The popular framing of human evolution, repeated across textbooks, museum exhibits, and most contemporary writing on the subject, is that the human brain has been getting larger across the entirety of our evolutionary history. The story typically begins with the early hominins of approximately 6 million years ago, whose brains were comparable in size to those of modern chimpanzees, and proceeds through a series of steady increases in cranial capacity that culminated in the modern human brain, which is approximately three to four times larger than that of our earliest hominin ancestors and proportionally larger than that of any other primate.
The story is broadly correct for the first 5.99 million years of human evolution. It is, on the strongest current reading of the peer-reviewed evidence, wrong for the most recent chapter.
The average modern human brain is approximately 8.5 per cent smaller than the average late Pleistocene human brain, the difference is real and has been documented repeatedly across nearly ninety years of independent peer-reviewed research, and no one in the relevant scientific disciplines has yet been able to establish either when exactly the reduction occurred or what caused it.
A ninety-year-old observation
The first peer-reviewed observations that modern human brains were smaller than the brains of late Pleistocene humans were published in the 1930s and 1940s by Gerhardt von Bonin (1934) and Franz Weidenreich (1946), both working with the limited cranial samples available at the time. The observation was not initially considered controversial. The data was thin, the methodology was crude by modern standards, and the finding sat quietly in the background of physical anthropology research for several decades.
The observation began to attract more sustained attention from the 1970s onward, as larger cranial datasets became available and as better statistical methods for analysing them were developed. Phillip Tobias published comparative measurements in 1971. Maciej Henneberg, an Australian biological anthropologist who has spent his career studying global cranial datasets, published a series of papers from 1988 onward documenting the reduction across multiple population samples on multiple continents. By the early 2000s, the basic empirical claim that the modern human brain is smaller than the late Pleistocene human brain had been documented in approximately nineteen independent peer-reviewed studies, spanning samples from Europe, Africa, East Asia, Australia, and the Americas.
The average reported reduction across these studies is approximately 8.5 per cent. The late Pleistocene average, on the largest available compilations, was approximately 1,458 cubic centimetres. The modern average sits closer to 1,300 to 1,350 cubic centimetres. The difference of approximately 150 cubic centimetres is roughly the volume of a tennis ball.
The reduction is, in evolutionary terms, surprisingly large. Average hominin brain size increased by approximately 70 cubic centimetres per 100,000 years across the long arc of the Pleistocene, between approximately 800,000 years ago and 30,000 years ago. The Holocene reduction has occurred at a rate that is, depending on how the timing is set, between thirty and one hundred times faster than the earlier increase. In evolutionary terms, the human brain has been shrinking with unusual rapidity by the standards of its own previous trajectory.
The 2021 DeSilva analysis
In October 2021, a team led by Jeremy DeSilva at Dartmouth College, with collaborators James Traniello at Boston University, Alexander Claxton at Oklahoma State University, and Luke Fannin at Dartmouth, published a study in Frontiers in Ecology and Evolution that attempted to identify the precise timing of the brain-size reduction. The team compiled a dataset of 985 cranial measurements from across the full hominin fossil and recent human record, ranging from approximately 9.8 million years ago to the present, and applied a statistical method called change-point analysis to identify the moments at which the rate of brain-size change shifted.
The analysis identified three significant change points in hominin brain evolution. Two were positive, corresponding to known periods of brain-size expansion approximately 2.1 million years ago (coinciding with the early evolution of the genus Homo) and approximately 1.5 million years ago (coinciding with the appearance of Homo erectus and the development of more sophisticated stone tool industries). The third change point was negative. The DeSilva team’s analysis dated it to approximately 3,000 years ago.
The proposed timing was a substantial departure from earlier estimates, which had placed the reduction anywhere from 35,000 to 10,000 years ago. The 3,000-year figure placed the brain reduction in a period of well-documented human history, after the development of agriculture, after the emergence of the earliest cities, and during the rapid expansion of literate societies in the Bronze Age. The DeSilva team’s interpretation, drawn from a comparative analysis of brain evolution in ant species with similar social structures, was that the reduction reflected the externalisation of cognitive load into social structures, the distribution of knowledge across literate groups, and the consequent reduction of selective pressure on individual brain size.
