Graves is confident her gloomy prophecy won’t spell the end of mankind. She’s certain that the human male would simply evolve a fresh genetic trigger for his gonads. Other mammals have already done so. A spiny rat from Japan (Tokudaia osimensis) and a Transcaucasian mole vole (Ellobius lutescens) are just two species of mammal known to have completely lost their Y chromosomes, yet hung on to their testicles. Both males and females have a solitary X chromosome and sexual development is triggered by an entirely different, and as yet unidentified, master sex-determining gene
