Human brains may have become exceptionally large for no particular evolutionary advantage, scientists now suggest

A hyperrealistic, close-up view of a human brain, showcasing its detailed textures and vascular network under dramatic lighting. Image Cedit: TIL Creatives

Nearly 90 fossil skulls were studied by scientists

A larger brain doesn't necessarily translate into greater selection pressures

Image of a human brain and skull. Image Credit: Wikimedia Commons

It contradicts a well-established notion

Human evolution is seldom linear

The controversy is far from being settled

One of the most common stories in the field of human evolution is that over time, the brains of our ancestors got bigger because bigger brains allowed them to be more intelligent, enabling them to survive and create more advanced societies.But now, recent research has challenged the simplicity of this story. In a paper published in Nature Communications , it has been shown that natural selection didn't favor larger brain sizes in humans throughout the past two million years of their evolutionary history. Rather, the increase in brain size may have been driven by processes other than natural selection.The paper reports that the authors compared the cranial changes against six evolutionary scenarios and found the strongest support for neutral evolution and stasis, rather than a steady directional push toward bigger brains. In the neurocranial data, an unbiased random walk carried an Akaike weight of 0.961 for PC1, while the facial dataset likewise favoured stasis, strict stasis, and neutral evolution.These results challenge a common assumption about human evolution.The study, led by researchers Mark Hubbe of the University of Tennessee and Katerina Harvati of the University of Tübingen, analysed three-dimensional cranial measurements from 87 specimens, including 63 fossil members of the genus Homo and 24 recent Homo sapiens crania, to investigate how the human skull evolved over the past two million years.As reported in the journal Nature Communications , the scientists used six different models of evolution to see which one best explains the evolution of braincases and faces for about two million years.In the study, Hubbe and Harvati analysed three-dimensional landmark data from 87 Homo crania, 63 fossils, and 24 recent H. sapiens, and split them into neurocranial and facial datasets. They then tested six models, including gradual directional selection, stabilizing selection, neutral evolution, Ornstein-Uhlenbeck change, and punctuated equilibrium, across two lineages: H. sapiens and H. neanderthalensis. While there is no question that brains have grown larger and faces have become smaller over time, the data analysis did not back up the widely accepted belief that directional natural selection was the main force behind these changes.The paper does not argue that brain evolution had no purpose or no benefits for humans.Rather, the results imply that the evolution of larger brains may not have been subjected to evolutionary pressures throughout the evolution of the genus Homo. Random genetic drift, stabilizing selection, and biological limitations may have played a larger role than has been acknowledged before by scientists. As noted in a press release issued by the University of Tübingen , according to Professor Katerina Harvati, instead of trying to find out why humans developed larger brains, scientists should try to understand what enabled populations to evolve completely new features.Anthropologists have associated larger brains with increased cognitive, linguistic, tool-making, and social skills. But large brains are expensive from a biological point of view. Human brains comprise just 2 percent of body weight but use up to 20 percent of the body's resting energy consumption of the body, making it one of the most energy-intensive organs. Since maintaining brain tissue is very energy-intensive, many researchers have believed that such an expensive organ must have contributed greatly to the survival of its owners.Previous studies in the journal Nature have shown that humans developed a greater overall energy metabolism, which could sustain their energy-hungry brains. The study does not overturn the basic facts. On the contrary, it casts doubts on the notion that the increased size of the brain was always the goal of evolution.Although evolution is often described as a smooth march toward perfection, scientists say the process is much more complicated.Many traits evolve not just from mutations, evolutionary processes, environmental changes, and developmental constraints, but also because some traits evolve by association, and luck can play a role in the survival and reproduction of populations. According to the scientists, their models suggest that brain-size increases in Homo fit a more complex evolutionary picture than a simple story of gradual selection for intelligence.The authors do not argue that intelligence was irrelevant or that large brains offered no benefits. They mean that the fossil record cannot provide enough evidence for the idea of consistent growth of brains under selective pressures over millions of years.The results are likely to prompt debate among evolutionary biologists because they challenge a long-standing theory.The brain size itself cannot provide enough information to understand intelligence, behavior, and cognition. Homo sapiens, Neanderthals, and other extinct species had large brains, but their morphology, lifestyles, and evolutionary histories were very different. The study encourages scientists to rethink long-held assumptions without abandoning them entirely. Future research in fossils, genetics, archaeology, and developmental biology may show that human brain evolution was shaped by several processes working together.The paper suggests evolution is not always simple or linear. One defining feature of our species may not be the result of selection for ever-larger, more complex brains. Instead, its evolution may have been more complex than previously believed.