The hypothesis was provocative. It also, the team acknowledged in the paper, depended on the specific timing they had identified, which depended in turn on the specific statistical method they had applied to the specific dataset they had compiled.
The 2022 critique
In July 2022, Brian Villmoare at the University of Nevada Las Vegas and Mark Grabowski at Liverpool John Moores University published a peer-reviewed critique in the same journal. The Villmoare and Grabowski analysis reexamined the DeSilva dataset and identified what the authors considered substantial methodological problems with the original conclusion.
The first issue was sample distribution. The DeSilva dataset of 987 skulls was heavily weighted toward modern samples, with more than half of the specimens dating from the most recent 100 years of the 9.8-million-year time range. The 23 critical specimens from the period in which the brain reduction was supposed to have occurred were drawn from geographically heterogeneous locations, including England, China, Mali, and Algeria. Villmoare and Grabowski argued that this heterogeneity made the dataset unsuitable for detecting a fine-grained change point, because population-level variation between regions was being conflated with temporal variation across the species.
The second issue was the change-point methodology itself. Villmoare and Grabowski reanalysed portions of the same dataset using independent statistical methods and were unable to reproduce the 3,000-year change point. Their analysis found no statistically significant brain-size reduction during the time frame DeSilva had identified. The Villmoare and Grabowski conclusion was that the alleged reduction either did not occur, or that the available data was insufficient to demonstrate it.
The Villmoare and Grabowski critique did not reject the entire 90-year-old observation that modern brains are smaller than late Pleistocene brains. Their argument was specifically that the dramatic 3,000-year reduction the DeSilva team had proposed was not supported by the evidence, and that any actual reduction was likely to have occurred over a much longer time scale and through different mechanisms than the social-evolution hypothesis required.
The 2023 response
In 2023, the DeSilva team published a response to the Villmoare and Grabowski critique, also in Frontiers in Ecology and Evolution. The response acknowledged some of the methodological concerns raised in the 2022 paper and presented a revised dataset that addressed several of them. The revised dataset removed juvenile Neanderthal specimens that had been accidentally included in the original compilation, removed a duplicated entry, and dropped data from the historically problematic Morton collection. The revised dataset also added additional Pleistocene specimens to address the under-sampling concern.
The DeSilva team’s reanalysis with the revised dataset, they argued, still detected a statistically significant brain-size reduction in the recent Holocene. They acknowledged that the exact timing was uncertain and could reasonably be placed anywhere between approximately 5,000 and 3,000 years ago, but they maintained that the broader pattern of reduction was robust. The response also emphasised that the underlying observation, that modern human brains are smaller than late Pleistocene brains, was supported by approximately nineteen independent studies over nearly ninety years and could not reasonably be dismissed even if the specific 3,000-year timing was challenged.
The Villmoare and Grabowski position is that the brain may have reduced more gradually since the origin of Homo sapiens approximately 300,000 years ago, or may not have reduced at all in any statistically defensible sense. The DeSilva position is that a rapid Holocene reduction is real and recent. The dispute is genuinely ongoing in the peer-reviewed literature.
Why the brain might have shrunk
The peer-reviewed literature has produced at least six major hypotheses for why human brain size might have decreased in the Holocene, none of which has won scientific consensus.
The first is the social externalisation hypothesis advanced by DeSilva and colleagues. Cognitive load, on this view, has been progressively distributed across literate populations, writing systems, oral traditions, and social structures, reducing the selection pressure on individual brain size.
The second is the body-size hypothesis. Human body size has reduced measurably during the Holocene, possibly as a result of dietary changes following the agricultural transition, and brain size and body size are known to be correlated in mammals. On this view, the brain reduction is a passive consequence of overall body-size reduction. The Villmoare and Grabowski critique points out that body-size reduction can only account for a small fraction of the observed brain-size reduction, and that other factors must be involved.
