In biology, a group of organisms is said to have common descent if they have a common ancestor. The most recent common ancestor is the most recent population or species which is an ancestor of all of the species being referenced, whether that be polar bears and sun bears or woolly mammoths and African elephants or even all four of those species which ancestor would most likely be one of the earliest mammal. The broadest form of common descent is universal common descent, which is the theory that all life on Earth originated from one the same common ancestor billions of years ago.