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Article

The Evolution of Brain and Body Size in Genus Homo

by
Tesla A. Monson
*,
Andrew P. Weitz
and
Marianne F. Brasil
Department of Anthropology, Western Washington University, 516 High Street, Bellingham, WA 98225, USA
*
Author to whom correspondence should be addressed.
Humans 2026, 6(2), 12; https://doi.org/10.3390/humans6020012
Submission received: 15 November 2025 / Revised: 30 January 2026 / Accepted: 18 March 2026 / Published: 7 April 2026
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Abstract

Humans, and most other late Homo species, are characterized by large brains and bodies. However, the discovery of two small-brained Homo species—H. floresiensis and Homo naledi—has cast doubts on large brain size as a defining feature of our genus. We reevaluated brain and body size scaling using data for 225 extant primates and 16 fossil hominid taxa, including one of the most diminutive species in genus Homo, H. floresiensis. Brain and body size are tightly correlated in genus Homo, varying along a positively allometric slope (R2 = 0.84, F(1,5) = 33, p < 0.01) that is significantly different from the slope characterizing extant primates (R2 = 0.94, F(1,222) = 3294, p < 0.001). Both small-bodied Homo floresiensis and Homo naledi have endocranial volumes (ECVs) that are consistent with their body size given the scaling relationship that characterizes genus Homo. Paired ECV and body mass estimates demonstrate considerable overlap of brain:body size proportions across fossil hominid taxa. Earlier hominids, Ardipithecus ramidus and Australopithecus anamensis, are characterized by ancestral brain:body size scaling; we discuss the hypothesis that a fundamental biological shift ca. 3 Ma altered the trajectory of encephalization—potentially linked to changes in fetal growth and gestation in Pleistocene fossil hominids—and may be directly implicated in the evolution of complex symbolic behavior in our lineage.

