Charles Darwin wondered if animals were aware of themselves. Allowed to visit a rare orangutan in the London Zoo, he brought a mirror and observed the ape apparently make faces at its own reflection. It’s hard to say for sure that the orangutan really was aware that its reflection was its own. Over a century later, a scientist named George Gallup turned Darwin’s idea into a more rigorous test. He would secretly put a mark on an animal’s forehead and see if it noticed the difference the next time it passed a mirror.

Human adults pass this test, but young children don’t, suggesting that our self-recognition takes time to develop. Some chimpanzees (our closest relatives) seem to pass this mirror test,