Urban legend. Almost all of the gene loss happened 200 million years ago in early mammals. In the past 40 million years, our ancestors lost one Y-chromosome gen

Urban legend. Almost all of the gene loss happened 200 million years ago in early mammals. In the past 40 million years, our ancestors lost one Y-chromosome gene one single time about 20 million years ago. In the 7 million years since we separated from chimps, there have not been any genes lost from the Y. Note: a similar shrinking pattern is seen in birds, where females have a small W chromosome and males have two large Z's.