The remarkable size of the human brain, a defining characteristic of our species, may not be the direct result of natural selection favouring increased intelligence, according to a new analysis of fossil skulls. Instead, researchers propose that brain size may have increased more or less randomly during periods when evolutionary pressures to maintain a consistent size temporarily diminished.
Dr. Katerina Harvati from the University of Tübingen in Germany, alongside Mark Hubbe from the University of Tennessee, Knoxville, examined 87 hominin skulls dating back two million years. This extensive sample included skulls from early humans like Homo habilis and Homo erectus, Neanderthals (Homo neanderthalensis), and recent Homo sapiens. Their investigation focused on the changing size and shape of both the braincase and the face over this vast evolutionary timescale.
Their findings, which have been peer-reviewed, indicate a clear trend: human braincases became considerably larger, accommodating bigger brains, while faces simultaneously became flatter with less prominent jaws and brow ridges. However, when testing six different evolutionary scenarios using a mathematical model, the changes in braincase size and shape were best explained by 'neutral evolution'. This suggests that random genetic mutations accumulated over time, leading to larger brains without necessarily conferring a significant survival advantage over smaller-brained ancestors.
The study also found evidence of 'stasis' – an evolutionary pressure that often worked to keep braincases roughly the same size and shape. It appears that only when this constraint eased, for reasons yet unknown, did brains have the opportunity to grow larger. Similar patterns were observed in facial changes, although the pressure for facial features to remain constant seemed even more pronounced.
This research challenges the long-held hypothesis that natural selection consistently favoured greater cognitive abilities, leading to a gradual increase in brain size. While acknowledging that the study's reliance on skull data limits insights to overall brain size and shape, not internal reorganisation, experts like Amélie Beaudet from the French National Centre for Scientific Research in Poitiers commend the study for its deep dive into evolutionary mechanisms. The findings offer a fresh perspective on the complex journey of human evolution.