Charles Darwin ushered in modern biology with his explanation of how different species evolve. But his work leaves us with a paradox: Why should dozens or even thousands of species coexist in a single habitat? The theory suggests they ought to duke it out until just a few winners dominate. And yet we have such magnificent biodiversity all over. More than 2,000 species of trees share a single acre of rainforest in the Amazon. So what gives?

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The answer might lie in a game you probably mastered before you were 12: rock, paper, scissors. Any pairing of two species (say, “rock” tree and “paper” tree) will almost always lead to the weaker one going extinct (so long, “rock” tree). But introduce a third species – “scissors” tree – and you close up into a stable loop, where all three can coexist. This has been known for a while, and observed in natural settings among side-blotched lizards in California and bacteria growing in a dish.

University of Chicago ecologist Stefano Allesina scaled it up with a computer model, and showed it could indeed explain big, complicated systems like the Amazon jungle or underwater kelp forests. In fact, you can have as many species as you want coexisting, with one big caveat: Strangely, it has to be an odd number. That means no fourth throw in roshambo, though “rock, paper, scissors, lizard, Spock” is safe.

Dig even a little deeper, and it seems that rock, paper scissors describes a basic mathematical concept that appears in all kinds of systems, as shown in game theory. Whether it’s economics, political science or biology, any system where competitors have different advantages that can’t be ranked from best to worst probably has a little rock, paper, scissors tournament hiding in there somewhere.

Incidentally, actual rock paper scissors tournaments have been gaining steam, thanks largely to the efforts of the World Rock Paper Scissors Society. If you want to learn how to crush the competition (and never change a diaper again! Oh wait, that’s probably just in my household), check out their strategy tips. You can also practice against a robot here.

Finally, we inaugurate our recurring series, Ask an Ape, in which we answer science-y questions posed by listeners. Please weigh in with your own question in the comment section below, tweet us, post to our Facebook wall, or call our hotline: 312-893-2935.