of the Planet. 2018. https://blogs.ei.columbia.edu/2016/06/08/seeking-humanitys-roots/.
- Mascarelli, A. “Climate Change Drove Early Humans Out of Africa (and Back Again).” Sapiens. 2018. https://www.sapiens.org/biology/early-human-migration/.
- Reece, J. B., et al. Campbell Biology. Pearson Learning Solutions, 2014.