"Stone breadcrumbs" reveal humans who left Africa over 100,000 years ago

Until recently, scientists thought humans first left Africa between forty and seventy thousand years ago. But the archaeological record keeps painting a far different, more ancient picture, one that took humans deep into the Arabian peninsula 100,000 years ago.

The 40,000 to 70,000 year estimate comes from genetic evidence, which suggests all living humans are descended from people who either left or remained in Africa around that time. The point is, the genetic evidence strongly suggested humans had not left Africa before that time period. But this year has seen not one, not two, but three different discoveries that place modern humans outside of Africa at least a hundred thousand years ago.

The latest finds are a series of discoveries made in Oman, a country on the southeastern tip of the Arabian peninsula. Researchers from the UK's University of Birmingham say they have followed "stone breadcrumbs", a series of artifacts at different sites that trace the movements of these ancient humans from the northeastern tip of Africa right down to the bottom of the Arabian peninsula.

Based on the artifacts discovered, these ancient humans were part of a culture known as the Nubian Middle Stone Age, which lived in the Nile Valley many tens of thousands of years ago. This incredibly ancient culture of toolmakers was previously unknown outside of Africa, but these new discoveries place them across the Red Sea on the Arabian peninsula by at least 106,000 years ago, if not even earlier.

Project leader Dr. Jeffrey Rose comments:

"After a decade of searching in southern Arabia for some clue that might help us understand early human expansion, at long last we've found the smoking gun of their exit from Africa. What makes this so exciting," he adds, "is that the answer is a scenario almost never considered."

The unexpectedly ancient dates of these discoveries isn't the only remarkable thing about them. The tools were all found far inland. All previous theories had assumed that early humans in the Arabian peninsula would have hugged the coast, relying almost exclusively on fishing to provide food.

Coauthor Anthony Marks, Professor Emeritus at Southern Methodist University, observes just how unexpected these new findings really are:

"Here we have an example of the disconnect between theoretical models versus real evidence on the ground. The coastal expansion hypothesis looks reasonable on paper, but there is simply no archaeological evidence to back it up. Genetics predict an expansion out of Africa after 70,000 thousand years ago, yet we've seen three separate discoveries published this year with evidence for humans in Arabia thousands, if not tens of thousands of years prior to this date."

One reason why these ancient toolmakers were able to leave the coast and move inland is the radically different climatic conditions in the Arabian peninsula at that time. About a hundred thousand years ago, ancient Oman experienced strong rains that briefly transformed the area into lush grasslands. Dr. Rose describes the scene:

"For a while, South Arabia became a verdant paradise rich in resources — large game, plentiful freshwater, and high-quality flint with which to make stone tools."

These ancient humans weren't fishermen, then, but instead hunters who traveled throughout southern Arabia using the newly created river systems like a network of highways. Conditions were lush enough that they could easily survive just on hunting, but the good times could not last. With the arrival of the Ice Age, the Arabian peninsula would have become even more arid than it is today, and it's an open question how long these ancient immigrants could have survived there. To answer that, the archaeologists say they need to continue hunting for these "stone breadcrumbs" of artifacts that remain buried in the Arabian desert.

Via PLoS ONE. Image of modern Oman by Panoramas on Flickr.