A newly published paper in Proceedings of the Royal Society B by researchers from University College London (UCL) shows that the human skull evolved relatively rapidly compared to that of other apes. The evolutionary changes involve modifications in the size and shape of the facial and cranial bones.
This serves as a reminder of just how artificial and functionally useless the creationist concept of a “kind” is. It should also show creationists the fallacy of the frequent claim that biologists are abandoning the Theory of Evolution, since this paper discusses the results of evolution, not some infantile notion of magical intervention by an unevidenced supernatural entity.
Creationists are quite content to regard all cats—from domestic tabbies to tigers—as belonging to the same “kind”, even though the main difference between them lies in the size of their skeletons. Yet they balk at the idea that humans and the great apes could belong to the same “kind”, despite the fact that the key distinctions between us and them are also differences in size and proportion—most notably in the bones of the skull.
But then, “kind” is precisely the sort of term creationists favour because it has no fixed definition and can be expanded or contracted to suit whatever argument they are trying to make. The only consistent rule seems to be that whatever constitutes a “kind”, it must always exclude humans. This sometimes leads to the absurdity of defining an “animal kind” and a separate “human kind”.
The UCL team suggest that the rapid evolution of the human skull can be explained by the considerable advantage conferred by a larger brain and advanced cognitive abilities.
Our complex cognition allows us to communicate abstract ideas through both words and gestures—what we call “body language”—much of which depends on facial expression. A flat, forward-facing face enhances our ability to convey and interpret these subtle cues. As social animals, we identify acquaintances and strangers by their faces; we watch the faces of those who speak to us; and we instinctively read emotions such as pleasure, anger, confusion, or distress in their expressions.
In short, it is our large brain and expressive face that make us human — not the addition of new organs or limbs, as creationists often insist marks a change above the genus level, but rather differences in the size and shape of the bones of the skull. Given the close similarity of our genomes to those of other apes, these differences arise not from the amount of genetic information, but from the way that information is regulated during embryonic development.
Where we differ from other apes. How the Human Skull EvolvedThe UCL team’s findings are also summarised in a news release from UCL.
Compared with the other great apes, the human skull has changed remarkably over a relatively short evolutionary timespan. Our ancestors’ skulls became more rounded, with a larger cranium to accommodate an expanding brain, while the face flattened and the jaws and teeth reduced in size. Some of these changes, such as the rotation of the face to hang beneath the cranium, are directly related to the adoption of a fully upright gait — the skull had to balance atop the spine rather than project forward, as in a quadruped.
Ape Skulls vs. Human Skulls: What Changed?
The great apes retain long, projecting faces, large brow ridges and smaller braincases. In contrast, Homo sapiens has a vertical forehead, forward-facing eyes and a globular skull, all of which improve balance, depth perception, facial recognition and communication — vital advantages for a social, bipedal species.
Regulating Genes, Not Adding Information
The genetic differences behind these anatomical transformations are subtle. Humans share over 98% of their DNA with chimpanzees, and the divergence lies chiefly in how genes are switched on and off during embryonic development. It is these regulatory differences, not any increase in “genetic information,” that produced the distinctive human skull and the larger brain it houses.
Humans evolved fastest amongst the apes
The discovery of a human facial fragment aged over one million years represents the oldest known face in western Europe and confirms the region was inhabited by two species of human during the early Pleistocene, finds a new study involving a UCL researcher.
Humans evolved large brains and flat faces at a surprisingly rapid pace compared to other apes, likely reflecting the evolutionary advantages of these traits, finds a new analysis of ape skulls by UCL researchers.
The paper, published in the journal Proceedings of the Royal Society B, analysed how the evolutionary diversity of the skulls of humans and other related apes evolved over millions of years. Researchers from UCL Anthropology and UCL Biosciences found that the skull structure for humans evolved substantially faster than that of any other closely related ape species.
Evolution across great apes (orange) and gibbons (blue).
Not to scale.Of all the ape species, humans have evolved the fastest. This likely speaks to how crucial skull adaptations associated with having a big brain and small faces are for humans that they evolved at such a fast rate. These adaptations can be related to the cognitive advantages of having a big brain, but there could be social factors influencing our evolution as well.