The third is the agricultural diet hypothesis. The transition to grain-based diets, which began approximately 12,000 years ago in different regions, reduced overall nutritional quality compared with the hunter-gatherer diet that preceded it. Brain tissue is metabolically expensive to maintain, and reduced nutritional intake may have produced selection pressure for smaller brains.
The fourth is the self-domestication hypothesis. Multiple peer-reviewed analyses have argued that humans have undergone a process of self-domestication during the Holocene, with measurable reductions in aggression-related neural systems and corresponding changes in facial structure. Domesticated animals consistently show brain-size reductions of approximately 10 to 15 per cent compared with their wild ancestors, and the human brain reduction may reflect the same mechanism.
The fifth is the energetic efficiency hypothesis. Smaller brains may have been selected for because they require less metabolic energy to operate. The human brain consumes approximately 20 per cent of total body energy, and even modest reductions in brain size can produce substantial energy savings that may have been advantageous under conditions of dietary stress or population pressure.
The sixth is the cognitive efficiency hypothesis, distinct from but related to the social externalisation argument. On this view, the human brain has not become less capable but has become more efficient, with the same cognitive functions implemented in a smaller volume through changes in neural organisation, white-matter density, or synaptic efficiency.
The hypotheses are not mutually exclusive, and most researchers in the field accept that some combination of factors is probably responsible. The disagreement is about which factors matter most and how they interact.
The honest limitations
Several methodological caveats apply to the literature described above.
The first is that cranial capacity is an indirect proxy for brain size, not a direct measurement of it. The brain does not fill the entire cranial cavity, and the relationship between endocranial volume and actual brain mass varies between populations and individuals. The Holocene reductions identified in the literature are based on cranial measurements, not direct brain measurements, and the relationship between the two is not entirely settled.
The second is that the available fossil and recent human cranial samples are not evenly distributed across time, geography, or population. The Pleistocene samples come from a relatively small number of well-preserved individuals from specific regions. The Holocene samples come from much larger numbers of individuals from much wider geographic and population coverage. Comparing the two requires statistical adjustments that involve assumptions about how representative each sample is, and different researchers have made different assumptions.
The third is that the relationship between brain size and cognitive ability is not direct. Larger brains do not, on the contemporary evidence, reliably produce greater intelligence within or between populations. The Holocene reduction, even if real, does not necessarily indicate that modern humans are less intelligent than late Pleistocene humans. The relationship between brain volume and cognitive function is mediated by neural organisation, connectivity, and efficiency in ways that volume measurements cannot capture.
What it means
Several things follow from the brain-shrinkage evidence that are worth saying clearly.
The first is that the popular framing of human evolution as an ongoing march toward larger brains is not supported by the most recent peer-reviewed data. The available evidence, on the strongest current reading, indicates that the human brain has been shrinking, slowly, for somewhere between 3,000 and 300,000 years. The standard textbook narrative of ever-expanding cranial capacity ends approximately 30,000 years ago, in the late Pleistocene, when the maximum average human brain size was reached. Everything since has been a slow contraction.
The second is that the cause of the reduction is genuinely unknown. At least six major peer-reviewed hypotheses are currently in active scientific competition, and the dispute about which one is correct, and whether the reduction occurred 3,000 years ago or 30,000 years ago or across the whole Holocene, is one of the more interesting open questions in physical anthropology.
The third is that the finding sits uncomfortably with the broader cultural narrative about human progress. The 3,000-year time frame that DeSilva proposed, in particular, coincides with the rise of writing, the emergence of cities, the development of complex political institutions, and the cumulative cultural achievements that most accounts of human history treat as the defining markers of advancement. If the DeSilva interpretation is correct, the average human brain has been getting smaller throughout the entire period of recorded civilisation, and the cause may be the very social and technological developments that civilisation is usually understood to consist of.
The fourth, on the strongest current reading of nearly ninety years of peer-reviewed evidence, is that the human brain may be getting smaller because we no longer need it to do quite as much as it once had to.
The peer-reviewed scientific literature has not yet established whether this is true.
It is, however, on the strongest available evidence, not impossible.
