The famed molecular biologist Francis Crick turned to neuroscience in the 1970’s. But by 1993, he was so chagrined by the ignorance of his new field that he penned an editorial in the journal Nature. “It is intolerable that we do not have [a connection map of] the human brain,” he wrote. “Without it there is little hope of understanding how our brains work except in the crudest way.”

There was no such map in 1993 because the only way to get one was to use anatomical methods: inject dye into the brain of an organism, kill it, and trace the color trail in the neurons with microscopes. Of course ethics rule out this sort of experimentation on humans.