How does the brain tell the body to walk backward? Scientists don’t know. To find out, researchers developed 3,500 different strains of fruit flies. Each genetically-modified fly carried genetic switches in different parts of the brain that are activated by heat, a technique called thermogenetics. When these genetic switches were activated, they caused different neurons to fire. The scientists finally found a fly that began walking backward when its genetic switches were activated by heat, and from there they found which neurons were firing.

As it turns out, walking backward (at least in fruit flies) is controlled by two neurons. As Science Magazine explains:

The first neuron appears to be the “initiator,” compelling the fly to “moonwalk” when it encounters certain cues, like an obstacle. And the second gene acts like a brake, perhaps preventing it from backing into something. The researchers, whose study was published in Science, will use these neurons as a starting point to find other networks involved in walking backward, such as those responsible for touch, sight, and smell.