Understanding how other animals dream could help us figure out why it's so important to the human brain, and why it may have been preserved throughout history.

Fruitflies, octopuses, birds, and humans don't seem to have much in common. Some live on land, others are aquatic. Some fly, while others are earthbound. Some are vertebrates, others lack backbones. These creatures evolved separately and their common ancestors are far, far back in the evolutionary chain. But they may share one fundamental feature: They dream.

Nearly all creatures sleep, though there's some debate as to whether single-celled organisms like paramecium do. But no one really knows why. For years, researchers have bandied about theories that sleep helps with memory, growth, and learning - and it's clear that humans need sleep to function properly - but there's little else that's well understood. "Sleep is this big black box," says Marcos Frank, a neuroscientist at Washington State University. Frank likens sleep to a mysterious organ: It's clear that it exists and is vital to animals' health, but it's exact function and the mechanisms that control it are still unknown.

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