The speed and character of human dispersals changed significantly around 100,000 years ago, and our dark side deserves a thanksgiving for that; a new paper suggests that betrayals of trust were the missing link in understanding the rapid spread of our species around the world.
Early species of hominin were limited in distribution to specific environments such as grasslands and open woodland. The expansion of Homo erectus out of Africa into Asia around 1.6 million years ago appears to have been caused by the need to find more large scale grasslands. By contrast, Neanderthals occupied cold and arid parts of Europe. All archaic species adapted slowly to new opportunities for settlement and were often deterred by environmental and climatic barriers.
Before 100,000 years ago, movement of archaic humans were largely governed by environmental events due to population increases or ecological changes. Afterwards populations spread with remarkable speed and across major environmental barriers.
Dr. Penny Spikins, a senior lecturer in the Archaeology of Human Origins at University of York, speculates in Open Quaternary that neither population increase nor ecological changes provide an adequate explanation for patterns of human movement into new regions which began around 100,000 years ago. Instead, as commitments to others became more essential to survival, and human groups ever more motivated to identify and punish those who cheat, the 'dark' side of human nature also developed. Moral disputes motivated by broken trust and a sense of betrayal became more frequent and motivated early humans to put distance between them and their rivals.
According to Spikins, the emotional bonds which held populations together in crisis had a darker side in heartfelt reactions to betrayal which we still feel today. Larger social networks made it easier to find distant allies with whom to start new colonies, and more efficient hunting technology meant that anyone with a grudge was a danger but it was human emotions which provided the force of repulsion from existing occupied areas which we do not see in other animals.
And so dispersal into distant, risky and inhospitable areas became relatively more common compared with movements into already occupied regions. Most notably, the spread of modern human populations was not inhibited by biogeographical barriers. Populations moved into cold regions of Northern Europe, crossed significant deltas such as the Indus and the Ganges, deserts, tundra and jungle environment and even made significant sea crossings to reach Australia and the Pacific islands.
Spikins argues that betrayals of trust resulting from moral disputes were a significant reason for such risky dispersals into apparently unwelcoming environments with a desire to avoid physical harm from disgruntled former friends and allies being a key motivation. Offenders and any allies within their social network would feel driven to get out of harm's way.