Scientists have made an enormous step toward understanding the human brain by building a neuron-by-neuron and synapse-by-synapse roadmap -- scientifically speaking, a 'connectome' -- through the brain of an adult fruit fly (Drosophila melanogaster). Previous researchers have mapped the brain of a C. elegans worm, with its 302 neurons, and the brain of a larval fruit fly, which had 3,000 neurons, but the adult fruit fly is several orders of magnitude more complex, with almost 140,000 neurons and 50 million synapses connecting them. The full brain map is the result of a collaboration between cutting-edge artificial intelligence, gamers, professional tracers, and neuroscientists. This connectome is important not only because the adult Drosophila has the most complex brain ever mapped, but also because fruit flies share many behaviors with humans as well as 75% of the genes that cause genetic diseases.