First Steps: Simulating the Migratory Patterns of Early Humans
By Ryan Cimmino
The study of human origins is notoriously imprecise. In a field that attracts the attention of archaeologists, geneticists, anthropologists, and even geologists, a clear and consistent narrative of our earliest ancestors remains elusive. While experts agree that the first anatomically modern humans arose in Africa, we have yet to answer two central questions: when did they leave, and where did they go? Faced with limited physical evidence and contradictory interpretations, researchers have now turned to computer science in hopes of discovering a novel gateway to the human past.
In September 2016, a group of researchers from the University of Hawaii at Manoa pioneered a new study of early human migrations that uses computer simulations and numerical methods to analyze the contribution of climate fluctuations to human migratory patterns. In their paper published in Nature, Axel Timmermann and Tobias Friedrich propose a model that clarifies our understanding of the exodus from Africa.1 Their work builds on the foundations of research conducted over the last decade, which has utilized genetic and archaeological evidence to question the once-dominant idea of a single window of human migration around 60,000 to 80,000 years ago. Rather, Timmermann and his team propose a scenario in which humans left for the Eurasian continent in four successive waves of migration, separated by approximately 20,000 years and coinciding with periodic climate shifts.1 Their work may prove to be a watershed moment in our understanding of these early migrations.
The team’s novel approach is among the first to numerically simulate the spread of early human populations in the context of long-term climate changes.2 Over many thousands of years, the earth’s orbit undergoes gradual changes in its shape and orientation in space; these adjustments, along with the periodic ‘wobble’ of the earth’s axis, cause long-term climate fluctuations called Milankovitch Cycles.3 The cycles cause adjustments in glacial cover, precipitation patterns, and seasonal conditions, among other factors.3 A long-term mathematical model of these effects found that North Africa saw increased precipitation and vegetation over 20,000 year intervals, conditions which likely encouraged the spread of early humans through so-called ‘green gateways.’2
While many prominent models of early human migration place the date of departure around 60,000 years ago, Timmermann’s model predicts four major waves of migration around 106-94, 89-73, 59-47, and 45-29 thousand years ago.1 Dispersing from two points of exit–the Sinai Peninsula in North Africa and the Horn of Africa–these groups of early human migrants would have likely split into two populations, with some following the southern Asian coast and others heading north from the Arabian peninsula.
By the time they reached southern Europe, the land would have already been inhabited by Neanderthals, a so-called ‘archaic human’ species.1 If correct, Timmermann’s model would have profound consequences for our understanding of human-Neanderthal interaction and admixture. Specifically, his predictions pre-date the earliest European human fossils by 45,000 years.1,4 This discrepancy may shed light on previously overlooked cultural and biological contacts between modern humans and their archaic relatives in Europe. A lack of fossil evidence, Timmermann suggests, may imply that the earliest human migrants were subsumed into the existing archaic population.1
Perhaps more surprisingly, Timmermann’s study supports the idea that certain human populations migrated back to Africa from the Eurasian continent, causing “climate-mediated dispersal and mixing waves across the Africa/Asian nexus.”1 This would have led to a genetic backflow, in which genes originally expressed in Europe made their way back into African populations. This phenomenon is further explored in a 2016 study, in which geneticists were able to identify an ancient individual as a member of a certain genetic lineage called ‘U6 basal,’ which is known to be an ancestor of a modern lineage found in North Africa 35,000 years ago.5
A consensus concerning the earliest human departures from Africa remains out of reach for the time being. However, Timmermann’s work opens the door to further simulation-based research into such questions. This research represents an important step towards a model that integrates the findings of many different scientific and social scientific disciplines, whether genetic, anthropological, or climatological. While our ancestors left few unmistakable clues of their whereabouts, their story need not remain a mystery for long.
- Timmermann, Axel and Tobias Friedrich. “Late Pleistocene climate drivers of early human migration.” Nature 538, 6 October 2016, pp. 92-95. doi: 10.1038/nature19365.
- Lentz, Rachel. “Past climate swings orchestrated early human migration waves out of Africa.” Phys.org, September 22, 2016, https://phys.org/news/2016-09-climate-orchestrated-early-human-migration.html.
- Timmermann, Axel and Tobias Friedrich. “Astronomical Theory of Early Human Migration (Milutin Milankovic Medal Lecture).” Geophysical Research Abstracts 19, April 2017. http://meetingorganizer.copernicus.org/EGU2017/EGU2017-2276.pdf.
- Wade, Nicholas. Before the Dawn: Recovering the Lost History of Our Ancestors. New York: Penguin, 2006.
- “Migration back to Africa took place during the Palaeolithic.” Phys.org, May 26, 2016. https://phys.org/news/2016-05-migration-africa-palaeolithic.html.