The Doctors: Organ Pain
A woman asked why the human body has two kidneys but just one liver. Dr. Travis Stork said this is due to evolution. Vital organs keep you alive and they rely on each other to keep the body functioning.
Scientists believe that animals developed one heart and two lungs about 300 million years ago, when animals migrated to land from the sea, in search of safety and food. Not everything can be explained medically, and every evolutionary change happens slowly.