Tiny Boltzmann Brains and Big Brains
The potions master starts with a tiny bit of skin and in a carefully calibrated series of steps adds a little bit of this and a little bit of that, a little molecular magic, and, presto the former skin cells become neural stem cells. Next, a cell or two is placed into tiny wells, and, with a bit more molecular magic, they start growing into a tangled mass of neurons. Tangles? Well, not really, because they organize themselves into circuits, and start sending out random pulses which eventually synchronize into something like brain waves. The experimenter drums his fingers on the table, and the tiny Boltzmann brains, er, neural organoids synchronize with the drumming.
These potions masters are not at Hogwarts, but in university labs. One interesting thing they have discovered in their experiments is that a single switch seems to be mainly responsible from brain size. Organoids from chimp and gorilla brains stop growing much sooner than human brain organoids. Not coincidentally, human brains are three times as large as gorilla brains and almost four times as large as chimp brains. This single gene seems to terminate brain growth in each, and, say the investigators, explains nearly all of the difference in brain size among the three apes.