Today, organoids that resemble different regions of the human brain are routinely spun up from stem cells in large batches in laboratories around the world. Researchers have refined their recipes since the technique was first described five years ago, but the process is surprisingly hands-off: after a few nudges from scientists, stem cells grow into spheres with about a million neurons through a naturally occurring choreography that mirrors early brain development in the womb. At Day 100, Qian’s minibrains resemble a portion of the pre­natal brain in the second trimester of pregnancy.

“People are more worried about if they reach a certain level — if it’s really like a human brain. We’re not there; we’re very far from there,” said Hongjun Song, who leads the laboratory at Penn’s Perelman School of Medicine, where Qian works. “But the question people ask is, ‘Do they have consciousness?’ The biggest problem I have so far is I think, as a field, we don’t know: What is consciousness? What is pain?”