The idea of extinction is pretty straightforward — a species is there, until it’s not. But modern attitudes towards endangerment and extinction are hardly that simple. In fact, it wasn’t long ago that the idea of endangered animals didn’t even exist.

The concept of caring about or quantifying threats to animals is actually fairly modern — but it started earlier than you might think. Though early colonists reacted to the sheer abundance of American wildlife with shock and delight (Captain John Smith boasted of “diverse sorts of wild beasts as fat as we could eat them”), people soon started to notice the impacts of settlers on animals. “I have heard a hunter assert, he saw above one thousand buffaloes at Blue Licks at once;” wrote John Filson in 1784, “so numerous were they before the first settlers had wantonly sported away their lives.”

Growing populations and unchecked hunting quickly left their mark. In 1857, citizens concerned about dwindling numbers of passenger pigeons turned to the Ohio Senate, but were dismissed. “The Passenger Pigeon needs no protection,” the Senate scoffed. “No ordinary destruction can lessen them.” Not so — in 1914, the very last passenger pigeon died in a Cincinnati zoo.

Early conservation attempts aimed to preserve game for settlers rather than protect animals per se — the Lacey Act, which was passed in 1900 and was the first federal law protecting wildlife, focused primarily on poaching and hunting. But by the turn of the century, a Progressive conservation movement was underway. Imbued with a romantic appreciation of nature and alarmed by declining animal populations, grassroots efforts to protect animals began.

In 1973, the Endangered Species Act enshrined both animal endangerment and endangered species conservation in American law. Today, both the ESA and the International Union for Conservation of Nature and Natural Resources’ Red List define endangered species and identify extinct ones.

Pat Deibert, national sage-grouse coordinator for the U.S. Fish and Wildlife Service, points out that the Endangered Species Act enables conservation policy within the United States. “We tie together the threats to a species with the population trend” to determine whether a species is endangered, she tells Smithsonian.com. Once endangerment is identified, the act enables Fish and Wildlife to take steps to conserve a species using local laws and recovery plans. The act also lists some “foreign species” as endangered in an effort to increase awareness, enable laws about the import of foreign animals and free up funds for international wildlife conservation. Today, 1,345 species are listed under the Endangered Species Act.

This differs from the IUCN’s methods. “It’s very much a probabilistic system,” Craig Hilton-Taylor, head of the ICUN’s Red List unit, tells Smithsonian.com. He works with thousands of scientists worldwide to try to determine the probability of a species becoming extinct in the wild — a process that’s painstaking, lengthy and that involves a complex web of data and mathematical models. The IUCN’s list is much larger than that of the ESA: Today, it lists over 20,000 species as threatened.

Both systems have their challenges, especially given the growing impact of things like climate change and industrial development. But there are successes, too, like when the Virginia northern flying squirrel was taken off the list of endangered species in the United States after its population grew from just ten to over 1,100. Not all success stories are that dramatic: For example, the IUCN was able to move the Iberian lynx from “critically endangered” to “endangered,” but it still faces threats from hunting and scarce food sources.

“Lots of people think that extinction is a natural process, which it is,” says Hilton-Taylor. But humans play a role, too, speeding up extinction as modern lifestyles disrupt animal habitats and speed up processes like climate change.

“It all comes down to a value judgment,” agrees Deibert. That and the perceived desirability of a species. “Conserving a sand flea is a little more challenging than a charismatic bird,” she admits.

Despite better conservation laws and growing awareness of the threats that face animals, says Krithika Srinivasan, a social scientist who specializes in social, ecological and animal justice, “we often cause harm even when we want to care.” By marking some animals as endangered, she tells Smithsonian.com, humans can ignore their responsibilities to all animals — and downplay their own contributions to threats and extinction.

“The ironic part of this is that in order to be endangered, you first need to be harmed,” says Srinivasan. “We seem to only want to protect those things that are not there in large numbers,” she says — a lesson that, though exemplified by the extinction of the once-abundant passenger pigeon, doesn’t seem to have sunk in to the collective conscious. Until humans accept responsibility for their role in causing and perpetuating endangerment, says Srinivasan, the list will continue to grow. Perhaps that’s the next frontier in modern attitudes towards endangered animals — broadening the definition before it’s too late.