http://www.newsweek.com/y-chromosome-ancient-clans-wars-950587

Recent genetic research suggests that the number of reproducing men plummeted across Africa, Asia and Europe around 5,000 to 7,000 years ago. In fact, scientists think this population dropped by some 95 percent during this time.

Now, researchers writing in Nature Communications think they might have an explanation. Warring patrilineal clans—where membership follows the male line—could be to blame.