The answer to a Stone Age event that has puzzled anthropologists, biologists and sociologists alike has been found by two Singaporean students who recently graduated from Stanford University.

The event, known as the Neolithic Y-chromosome bottleneck, happened some 7,000 years ago and saw male populations across Africa, Europe and Asia dwindling down to one-twentieth the original size, so much so that the genetic diversity of the Y-chromosome in males - passed down from fathers to their sons - plummeted with it.