1. Introduction

Large brain size has traditionally been cited as a central feature of genus Homo; brain size began increasing early in hominid evolution (de Sousa et al., 2023) and increased throughout the evolution of genus Homo (although not in all taxa, Figure 1; Aiello & Dean, 1990). However, the descriptions of geologically recent small-brained species of genus Homo (e.g., H. floresiensis, H. naledi, and potentially H. luzonensis) have challenged canon in paleoanthropology by demonstrating that increasing absolute brain size is not a defining characteristic of the genus. Likewise, these geologically younger, small-bodied hominids (e.g., Homo floresiensis, Brown et al., 2004, and H. naledi, Berger et al., 2015) have prompted some researchers to reevaluate the importance of absolute brain size for the evolution of complex behaviors like tool use.
Humans 06 00012 g001
Figure 1. Average hominid endocranial volume in cubic centimeters plotted against average geologic age of each taxon. Colors distinguish genera, and shapes distinguish species. Ma is millions of years. Data are presented in Table 1.
Figure 1. Average hominid endocranial volume in cubic centimeters plotted against average geologic age of each taxon. Colors distinguish genera, and shapes distinguish species. Ma is millions of years. Data are presented in Table 1.
Humans 06 00012 g001
Table 1. Average geologic age, endocranial volume, and body mass for the hominid taxa included in this study.
Table 1. Average geologic age, endocranial volume, and body mass for the hominid taxa included in this study.
TaxaAvg 1 ECV (cm3)SE ECV (cm3)Avg Body Mass (g)SE Body Mass (g)Avg Age (Ma)n = (ECV; Body Mass)
Ardipithecus ramidus300.00NA46,466.67733.334.40(1; 4)
Australopithecus aethiopicus410.00NA37,666.00NA2.50(1; 1)
Australopithecus afarensis457.2538.7543,718.332041.853.23(4; 38)
Australopithecus africanus464.8621.1536,124.291444.882.48(7; 41)
Australopithecus anamensis367.50NA56,983.334169.174.09(1; 4)
Australopithecus boisei505.718.4852,425.003936.371.69(7; 9)
Australopithecus garhi450.00NA36,500.00NA2.50(1; 1)
Australopithecus robustus530.00NA35,326.392007.571.75(1; 21)
Australopithecus sediba420.00NA32,372.224964.341.98(1; 6)
Homo erectus948.6932.5761,574.851949.650.96(29; 38)
Homo floresiensis417.00NA29,750.001650.000.39(1; 3)
Homo habilis609.3325.6236,803.172422.601.82(6; 8)
Homo heidelbergensis1282.3339.8760,513.00NA0.29(9; 1)
Homo naledi545.0042.5242,421.38983.260.27(3; 39)
Homo neanderthalensis1409.8138.6587,570.005254.930.08(21; 6)
Homo sapiens (fossil)1469.4414.8265,398.335093.810.04(71; 9)
1 Definitions: Avg is average, ECV is endocranial volume, SE is standard error, cm3 is cubic centimeters, g is grams, Ma is millions of years. Sample size (n) is for the individual estimates (ECV; body mass); some fossils have more than one estimate—in these cases, the average was included in statistical analyses. See Section 2 for details. All raw values are presented in Supplementary Materials.
Beyond measurements of absolute brain size, another common method of assessing brain size in mammals is to quantify the size of the brain relative to the body, termed ‘encephalization’. Using this definition, larger brains relative to body size are more encephalized, and smaller brains relative to body size are less encephalized. Patterns of encephalization are phylogenetically conserved, and brain:body size relationships are tightly correlated across mammals, including primates (Henneberg, 1998; Martin, 1981, 1982; Martin & Harvey, 1985; Sansalone et al., 2020; Smaers et al., 2021). It has been theorized that different mammalian lineages have followed different trajectories of brain:body size evolution, and that brain structure may play an important role in cognitive behaviors (Tartarelli & Bisconti, 2006).
It is well documented that primates are more encephalized than the general mammalian pattern, as are many cetaceans, elephants, and canids. Humans are distinct in being highly encephalized relative to other extant mammals, meaning that humans have a larger brain than would be predicted from body size alone (Striedter, 2005). Although there have been several different scaling factors proposed to explain interspecific allometry of brain:body size, all methods consistently demonstrate a larger brain relative to body size in humans compared to other extant mammals (Aiello & Dean, 1990; Boddy et al., 2012).
This pattern of relatively larger brains extends into the fossil record: a growing body of work from neuroanatomy and evolutionary biology—using both linear allometric analyses as well as comparative phylogenetic methods—supports Pilbeam and Gould’s (1974) early hypothesis that brain:body size scaling is derived in Australopithecus and Homo relative to other primates (Buckner & Krienen, 2013; Du et al., 2018; Gingerich, 2022; Miller et al., 2019; T. A. Püschel et al., 2024). This derived brain:body size scaling results in the greater encephalization of these fossil hominids relative to extant primates.
However, brain:body size scaling in a sample that includes both the largest and smallest species of genus Homo has not been comparatively assessed to date. Recent fossil discoveries underscore the range of hominid brain and body sizes that evolved during the Pleistocene. Homo floresiensis, known from approximately 60,000 to 200,000 years ago (and potentially as early as 700,000 years ago; van den Bergh et al., 2016), and found exclusively in Indonesia on the island of Flores (Brown et al., 2004), had a small brain and is associated with technology use (e.g., stone tools, butchery, and potentially use of fire; Moore & Brumm, 2009; Morwood et al., 2005). In contrast to some other small-bodied species like H. naledi, H. floresiensis possesses a suite of clearly Homo-like anatomical traits but is characterized by very small brain and body size (Brown et al., 2004). H. floresiensis is so Homo-like in its anatomy that the announcement of this species was followed by nearly a decade of debate about whether the individuals assigned to H. floresiensis represent early pathological humans or a derived species that underwent island nanism (Aiello, 2010; Argue et al., 2006; Falk et al., 2005; Kubo et al., 2013).
For some, as new hominid fossils are discovered and described (DeSilva & Lesnik, 2008; Grabowski et al., 2016; Haile-Selassie et al., 2016), there is increasing reason to question whether encephalization as traditionally defined should be used as a hallmark of our genus. On the other hand, many researchers continue to claim that Homo sapiens is more encephalized than other hominids, and that this distinctive encephalization is critical for explaining our evolutionary ‘success’ (i.e., population expansion, technological innovation, and ecological dominance; Lefebvre, 2012; Potts, 1998; Rightmire, 2004).
To our knowledge, none of the previous studies on brain:body size scaling considered the smallest known species of genus Homo, Homo floresiensis and Homo naledi, with endocranial volumes of just 417 cm3 and 545 cm3 respectively, more comparable to chimpanzees than humans (note that Homo luzonensis is also estimated to be among the smallest known species, but no cranial remains have yet been recovered; Berger et al., 2015; Brown et al., 2004; Kubo et al., 2013). This is in part because they were published as new species relatively recently, but perhaps also because their anatomy is so different from what has traditionally been considered typical of genus Homo. Whether or not small-bodied species of genus Homo have the same brain:body size scaling as other species in the genus remains undetermined. To address this gap, we collated data on endocranial volume (cm3; ECV) and estimated body mass (g) for n = 16 fossil hominid taxa spanning the last five million years (Table 1), as well as 225 extant primate species (Table S1). Using these data, we tested the following hypotheses:
H1. 
Brain and body size are correlated in fossil Homo (even when considering the most diminutive taxa).
H2. 
Brain:body size scaling of genus Homo is significantly different from brain:body size scaling in extant primates.

