The procedure is the culmination of decades of research and marks a critical advance in lessening the reliance on human organ transplants. The donor heart came from a 1-year-old pig that had been genetically modified to decrease the likelihood Bennett’s body would reject it. More than 100,000 Americans are on organ transplant lists, and 6,000 patients die each year while waiting.

The success follows two surgeries in which pig kidneys were successfully transplanted to human hosts—however, the procedures were demonstrations involving clinically brain-dead patients. Read more about xenotransplantation here.