Human evolution may not have followed a straight path toward a bigger brain

The fossil record appears to show a familiar progression. Across the genus Homo, the braincase became larger and more rounded, while the face became less projecting and robust. Yet a visible trend does not by itself show that natural selection kept pushing our ancestors toward bigger brains and modern human faces for nearly two million years.

The conventional explanation is plausible. Larger brains may have improved cognitive abilities and therefore been favoured by sustained directional selection. Tools, food processing and cooking reduced the work performed by teeth and jaws, potentially making a large face unnecessarily costly. Mark Hubbe and Katerina Harvati tested whether fossil skull morphology actually fits this scenario better than competing evolutionary models.

Their dataset contained 63 fossil Homo crania and 24 recent human skulls, spanning roughly 1.9 million years to the Holocene. It included early Homo, Homo erectus, Homo heidelbergensis, Neanderthals and Homo sapiens. Of the fossil specimens, 57 could be used in the neurocranial analysis and 47 in the facial analysis.

The researchers recorded three-dimensional anatomical landmarks and arranged the specimens into eight chronological and taxonomic groups. They then compared the observed variation with six models representing gradual directional change, neutral random change, different forms of evolutionary stasis, movement toward an adaptive optimum, and punctuated equilibrium. The analyses were run separately for evolutionary sequences leading to modern humans and Neanderthals.

A neutral random walk describes changes that accumulate without a persistent shift in one direction. Stasis describes a trait remaining within a restricted range for long periods, potentially because stabilizing selection or biological constraints limit variation.

For the main neurocranial trend in the Homo sapiens lineage, the neutral model received an Akaike weight of 0.961, compared with 0.036 for gradual directional selection. In the Neanderthal lineage, the corresponding values were 0.931 and 0.024. Facial evolution also fit combinations of stasis and neutral change better than sustained directional selection. For the principal facial trend in the Homo sapiens lineage, the weights were 0.571 for stasis, 0.387 for neutral change and 0.041 for gradual directional change. These values measure relative support among the models tested; they are not probabilities that any one model is definitively true.

The results suggest that a larger, more rounded braincase and a flatter face could emerge without uninterrupted selection toward a predetermined modern form. The authors propose that major transitions may instead have occurred when existing constraints were relaxed. Those constraints could have involved development, cranial architecture, metabolism or energy availability. Tools, new food resources and cooking might have helped release some of them, but the study did not directly test cultural behaviour as a causal mechanism.

There are substantial limits to the inference. The study examined external cranial shape and overall skull size, not brain volume relative to body size, which is the strict definition of encephalization. Some groups contained only a few fossils, missing landmarks had to be reconstructed, and the results were sensitive to how specimens were grouped into evolutionary sequences. Alternative Neanderthal groupings sometimes favoured more complex models. With so few chronological groups, brief episodes of strong selection may also be difficult to detect.

The study does not remove natural selection from human evolution. It draws a more precise distinction: the direction in which anatomy changed is not necessarily the process that caused it to change.