Just published in Nature is a study revealing the origins of maleness in mammals. The team of Prof. Henrik Kaessmann at the Center for Integrative Genomics and the Swiss Institute of Bioinformatics concludes that the key male-differentiating features in the Y chromosome appeared 180 million years ago in placentals and marsupials with the arrival of the sex-determining gene, SRY. Curiously, the AMHY gene, performing the same function in monotremes, appeared independently 175 million years ago. In other words, testicles have been around for a good long while, but not as long as one might have imagined.
The study, which required 29,500 computing hours, was done by isolating special Y chromosome genes among fifteen mammalian species. It is the largest study of the Y chromosome to date. More information can be found in the current issue of Nature or here.