Scientists have cast doubt on the conventional view of human evolution

Scientists from the University of Tennessee and the Senckenberg Centre for Human Evolution and Palaeoenvironments at the University of Tübingen analysed the skulls of ancient members of the genus Homo and concluded that: brain growth and facial reduction were not necessarily the result of constant, directed selection. Random genetic changes, long periods of near-stagnation and biological constraints may have played an important role.

This does not mean that natural selection has been ‘ruled out’. The authors do not dispute that, overall, the brains of Homo species grew larger, whilst their faces and jaws became less robust. What they are saying is this: these changes may not have followed a straight path towards ‘Homo sapiens’, but rather a more complex process involving pauses, constraints and periods of acceleration.

Details

The genus Homo first appeared around 2.5 million years ago. Today, it is represented solely by modern humans, but in the past it included various species: Homo habilis, Homo erectus, Homo heidelbergensis, Neanderthals and other lineages. Overall, this group is indeed characterised by two major trends: an increase in brain size and a reduction in the size and bulk of the face and jaws.

For a long time, these changes were explained quite simply: a larger brain provided cognitive advantages, whilst smaller jaws and a smaller face became possible because humans made greater use of tools and processed food differently. In other words, the body no longer needed to rely so heavily on a powerful masticatory system.

However, a new study has examined the extent to which this widely accepted explanation aligns with actual data from fossilised skulls. To do this, Mark Hubbe and Katerina Harvati used three-dimensional measurements of 87 fossil Homo specimens — ranging from early species such as Homo habilis and Homo rudolfensis to Homo erectus, Homo heidelbergensis, Neanderthals, and early and modern Homo sapiens.

The researchers then compared the data with six evolutionary models. These included directed natural selection, neutral evolution, prolonged periods of almost imperceptible change, and the ‘punctuated equilibrium’ model, in which species remain stable for long periods before changing relatively rapidly.

What the scientists discovered

The analysis confirmed the fact itself: the cerebral cortex in members of the Homo genus generally increased in size, whilst the face became smaller. However, these changes were best explained not by a constant progression towards a single ‘goal’, but by a combination of neutral processes, stabilising selection and prolonged periods of evolutionary stasis.

Put simply, human evolution may not have been a staircase where each step leads to a larger brain and a more modern face. Rather, it was a path subject to constraints: at times the organism remained virtually unchanged; at other times, changes were held back by developmental factors, energy constraints and the environment; and at still other times, conditions allowed for a rapid transition to new traits.

Periods of cultural and technological change may have been particularly significant. According to the authors, culture acts as a kind of ‘buffer’: it helps us settle in new places, gain access to more resources and reduces the body’s dependence on strict adaptation to a specific environment. This may have opened the way for larger brains and new forms of behaviour.

What this means in simple terms

Evolution did not set out to create humans with larger brains. It did not aim to make faces smaller or intelligence greater.

Traits changed because ancient populations lived in different conditions, faced constraints, used tools, changed their diet, settled in new territories and developed social bonds. Some changes became established, some were held back, and some may have been a side effect of other processes.

Therefore, the new conclusion is this: humans did not simply ‘gradually become more intelligent’. Their evolution was far less linear and much more dependent on a combination of biology, environment and culture.

Why this matters

This research challenges the popular but overly simplistic view of human origins. In this view, Homo is often depicted as a line of progress: from a ‘primitive’ ancestor to modern humans. But palaeoanthropology increasingly shows that evolution was not a straight line, but a branching and uneven process.

The study also reframes the question itself. Instead of asking, ‘Why did humans consistently evolve towards larger brains and smaller faces?’, the authors suggest asking it differently: under what conditions could ancient populations break free from previous constraints and develop new traits?

This is an important shift in perspective. It does not negate the role of natural selection, but it dispels the notion that it constantly and purposefully ‘pushed’ humans towards the modern form.

Background

Brain enlargement is one of the best-known hallmarks of human evolution. It is associated with tool use, social life, language, learning and the ability to adapt to new conditions. But a large brain comes at a cost: it requires a great deal of energy, a long period of development and complex care for offspring.

The reduction in the size of the face and jaws was also long explained by changes in diet and technology. If food can be cut, crushed, cooked and processed using tools, the strain on powerful jaws may decrease. But new research shows that even such a logical explanation does not account for the whole picture.

Changes occurred differently across the various Homo lineages. For example, Neanderthals retained a more robust facial morphology for a long time, whereas in modern humans the face became significantly smaller compared with other lineages. The authors suggest that these later changes may have been linked to profound behavioural shifts following the emergence of our species.

Source

Study: Mark Hubbe, Katerina Harvati, “Evolutionary drivers of encephalisation and facial reduction in the genus Homo”, Nature Communications, 2026.