Dr Aida Gomez-Robles, lead author
Department of Anthropology
University College London, London, UK.
The researchers examined three dimensional virtual models of the skulls* of different modern species of primates, including seven hominids, or “great apes,” ** such as humans, gorillas and chimpanzees, and compared them to nine species of hylobatids, or “lesser apes,” *** such as gibbons.
Hominids and hylobatids evolutionarily split from each other about 20 million years ago. During that time, the anatomical diversity of hominids exploded, while that of hylobatids remained surprisingly restricted. As a result, the skulls of different species of hylobatids look extremely similar to each other, while species of hominids look much more distinct. However, even within hominids, humans evolved faster than other species.
To measure skull variation, the researchers broke down each species’ skulls into four principal sections, the upper face, the lower face, the front of the head and the back of the head. Using a computer to compare the 3D scans, they numerically identified how different each section was between species. It’s the most detailed comparative analysis of 3D skull structure across these closely related ape species.
Most great apes have big and forwardly projecting faces with relatively small brains, while humans have flatter faces and large round heads. Gibbons, in some ways similar to humans, likewise have relatively flat faces as well and a round head, but a much smaller brain.
The team used the slow evolution and low diversity of hylobatids as a kind of control to compare the variation in hominid skulls. By comparing the species, the researchers found that humans changed about twice as much as would be expected if there wasn’t some additional factor encouraging additional changes.
The researchers cautioned that while it can be tempting to conclude that the evolutionary advantages of greater intelligence from bigger and more complex brains was the primary driver for human’s rapid evolution, social factors could be affecting these changes as well.
After humans, gorillas have the second fastest evolutionary rate of their skulls, but their brains are relatively small compared to other great apes. In their case, it’s likely that the changes were driven by social selection where larger cranial crests on the top of their skulls are associated with higher social status. It’s possible that some similar, uniquely human social selection may have occurred in humans as well.
Dr Aida Gomez-Robles.
* These are virtual representation of actual skulls, obtained from CT-scans of the skulls.
**Great apes are humans and species of gorilla, orangutans, chimpanzees and bonobos.
***Lesser apes are gibbons, which consist of around 20 different species.
Publication:
Abstract
The level of craniofacial diversity of hominids (the group that includes great apes and humans) is much higher than that of their sister group, the hylobatids (also known as gibbons or lesser apes), despite the similar timeline of diversification and a similar level of genetic differentiation between the two clades. To shed light on the evolutionary dynamics shaping these varying levels of craniofacial diversity, we used three-dimensional high-density geometric morphometric approaches and phylogenetic comparative methods. We show that neurocranial diversity exceeds that expected from neutral evolution in the great apes with respect to the gibbons, whereas facial diversity does not. These results indicate that neurocranial diversity across the great apes has been shaped by genus-specific neurocognitive, social or ecological selective pressures that are not observed in the gibbons, whose neurocranial diversity is constrained by stabilizing selection and gene flow. However, facial diversity results from similar evolutionary dynamics across both clades. Within this general pattern of differences and similarities between the great and lesser apes, humans emerge as the only species that consistently shows the highest evolutionary rate across almost all craniofacial regions in both males and females, thus pointing to strong human-specific selective pressures in neurocranial and facial evolution.
1. Introduction
Human craniofacial anatomy differs substantially from that of the great apes. While the great apes have big and forwardly projecting faces and relatively small brains, humans are characterized by an orthognathic face and a globular neurocranium [1]. Some researchers consider that human craniofacial anatomy has evolved in response to the primary selective pressure on increasing overall brain size and the size of particular brain regions [2–4], but others consider that human neurocranial shape has evolved as a result of integration patterns linking a flatter face with a more globular neurocranium [5,6]. Interestingly, human craniofacial anatomy appears more similar to that of the lesser apes (also known as hylobatids or gibbons), who also have a relatively flat face and a more globular braincase [7–9], than to the great apes’, despite the greater evolutionary distance separating humans and gibbons. It has been suggested that patterns of morphological integration between the face and the neurocranium have made gibbons and humans evolve in similar ways despite their large evolutionary distance [9].
