CAMBRIDGE, ENGLAND—Chris Tyler-Smith of the Wellcome Trust Sanger Institute led an international study of the Y-chromosomes of more than 1,200 men from 26 populations around the world. The scientists then built a tree of the Y-chromosomes to show how they are related to one another. According to a report in The International Business Times, some parts of the tree were more like bushes, with many branches originating at the same point. “This pattern tells us that there was an explosive increase in the number of men carrying a certain type of Y chromosome, within just a few generations,” explained Yali Xue of the Wellcome Trust Sanger Institute. This increase was seen some 50,000 years ago across Asia and Europe, and 15,000 years ago in the Americas. These population increases may have been due to plentiful resources as people moved into new continents. Later expansions are seen in sub-Saharan Africa, Western Europe, South Asia, and East Asia, between 4,000 and 8,000 years ago. “What we think likely happened is that advances in technology led to more hierarchical societies led by small groups of men whose privileges allowed them to have a lot of sons,” Tyler Smith added.