Modern humans evolved in Africa about 200,000 years ago and later left the continent to populate the rest of the world. If you look at a map, it seems obvious that the best exit route would be through northern Egypt, across the Sinai Peninsula and into modern Israel and Jordan. But mounting evidence is now pointing to another possible path out of Africa: the so-called southern route, leaving from the Horn of Africa, crossing the Red Sea and entering into southern Arabia.
Recent genetic and archaeological evidence suggests that the southern route was a viable alternative to the more northerly course. Based on such evidence, it appears humans made it to Asia sometime between 80,000 and 60,000 years ago—not reaching Europe until a few tens of thousands of years later. Cutting across the southern portion of the Arabian Peninsula might have been the speediest way for our ancestors to get to southern Asia. This year, researchers found evidence that early humans did indeed make it to southeastern Arabia, and at a much earlier date than previously thought. Simon Armitage of Royal Holloway, University of London and colleagues reported finding stone tools in the United Arab Emirates at an archaeological site called Jebel Faya just 35 miles from the Persian Gulf. They unearthed stone tools, including hand axes and scrapers, dated to 125,000 years ago. It’s not yet known whether the people who made the tools went on to explore other new frontiers or just stayed put.
Geologists are also weighing in on the question of how modern humans departed from Africa. Although Arabia is a giant pile of sand today, it wasn’t always that way. Due to repeated climate cycles, the area has alternated between desert and grassland. By looking at what the environment was like when humans might have passed through, geologists can provide evidence for or against the southern route. In the newest issue of the journal Geology, a group of researchers led by Thomas Rosenberg of the University of Bern in Switzerland reports there were “windows of opportunities” when humans could have dispersed into Arabia while it was a hospitable savannah. The team found deposits in southwestern Saudi Arabia that record the presence of shallow, freshwater lakes in the region at 80,000, 100,000 and 125,000 years ago—a sign of a wetter climate. “Until now,” they wrote, “there was no solid evidence whether the environmental conditions would have allowed to disperse along the southern dispersal route into Asia.”
But just because the environment was favorable doesn’t necessarily mean it was feasible for humans to get there. The Red Sea does stand in the way. But like Arabia’s climate, the Red Sea has not stayed static over time; its sea level has risen and fallen as ice sheets have melted or formed, and as tectonic activity has raised or lowered the seabed. Geoffrey Bailey of the University of York in England and co-workers have looked at the issue by studying ancient sea-level records and the topography of the sea floor to reconstruct ancient shorelines. In at least the last 400,000 years, Bailey and his co-workers recently reported in the journal Quaternary Science Reviews, there has never been a land bridge connecting Africa and southern Arabia. But over the last 150,000 years, there were periods when the channel separating the two landmasses was less than 2.5 miles wide. With such short journeys, humans would not have needed sophisticated boats or seafaring technology; simple rafts would have sufficed. Furthermore, these favorable periods coincide with periods when the environment in Arabia was hospitable.
Researchers still have a lot of work to do before the southern route is well established; fossils and more stone tools would be helpful. But there isn’t a lot of archaeological research from this time period being done in the region. It’s a hard place to do fieldwork. Maybe all of the converging evidence will entice more archaeologists and paleoanthropologists to travel to southern Arabia—just as our ancestors might have done millennia ago.