The experiment gave bioethicists the heebie-jeebies: the mini-brain not only matured, but commandeered the mice’s circulatory system to feed themselves; they also formed functional synapses with the mice’s own neurons in a hybrid network that fired together.

Then, a few months later, another team offered evidence that the electrical activity of mini-brains resembles those of premature babies. Using electroencephalography (EEG), the team captured the surface electrical activity of mini-brains at six months, and recorded activity eerily similar to fetuses roughly 30 weeks after conception—unpredictable, chaotic bursts of synchronized electrical activity.