Why did so many ancient societies around the world make human sacrifices to appease the gods? A new study supports the theory that the practice may have played a key role in keeping the poor downtrodden and the elites in power.

And barbaric as it sounds, human sacrifice may have created one path for more advanced civilizations, according to the study.

"Unpalatable as it might be, our results suggest that ritual killing helped humans transition from the small egalitarian groups of our ancestors, to the large stratified societies we live in today," concludes the study published this week in Nature.

Anthropologists and archeologists had previously suggested that ritual human sacrifice might help build and sustain social class systems. Researchers at the University of Auckland, the Max Planck Institute for the Science of Human History and Victoria University, wanted to see if they could find evidence of that.

They looked at 93 Austronesian cultures, which share similar languages and were spread across the South Pacific from Madagascar to Easter Island to the Pacific Islands and New Zealand. About 40 of them practised some kind of ritual killing, including burning, drowning, strangulation, bludgeoning, burial, being cut to pieces, being crushed beneath a newly built canoe or being rolled off the roof of a house and decapitated.

The researchers looked at the class divide or "social stratification" in each culture and found:

About 25 per cent were egalitarian, without inherited wealth and status.

About 50 per cent were "moderately" stratified, where wealth and status were inherited, but there was some movement between classes.

The remaining quarter were highly stratified. "They had strict class systems and no matter how talented you were or how hard you worked, you couldn't do a lot about your social status," said Joseph Watts, the lead author of the new study.

Strict class systems linked to human sacrifice

Watts and his colleagues found that ritual human sacrifice was relatively uncommon among egalitarian societies, with only about a quarter of them engaging in it.

But the vast majority of highly stratified societies — about two-thirds — made human sacrifices to the gods.

Typically, the victims were those of low social status, such as slaves, and those conducting the sacrifice were people of high status, such as chiefs or priests. Sometimes victims were sacrificed as punishment for violating social or religious rules. Other times, sacrifices were made in an effort to prevent or end natural disasters.

"We know from accounts in these cultures that it was often those out of favour with the social elites who became the victims," said Watts, a PhD candidate in the School of Psychology at the University of Auckland, in an interview with CBC Radio's Quirks & Quarks.

Ritual killings may have enhanced the power of rulers. 10:01

The sacrifices may have been used both to justify the position of chiefs and to intimidate their inferiors, Watts said.

Class divisions aren't usually viewed in a positive light, especially by today's non-elites, the so-called "99 per cent."

But Watts said social stratification is "a pretty important first step in the emergence of complex societies."

"Are you saying that modern civilization is based on human sacrifice?" asked Bob McDonald, host of Quirks & Quarks.

"Well, I'm not saying it's the only way humans were able to build social stratification, but it may have played an important role there, yes," Watts said.

He noted, however, that human sacrifice is pretty rare in the modern world, suggesting that other ways to enforce class divides eventually replaced it.

The new paper also makes a note about religion and the role it plays in the development of human civilization. It's generally been seen as a positive force, by increasing co-operation, for example.

"Our findings suggest that religious rituals also played a darker role in the evolution of modern complex societies," the researchers wrote.