When the Endangered Species Act was approved on this day, Dec. 28, in 1973, it allowed the U.S. government to take several steps to protect the nation's native plants and animals. One of the most famous of those measures was creating the list of endangered and threatened species.

But the list actually traces its roots to the beginning of the 20th century, when the disappearance of the Passenger Pigeon led the government to try—in vain—to keep the birds alive by further regulating the hunting of game birds, as the U.S. Fish and Wildlife Service notes . Decades later, in 1966, Congress passed the Endangered Species Protection Act, choosing a group of 14 mammal species for the first-ever official U.S. endangered species list. (The list also included dozens of birds, a couple of reptiles and amphibians, and 22 fish species.)

Looking back through that original list, it's clear that the past half-century has been a mixed bag for the species on it. Many remain endangered, but others have been moved to "threatened" status or removed from the list entirely.

Captive Caribbean monk seal, Monachus tropicalis, at the New York Aquarium, ca.1910. New York Zoological Society

The unfortunate exception to the rule is that Caribbean Monk Seal, pictured at left. In 1979, the seal was removed from the list due to extinction. In fact, the seal had not actually been seen since 1952, more than a decade before the list was drafted, but it took decades for scientists to conclude that there were no members of the species left to look for. It remains the only mammal in the first class of listed species to have gone extinct.