Most people are familiar with the so-called “Out of Africa” (OoA) hypothesis according to which our early Homo sapiens ancestors dispersed around the world by leaving an African hotspot 60,000 years ago. But the story of how our species came to conquer the world has far more complex beginnings, scientists say.

Modern humans left Africa in multiple minor waves prior to a great migration which occurred 60,000-50,000 years ago. Credit: Pixabay.

In the past fifteen years, a great deal of archaeological and paleontological evidence, as well as genetic findings, have placed a question mark on the “Out of Africa” hypothesis. Homo sapiens fossils dated between 70,000 to 120,000 years old were discovered in China and southeast Asia, and some even as far as Australia dated to 60,000 years ago. Human fossils found in the Skhul and Qafzeh caves in Israel that predate the OoA timeframe are also worthy examples. If humans had barely begun to exit Africa 60,000 years ago, how can we explain these other findings?

At the same time, such fossils are few and far between, whereas there’s a treasure trove of fossils and artifacts that document a human dispersal out of Africa around 60,000 to 50,000 years ago.

The most plausible explanation is that humans indeed embarked on a massive wave of migration around that time, possibly spurred by a changing climate. However, before this truly massive undertaking, early hunter-gatherers must have migrated in smaller waves.

The baby steps

This is the conclusion of researchers from the Max Planck Institute for the Science of Human History and the University of Hawai’i at Manoa who reviewed the plethora of new discoveries reported from Asia over the past decade.

“The initial dispersals out of Africa prior to 60,000 years ago were likely by small groups of foragers, and at least some of these early dispersals left low-level genetic traces in modern human populations. A later, major ‘Out of Africa’ event most likely occurred around 60,000 years ago or thereafter,” explains Michael Petraglia of the Max Planck Institute for the Science of Human History in a statement. 

Various migratory pathways associated with modern humans dispersing across Asia during the Late Pleistocene. Credit: Bae et al. 2017.

Human migrations across the Old World are further complicated by the numerous proven interbreeding events. We now know for sure that humans not only interbred with Neanderthals but also with other close relatives like the Denisovans and a mysterious unidentified population of pre-modern hominins. All non-Subsaharan humans alive today have 1-4% Neanderthal heritage, while modern Melanesians have an average of 5% Denisovan heritage. Though not included in this present review, there’s evidence that around 2% of the Papua New Guinean genome contains traces of modern human dispersals earlier than 60,000 years.

These interactions suggest that humans interacted closely with different hominin populations present in Asia during the Late Pleistocene.

“Indeed, what we are seeing in the behavioral record is that the spread of so-called modern human behaviors did not occur in a simple time-transgressive process from west to east. Rather, ecological variation needs to be considered in concert with behavioral variation between the different hominin populations present in Asia during the Late Pleistocene,” explains Christopher Bae of the University of Hawai’i at Manoa.

The authors plan on conducting new research in areas across Asia which have yet to be investigated. There are still many unanswered questions but more multidisciplinary research might help fill the gaps in the evolutionary records. For instance, why did these initial minor dispersals fail? Were early populations assimilated by the later and larger migratory wave or did they just disappear in isolation? These are certainly exciting times to be a researcher in the field, that’s for sure, concludes Bae.

Scientific reference: C.J. Bae at University of Hawai’i at Manoa in Honolulu, HI el al., “On the origin of modern humans: Asian perspectives,” Science (2017).