The questions that Herculano-Houzel sought to answer go back more than 100 years, to a time when scientists were just starting to study the relationship between brain size and intelligence.

In August 1891, laborers working for the Dutch anatomist Eugène Dubois began excavating trenches along a steep riverbank on the Indonesian island of Java. Dubois hoped to find early hominin remains.

Over the course of 15 months, layers of sandstone and hardened volcanic gravel yielded the petrified bones of elephants and rhinos, and, most importantly, the skullcap, left femur, and two molars of a human-like creature thought to have died nearly a million years before. That specimen, named Pithecanthropus erectus, and later Java Man, would eventually come to be known as the first example of Homo erectus.

Dubois made it his mission to infer the intelligence of this early hominin. But he had only three fragments of seemingly relevant information: its estimated brain size, stature, and body weight. Would this be enough?

Zoologists had long noticed that when they compared different species of animals, those with bigger bodies had larger brains. It seemed as if the ratio of brain weight to body weight was governed by a mathematical law. As a start, Dubois set out to identify that law. He gathered the brain and body weights of several dozen animal species (as measured by other scientists), and using these he calculated the mathematical rate at which brain size expands relative to body size. This exercise seemed to reveal that across all vertebrates, the brain really does expand at a similar rate relative to body size.

Dubois reasoned that as body size increases, the brain must expand for reasons of neural housekeeping: Bigger animals should require more neurons just to keep up with the mounting chores of running a larger body. This increase in brain size would add nothing to intelligence, he believed. After all, a cow has a brain at least 200 times larger than a rat, but it doesn’t seem any smarter. But deviations from that mathematical line, Dubois thought, would reflect an animal’s intelligence. Species with bigger-than-predicted brains would be smarter than average, while those with smaller-than-predicted brains would be dumber. Dubois’s calculations suggested that his Java Man was indeed a smart cookie, with a relative brain size—and intelligence—that fell somewhere between modern humans and chimpanzees.

Dubois’s formula was later revised by other scientists, but his general approach, which came to be known as “allometric scaling,” persisted. More modern estimates have suggested that the mammalian brain mass increases by an exponent of two-thirds compared to body mass. So a dachshund, weighing roughly 27 times more than a squirrel, should have a brain about 9 times bigger—and in fact, it does. This concept of allometric scaling came to permeate the discussion of how brains relate to intelligence for the next hundred years.