Naturalists have been trying for centuries to catalog all of the species on Earth, and the effort remains one of the great unfinished jobs in science. So far, researchers have named about 2.3 million species, but there are millions — perhaps even billions — left to be discovered.

As if this quest isn’t hard enough, biologists cannot agree on what a species is. A 2021 survey found that practicing biologists used 16 different approaches to categorizing species. Any two of the scientists picked at random were overwhelmingly likely to use different ones.

“Everyone uses the term, but no one knows what it is,” said Michal Grabowski, a biologist at the University of Lodz in Poland.

The debate over species is more than an academic pastime. In the current extinction crisis, scientists urgently need to take stock of the world’s biological diversity. But even some of the best known species on Earth may not be what they seem.

Take the giraffe.

In 1758, the Swedish taxonomist Carl Linnaeus described a single species of giraffe: Giraffa camelopardalis. Although the species has declined in recent decades, 117,000 giraffes still survive across Africa, prompting an international conservation group to designate the species as vulnerable, rather than endangered.

But some conservation biologists argue that giraffes are in great peril, because what looks like one species is actually four. Genetic studies have found that giraffe DNA falls into four distinct clusters: the Northern giraffe, the reticulated giraffe, the Masai giraffe and the Southern giraffe.