Within hominoids (the clade including great and lesser apes), hominids (great apes and humans) show a large degree of anatomical diversity, spanning from small-faced and large-brained humans to large-faced and relatively small-brained gorillas. Hylobatids, however, show a low level of craniofacial diversity across species, with some species virtually indistinguishable based on craniofacial morphology, even when they show differences in their pelage, vocalizations, external features and karyotypes, among others [8,10,11]. Anatomical, genetic and biogeographical comparisons indicate that gibbons experienced rapid evolutionary radiation, hence their overall anatomical similarity [9,11,12]. This radiation is inferred to have happened approximately 7 million years ago (Ma), which coincides with the inferred divergence time between the chimpanzee and human lineages [12]. Also, genetic differentiation across all hylobatid genera is similar to the level of genetic differentiation between chimpanzees and humans [13]. However, chimpanzees and humans differ in craniofacial morphology substantially more than any two species of gibbons, so it is not clear whether gibbon craniofacial diversity has been kept at a low level through stabilizing selection and/or gene flow across species [14], or whether humans and chimpanzees have diverged anatomically more than expected based on their divergence time, hence pointing to species-specific selective pressures. Either way, the low level of craniofacial diversity observed in hylobatids makes them a useful ‘control’ clade against which variation within hominids can be compared to infer whether humans have evolved in an unusually fast way. Indeed, based on their taxonomic diversity, timing of diversification, frequent hybridization and reduced sexual dimorphism, hylobatids have been claimed to be appropriate evolutionary models for fossil hominins [15] (species that are more closely related to humans than to chimpanzees). Beyond chimpanzees and humans, these comparisons can be expanded to all the great apes, as they all show typical species-specific cranial morphologies that may have evolved in response to particular selective pressures [16]. Indeed, genetic analyses indicate that different species and subspecies of great apes have evolved population-specific genetic adaptations to their habitats [17].
Our understanding of the evolutionary processes that have driven hominoid craniofacial diversification is limited, as is the understanding of the selective factors shaping this diversity. Previous studies have explored the effect of integration on hominid craniofacial evolution [18–21], and others have measured the correlation between anatomical and genetic diversity [22]. Other studies have looked at the differences in developmental trajectories between humans and the great apes [23]. Also, some studies have assessed craniofacial diversity across primates [24], but very few have quantified the tempo and mode of craniofacial evolution in hominins [25–27], hominoids [16,28] or other primates [29]. Those studies have focused on a small number of anatomically homologous landmarks, leaving vast craniofacial regions undescribed. Conversely, detailed analyses of craniofacial evolution using high-density geometric morphometric approaches do not include humans, and their broader evolutionary scale makes it difficult to assess finer scale evolutionary trends within particular groups of primates [30]. To properly describe the tempo and mode of hominoid craniofacial diversity, as well as to understand the evolutionary pressures of the face versus the neurocranium, we used high-density three-dimensional geometric morphometric approaches to measure evolutionary rates for overall craniofacial morphology and for specific craniofacial regions along the evolutionary branches leading to most extant species of the hominoid phylogeny. In doing so, we aimed to detect whether there are fundamental differences in the evolutionary dynamics observed in the great and lesser apes, and whether humans have evolved their craniofacial morphology in a way that departs from that observed in the other apes.
A clear understanding of the patterns of long-term evolution across hominoids is obscured by the different levels of sexual dimorphism across the apes [31]. While some ape species, such as gorillas and orangutans, are highly sexually dimorphic, other species, such as gibbons, chimpanzees and humans, are minimally dimorphic [32]. These differing levels of sexual dimorphism are associated with social differences across species and mediated by hormonal factors that influence cranial development (reviewed in [33]). To understand the relationship between overall selective pressures, long-term evolution and sexual selection, we quantified sex-specific evolutionary trends in craniofacial evolution across the apes. A similar strategy has been used to compare brain evolution between male and female primates and to infer the relationship between brain evolution and behavioural and social evolution [34].