2. Materials and Methods

2.1. Fossil Materials

We collated data for fossil hominid taxa from the literature to assess the relationship between endocranial volume and body mass in primates (Table 1). We acknowledge that there are strong disagreements about hominid taxonomic designations (e.g., H. heidelbergensis, H. ergaster/H. erectus, and early Homo), and that certain taxonomic names are either not represented in this analysis or were lumped together with other hominid taxa when averages were originally calculated. Our taxonomic designations are based on the original data sources to maintain consistency and do not necessarily reflect our taxonomic views. We use these taxonomic designations to facilitate communication and maintain consistency, not because we necessarily agree with the biological implications of these labels.
The few taxonomic differences from source publications are as follows: H. ergaster is subsumed in H. erectus (sensu lato); Dali 1 and Jinnuishan 1 are removed from analyses due to debate about taxonomic affinities (Liu et al., 2022; Ni et al., 2021); and, the taxonomy of Skhul and Jebel Irhoud is updated to fossil H. sapiens from H. neanderthalensis (Hublin et al., 2017; Shea, 2003). These taxonomic decisions have been made using what we identify as best practices in recent scientific literature. That said, there is not unified consensus about hominid taxonomy or phylogeny at this time, and we recognize that some readers may inevitably disagree with the classifications used here.
In total, we collected n = 229 body mass estimates and n = 164 ECV estimates for fossil hominids, and we generated averages of ECV and body mass for each taxon. This yielded a sample size of n = 16 fossil hominid taxa. We report sample sizes, standard errors, averages, and ranges for ECV and body mass in Table 1. The raw data and references for all original sources that were used to compile our comparative hominid dataset are reported in Supplementary Materials.
We curated the majority of ECV data from Holloway et al. (2004). Some taxa were unavailable in Holloway et al. (2004)—data for Ar. ramidus, Au. aethiopicus, Au. anamensis, Au. sediba, H. floresiensis, H. heidelbergensis (fossils from Sima de los Huesos), and H. naledi were taken from alternate publications (see Supplementary Materials for details). We did not include any specimens that have been reported to be juvenile (e.g., KNM-WT 15000), nor any specimens that have been disputed in publication due to significant distortion (e.g., KNM-ER 1470). The exact specimens from which ECVs were derived are reported in Supplementary Materials.
Body mass estimates can differ based on regression models and methods used. Because of the range of methods used to estimate body mass from fossils, we collated body mass data from several different publications and authors, derived from different methods and different elements, to generate the most inclusive average possible. When multiple estimates were available for the same fossil specimen, but estimated by different researchers using different methods, we averaged these estimates per specimen and used the average value in our analyses. We report the range of body mass estimates in Table 1. Associated ECV and body mass estimates—derived from cranial elements of the same fossil individual—are available for a subset of our data (n = 32; Table S2). Because these paired estimates come from single individuals, in contrast to data averaged across unassociated cranial and post-cranial elements, this intra-populational analysis offers an alternate, and potentially more taxonomically reliable, assessment of allometric variation. Results from the assessment of individual variation are presented in Section 3 below.

2.2. Extant Materials

We used the most comprehensive dataset available from the literature (Powell et al., 2017) to visualize the interspecific allometric relationship between endocranial volume and body mass across extant primates (n = 224 non-human primate species). Endocranial volume is preferred over brain mass because of the variation in preservation techniques that are used to preserve brains and estimate mass, and because ECV can be estimated from fossil remains (Isler et al., 2008). We included average values for modern Homo sapiens (Grabowski et al., 2016; Walpole et al., 2012), yielding a total of 225 extant species (Table S1).

