There Were 6,000 Animals We Could’ve Featured on the Cover—How We Chose 10.

Photographer Joel Sartore is over halfway through his mission to document the 12,000 animal species in captivity … before it’s too late.

“For many of Earth’s creatures, time is running out,” he says. “Half of the world’s plant and animal species will soon be threatened with extinction.”

He began his Photo Ark project in 1995 to create a visual record of what might soon be lost. But he also makes these images because, for many of these creatures, it’s not too late to shift the dangerous trend toward extinction.

Photo Ark is the cover story for the April issue of National Geographic magazine. This spotlight moment presented an amazing opportunity for exposure, along with a unique challenge: How do you narrow down 6,000 animal portraits to just one? And what one creature can stand in for thousands of others?

It helped that there were a few concrete requirements. The cover photo needed to be on a white background (Sartore photographs the animals against black or white backdrops, sometimes both). Creative director Emmet Smith suggested they not opt for a creepy-crawly creature. And, lastly, the animal needed to be making eye contact with the viewer. “I felt that eye contact was critical in terms of being true to the goal of Photo Ark,” says Sartore’s photo editor, Kathy Moran. “But also just the sense that on the newsstand you want that eye contact to pull in a viewer.”

So Moran started pulling portraits she liked that met these criteria. She and Smith pinned them all up on the wall and began whittling it down. They had about 12 photos left when they asked the editor in chief of the magazine, Susan Goldberg, to weigh in. “She was so enthusiastic,” says Moran. “She was like, ‘Oh, I like this. I don’t like that.’ And then out of nowhere she said, ‘We should do multiple covers! Collect them all, like baseball cards.’ It just took off from there. It was really this magic moment sitting in Emmet’s office saying, ‘Let’s do this. Is there any reason that we can’t do that?'”

And there wasn’t.

But having ten covers meant they needed to go back to the drawing board and choose images that represented a diverse range of species. Which meant going back on their “no creepy crawlies” requirement. “If we were going to do ten covers, you have to have amphibians, you have to have herps,” says Moran. “I went back in and started pulling, looking for greater species diversity.”

This created a sort of species puzzle: If you have a tiger, you can’t have another big cat. If you have a lemur, you aren’t going to get another primate in there. So they worked through the images that way, finding the right mix of animals to represent the scope of the project while still looking for photos that people could connect with. “Charisma, eye contact, [and] empathy,” Smith says, were still top priorities.

Finding, out of 6,000 options, ten animals photos that worked both as individuals and as a whole was like finally completing a puzzle. It’s a surprising, adorable, and varied ark—a waxy monkey tree frog along with a Brazilian porcupine—but they all belong there.

“I would have liked to have printed every animal Joel has photographed,” says Smith. Now that might have presented some logistical challenges. Ten isn’t 6,000, but it’s a start.

This range of species gets at the heart of Photo Ark’s mission to preserve biodiversity. “I think the biggest challenge in having only one cover would have been finding the balance between big and charismatic versus the small and perhaps not as immediately lovable,” Moran says. “So easy to go with a tiger, but the mission of Photo Ark is to give all species equal value. A mouse is just as important as a big cat.”

Which of these animals is most like you? Take our quiz.

See more Photo Ark in the April 2016 feature “Every Last One.”

Joel Sartore shot this month’s cover images at (in grid, from left, by row) Rolling Hills Zoo, San Antonio Zoo, Zoo Atlanta, Raptor Recovery Nebraska, Omaha’s Henry Doorly Zoo, Saint Louis Zoo, Lincoln Children’s Zoo (two), Australia Zoo Wildlife Hospital, Houston Zoo.