Steps of the Ziggurat of Ur built by the Neo-sumerians, in the reign of King Nabonidus 556 to 539 B.C. on the rubble of an older Sumerian structure constructed by Sumerian King Ur-Nammu ca. 2100 B.C.

At the dawn of civilization, just as clans of Stone Age peoples were banding together to form increasingly complex societies about 7,000 years ago, a strange thing happened.

The men began to disappear.

At least their lineages did. A genetic bottleneck in most corners of the Eastern Hemisphere meant that roughly one genetic variety survived out of 20, based on the genetic networks before and after the DNA collapse.

The collapse was due to generation after generation of total war between male-driven clans, which meant the losers were essentially pruned from the family tree, removing their genetic futures from the rest of human history, according to a new study by Stanford University scientists.

“Our hypothesis represents an attempt to synthesize genetic, anthropological and archaeogenetic data to create a synoptic view of social dynamics, or the social process, as a braided stream in time and space,” the scientists state.

The basis of the bottleneck was a 2015 study of 125 Y chromosomes, which showed a reduced diversity that had started 7,000 years ago and persisted for about two millennia, to the rise of the first civilizations in Mesopotamia.

“The inferred bottleneck and associated star-like expansions on the phylogenetic tree were confined to male-inherited Y chromosomes and were not apparent in female-inherited mitochondrial DNA,” the researchers said.

The Stanford scientists, two undergraduates and a professor of biology, tested three hypotheses: that environmental factors such as disease could have wiped out predominantly males; that small populations of Neolithic pioneer males prompted wide population expansions; and that intragroup relations and material inequalities increased reproductive variance among Stone Age groups.

Ultimately, they dismissed those theories, based on an interdisciplinary review incorporating mathematical models and archaeological history.

Instead, the researchers said it all shows how patrilineal kin groups, and their fierce competition at the beginnings of the first complex settlements, created less diversity through the loss of entire clans of men, but not of women. The mitochondrial DNA showed robust diversity in the run-throughs, while the Y chromosomes became less varied over the millennia.

Based on the computational model, all was not equal across the world, and the most extreme intensity of the bottlenecks were traced back through West Asia, Europe and South Asia.

“This may imply that prehistoric interactions between farmers and herders in the Middle East and South Asia involved interesting social patterns,” the team of scientists said. “Farmer-herder interactions have been recognized as playing a key role in cultural change, such as in the creation of ideals of personal, mobile and alienable property, or the rapid development.”

Among the male lineages that survived the bottleneck, and into humanity today, a few of them underwent dramatic expansions. But others did not. Taken together, these observations bolster the patrilineal clan model, the scientists conclude.