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For decades, the standard method for counting brain cells was stereology: slicing up the brain, tallying cells in thin sheets of tissue splayed on microscope slides and multiplying those numbers by the volume of the relevant region to get an estimate.

Using stereology to take a census of the human brain would require a daunting amount of time, resources and unerring precision.

Stereology is a laborious technique that works well for small, relatively uniform areas of the brain.

In a study from the 1970s, Herculano-Houzel discovered a curious proposal for an alternative to stereology: Why not measure the total amount of DNA in a brain and divide by the average amount of DNA per cell?

But many species have brains that are simply too big, convoluted and multitudinous to yield to stereology.