In 2011, Ben Trumble emerged from the Bolivian jungle with a backpack containing hundreds of vials of saliva. He had spent six weeks following indigenous men as they tramped through the wilderness, shooting arrows at wild pigs. The men belonged to the Tsimane people, who live as our ancestors did thousands of years ago — hunting, foraging and farming small plots of land. Dr. Trumble had asked the men to spit into vials a few times a day so that he could map their testosterone levels. In return, he carried their kills and helped them field-dress their meat — a sort of roadie to the hunters.