Betrayals of trust resulting from moral disputes among our ancestors forced them to avoid conflicts and spread quickly to new regions on the Earth around 100,000 years ago, find researchers.
The findings suggest that the evolution of human nature's dark side sent them into risky dispersal into apparently unwelcoming environments with a desire to avoid physical harm from disgruntled former friends and allies being a key motivation.
"Betrayals of trust were the missing link in understanding the rapid spread of our own species around the world and the speed and character of human dispersals changed significantly around 100,000 years ago," said archaeologist Dr Penny Spikins from the University of York.
Before then, movement of archaic humans were slow and largely governed by environmental events due to population increases or ecological changes.
Afterwards populations spread with remarkable speed and across major environmental barriers.
Dr Spikins relates this change to changes in human emotional relationships.
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," she added.
According to Dr Spikins, the emotional bonds which held populations together in crisis had a darker side in heartfelt reactions to betrayal which we still feel today.
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.
After 100,000 years ago, however, dispersal into distant, risky and inhospitable areas became relatively more common compared with movements into already occupied regions.
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.
"Moral conflicts provoke substantial mobility -- the furious ex ally, mate or whole group, with a poisoned spear or projectile intent on seeking revenge or justice, are a strong motivation to get away, and to take almost any risk to do so," Dr Spikins noted.
"While we view the global dispersal of our species as a symbol of our success, part of the motivations for such movements reflect a darker, though no less 'collaborative', side to human nature," she concluded in a paper published in the journal Open Quaternary.
