Replacing a damaged liver—the organ that helps control blood clotting and filters blood—is so tricky that nearly a decade after doctors had successfully transplanted kidneys in humans, they still hadn’t attempted the same procedure with a liver. That changed 55 years ago, on March 1, 1963, when the University of Colorado Hospital’s Dr. Thomas Starzl performed the world’s first liver transplant. The patient’s unfortunate death from hemorrhage was not in vain: Starzl continued to refine the procedure and successfully transplanted a liver in 1967. Today, the liver is the second-most transplanted organ in the body. “The innovation didn’t stop in 1963,” says Dr. Liz Pomfret, chief of transplant surgery at UCHealth. “We can do a liver transplant now with as little as 10 units of blood; we would have used hundreds of units back in the day.” All of which is why Dr. Starzl, who died last year at the age of 90, is still called the “father of transplantation.”