One of the defining features of our species is our notably large brain. However, a recent analysis of fossil skulls indicates that evolution hasn’t significantly favored larger brains during the past two million years of human evolution.

“Brain size has indeed increased over time, while face size has actually reduced,” states Katerina Harvati from the University of Tübingen, Germany. Yet, this development does not appear to be the result of natural selection.

Rather, the evolution of the human brain seems to have been influenced by various evolutionary pressures, which maintained brain size. Periodically, these constraints were relaxed, enabling the brain to grow larger.

Dr. Mark Hubbe from the University of Tennessee, Knoxville, along with Harvati, examined 87 hominin skulls, 24 of which belonged to modern Homo sapiens. The remainder were from extinct human species, such as Neanderthals (Homo neanderthalensis), Homo erectus, and Homo habilis. They gathered data on skull shape and size, concentrating on the braincase and facial features.

Over the past two million years, the human braincase has enlarged significantly, providing room for larger brains. Concurrently, human faces became flatter, characterized by reduced jaw prominence and smaller eyebrow ridges.

“There’s a pervasive hypothesis suggesting that increased cognitive ability drives selection,” says Harvati. This concept posits that intelligent individuals were favored, leading our ancestors to gradually develop larger brains over time.

To test this theory, Haverty and Hubbe employed a mathematical model that evaluated six different evolutionary scenarios influencing changes in skull shape and size. One scenario suggested gradual natural selection pressure favoring larger brains, while another scenario presented a more neutral approach. The remaining scenarios included fluctuating constraints around an optimal design and “punctuated equilibrium,” where long periods of stasis were punctuated by brief bursts of rapid evolution.

Findings indicated that changes in braincase size and shape are most effectively explained by neutral evolution, suggesting that random mutations impact brain size and shape without providing a significant advantage over previous designs. Evidence of stagnation further indicates that evolution exerts pressure on braincases to maintain similar dimensions.

While human faces underwent changes, the evolutionary constraint to preserve their shape appeared to be stronger.

This study provides insight into the mechanisms that have steered our evolutionary journey. Amelie Baudet at the French National Center for Scientific Research in Poitiers states, “This is precisely the kind of analysis we need right now.”

Although the study focused on skulls, it primarily addresses overall brain size and shape, which may not encapsulate the entire picture. Bode noted, “It’s not just about brain size; it likely involves brain reorganization.” She studies the imprints left by the brain within the skull, indicating that crucial areas like Broca’s area, associated with language, have evolved considerably over the last two million years.

Some researchers, like Gerhard Weber at the University of Vienna, argue that the sample size is insufficient for definitive conclusions. He remains skeptical about Harvati and Hubbe’s assertions that evolution kept our ancestors’ brains consistently sized. “As social animals, prehistoric societies likely featured varied divisions of labor,” he adds. In such contexts, exceptional intelligence may not have been a significant advantage.

Weber is intrigued by the notion that our brains grew not due to natural selection, but rather when constraints were lifted. “That’s a compelling idea,” he remarks.

Harvati suggests cooking may have played a critical role in this evolution. Since the brain demands substantial energy to function, and cooked food offers higher caloric value than raw, the advent of cooking could have provided the necessary energy boost for larger brains.