To test orangutans, one of van Schaik’s team members, Sofia Forss, built fake orangutan nests in the Sumatran canopy. She then filled them with items that the apes would never have seen before—a Swiss flag, plastic fruit, and even an orangutan doll. Footage from motion-sensitive cameras revealed that wild orangutans walked around the items for months. Only two adolescents ever actually touched the unfamiliar items. When another team member, Caroline Schuppli, repeated the same experiment in several zoos, she got completely different results. Within minutes, the orangutans had wrecked the nests.

Meanwhile, Laura Damerius did a similar experiment with 61 orangutans who lived in Indonesian rehabilitation stations, assessing their responses to unfamiliar objects like a human stranger or a lump of purple-dyed food. She found that apes who had spent more time with humans before arriving at the stations behaved more curiously—that is, they actively sought out new things, and explored them with gusto. And this, she found, influenced their mental abilities. On a battery of challenges designed to test their problem-solving skills, the curious orangutans scored higher than their incurious peers.

Scientists rarely study curiosity in other animals, and perhaps for good reason. It’s “a difficult concept to define, even for humans,” says Jill Pruetz from Texas State University, “but it has very intriguing implications for understanding human evolution.”

For example, our ancestors had developed large brains, upright bodies, and basic tools hundreds of thousands of years before they acquired language, art, and other more sophisticated cultural innovations. “We’ve always wondered what unleashed that, and it may have something to do with curiosity,” says van Schaik. Perhaps some change in our society, whether larger groups or the advent of new weapons, afforded us the safety that zoos provide to orangutans. That, in turn, could have unlocked the latent curiosity in our own minds, turning us into explorers and innovators.

Of course, “it’s very hard to test this,” van Schaik says. But orangutans provide some support for the idea. Wild ones learn almost all of their skills by copying their mothers and selected role models. “They’re not going around like Curious George and turning everything over,” says van Schaik. That makes sense. Curiosity, as they say, kills the orangutan. In a world of strangers and dangers, it’s more efficient and less risky to take your cues from experienced peers.

But this means that wild orangutans are conservative animals that only innovate by accident. They try something routine, it has unexpected consequences, they keep on doing that, and other orangutans copy them. “They’re not focused on innovation at all,” says van Schaik. Over decades of studying these animals, he’s seen very few examples of entirely new behaviors.