To reverse engineer an entire nervous system

There are many theories of how behavior may be controlled by neurons. Testing and refining these theories would be greatly facilitated if we could correctly simulate an entire nervous system so we could replicate the brain dynamics in response to any stimuli or contexts. Besides, simulating a nervous system is in itself one of the big dreams in systems neuroscience. However, doing so requires us to identify how each neuron's output depends on its inputs, a process we call reverse engineering. Current efforts at this focus on the mammalian nervous system, but these brains are mind-bogglingly complex, allowing only recordings of tiny subsystems. Here we argue that the time is ripe for systems neuroscience to embark on a concerted effort to reverse engineer a smaller system and that Caenorhabditis elegans is the ideal candidate system as the established optophysiology techniques can capture and control each neuron's activity and scale to hundreds of thousands of experiments. Data across populations and behaviors can be combined because across individuals the nervous system is largely conserved in form and function. Modern machine-learning-based modeling should then enable a simulation of C. elegans' impressive breadth of brain states and behaviors. The ability to reverse engineer an entire nervous system will benefit the design of artificial intelligence systems and all of systems neuroscience, enabling fundamental insights as well as new approaches for investigations of progressively larger nervous systems.