1905: The pace of xenotransplantation picks up, and physicians begin to graft animal tissues into humans with some regularity. For example, a French surgeon transplants slices of a rabbit kidney into a 16-year-old boy suffering from kidney failure. "The immediate results were excellent," he declares. But the patient dies two weeks later.

Over the next 20 years, doctors try to transplant organs from pigs, goats, lambs, and monkeys into various patients. All the grafts fail, but no one understands why.