Figure 1. Phylogeny and configuration of landmarks and semilandmarks. (A) Time-calibrated hominoid phylogeny used for analyses indicating node ages (in Ma). One cranium per genus is represented (not to scale), with hominids shaded in orange and hylobatids shaded in blue (from top to bottom: Pongo, Pan, Homo, Gorilla, Nomascus, Hylobates, Hoolock, Symphalangus). (B) Chimpanzee model showing the studied configuration of landmarks and semilandmarks, with the different cranial regions represented in different colours: posterior neurocranium, green; anterior neurocranium, blue; upper face, purple; lower face, yellow.
Figure 2. Principal component analyses (PCA) of overall craniofacial variation and corresponding evolutionary rates overlaid on the hominoid phylogeny. (A) PCA plot of male shape variation. (B) Phylogenetic tree with overlaid branch-specific rate values for males. (C) PCA plot of female shape variation. (D) Phylogenetic tree with overlaid rate values for females. In (A) and (C), landmark plots next to and underneath PCA plots show the patterns of variation corresponding to the extreme values of each principal component (PC) within the observed range of variation. In (B) and (D), numbers underneath branches of the phylogenies indicate evolutionary rates for each branch measured as excess change with respect to a neutral expectation, with yellow representing small change (<1), grey representing expected change (≈1), and blue representing large change (>1). Branch thickness is proportional to the evolutionary rate.
Figure 3. Evolutionary rates observed for each branch of the hominoid phylogeny and each craniofacial region. (A) Posterior neurocranium in males. (B) Posterior neurocranium in females. (C) Anterior neurocranium in males. (D) Anterior neurocranium in females. (E) Upper face in males. (F) Upper face in females. (G) Lower face in males. (H) Lower face in females. Numbers underneath branches indicate evolutionary rates for each branch measured as excess change with respect to a neutral expectation, with yellow representing small change (<1), grey representing expected change (≈1) and blue representing large change (>1). Branch thickness is proportional to the evolutionary rate.
Figure 4. Comparison of morphological disparity and evolutionary rates between hylobatids and hominids for the four studied craniofacial regions. (A) Morphological disparity measured as pairwise Procrustes distances within each clade in males. (B) Morphological disparity measured as pairwise Procrustes distances within each clade in females. (C) Evolutionary rates measured as excess change with respect to a neutral expectation for each branch in males. (D) Evolutionary rates measured as excess change with respect to a neutral expectation for each branch in females. In (C) and (D), the data points corresponding to the human rate for each craniofacial region and sex are highlighted with a black circle.
Figure 5. Correlations between evolutionary rates for the four craniofacial regions for hylobatids and hominids. (A) Males and (B) females. Corr, correlation obtained for the combined hylobatid–hominid sample; Hom, correlation found in hominids; Hyl, correlation found in hylobatids. Asterisks indicate significant correlations at p < 0.05 (*), p < 0.01 (**) or p < 0.001 (***).
Gómez-Robles Aida, Drennan Amy, Basa Maricci and Gleeson Alfie 2025
Accelerated evolution increased craniofacial divergence between humans and great apes
Proc. R. Soc. B. 292: 20251507 http://doi.org/10.1098/rspb.2025.1507
Copyright: © 2025 The authors.
Published by The Royal Society. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
Far from being “abandoned by mainstream biologists”, as creationists routinely assert, the Theory of Evolution continues to underpin and drive modern biological research. This study is yet another example of evolution being used as the explanatory framework for new discoveries, helping us to understand not only how species change over time, but why specific anatomical traits emerged in response to selective pressures. It is precisely because evolution is so robust, predictive and fruitful as a scientific theory that researchers employ it — rather than the vague and untestable assertions of creationism, which make no useful predictions and cannot explain the evidence.
Moreover, these findings highlight just how contrived the creationist notion of separate “kinds” really is. The differences between humans and the other great apes are not a matter of separate origins or magically distinct categories, but of gradual change in size, shape and development of shared anatomical structures from a common ancestor. The human skull did not suddenly appear in its modern form — it evolved from an ape-like skull by known biological mechanisms. The more we learn, the clearer the continuity becomes, and the less room there is for the artificially imposed barriers that creationists insist upon.
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