2.3. Statistical Analyses

We applied linear regression models to natural log-transformed values of the extant and fossil datasets, as well as to the paired subset data. We also ran a type II ANOVA using the car package (Fox & Weisberg, 2019) to test for significant differences between allometric slopes of ECV and body mass between (a) genus Homo and extant primates, (b) genus Homo and other fossil hominids, and (c) all fossil hominids and extant primates.
All statistical analyses were performed in R version 4.3.0 (21 April 2023) (R Core Team, 2023) using R Studio software version 2023.03.1 + 446 (Posit Team, 2023). Data visualizations were made using the ggplot2 package (Wickham, 2016), and calculations of averages and log-transformed values were made using the dplyr package (Wickham et al., 2023). All data used to produce the results presented here are available in this manuscript and associated Supplementary Materials.
As is almost always the case in paleontological studies, it is important to note that our fossil samples are not sex-balanced, and estimating sex is inherently difficult for fossil hominids because of the low levels of sexual dimorphism relative to many other primates. A skewed sex sample may influence these results and contribute to greater similarities or differences than would be seen with a sex-balanced sample, and sexual dimorphism may lead to sex-based differences in anatomy. Additionally, many of these taxa are represented by very few individuals, and sometimes only a single fossil. It is impossible to know whether these individuals represent average brain and body size for their respective taxa, or whether they are outliers (such as AL-288-1, Lucy, who remains the shortest-statured Au. afarensis discovered to date). Ecogeography may also impact changes in brain and body size, resulting in patterns of global variation (e.g., Ruff et al., 1997; Stibel, 2025). Only increased sampling through field recovery efforts will fully resolve these uncertainties; at present, as with any fossil-based study, we are limited to drawing conclusions from the available data.

3. Results

3.1. Brain:Body Size Scaling in Fossil Hominids

Brain and body size are significantly correlated in genus Homo (n = 7, R2 = 0.84, F(1,5) = 33, p < 0.01) across a sample that includes both modern humans and one of the smallest known fossil species, Homo floresiensis. The slope of the brain:body size relationship in genus Homo is positively allometric (y = 1.27x − 7.00), where brain size (ECV) increases at a greater rate than body size across the genus (Figure 2).
Brain and body size are also significantly correlated across non-human extant primates (n = 224, R2 = 0.94, F(1,222) = 3294, p < 0.001), with markedly few deviations from the negatively allometric slope that characterizes the Order (y = 0.8x − 2.62). Across extant primates, larger ECV is correlated with a relatively larger increase in body size (in contrast to fossil hominids, where larger ECV is correlated with relatively smaller changes in body size). The slope of the line that characterizes fossil Homo is significantly different from the slope of the line that characterizes non-human extant primates (ANOVA, F(1,228) = 19.9, p < 0.001). The slope of the line that characterizes fossil Homo is also significantly steeper than the regression line calculated for Australopithecus alone (F(1,12) = 9.94, p < 0.01). This aligns with previous work that has documented positively allometric scaling of brain and body size in hominids that is significantly different from other primates (Martin, 1982; Martin & Harvey, 1985; Miller et al., 2019; Pilbeam & Gould, 1974; Smaers et al., 2021).
Brain and body size are significantly correlated across fossil hominids broadly (n = 16, R2 = 0.48, F(1,14) = 14.86, p < 0.01; y = 1.19x − 6.37). The slope of the line that characterizes fossil hominids aligns with previously reported values (e.g., 1.10, Smaers et al., 2021) and is significantly steeper than the slope of the line that characterizes brain:body size scaling in non-human extant primates (ANOVA, F(1,237) = 25.5, p < 0.001). Brain and body size are less strongly associated when including all fossil hominids due to the relatively small endocranial volumes and large body masses estimated for Australopithecus anamensis, Au. boisei, and Ardipithecus ramidus. The brain and body size relationships of these three taxa are more similar to extant non-human primates than to other fossil hominids in this study. Using all definitions of encephalization (relative brain size), Ardipithecus, and early Australopithecus are less encephalized than later hominids, including small-bodied species of Homo.

3.2. Specimen-Based Assessment of Morphological Overlap

In contrast to previous studies, our work also includes an analysis of individual, specimen-based variation in fossils with both ECV and body mass estimates (Figure 3). Body mass can be estimated from measurements of the cranium—including measurements of the orbits—independent of endocranial volume (Kappelman, 1996); thus, both ECV and body mass can be approximated from a single fossil cranium, eliminating some of the taxonomic uncertainty introduced by using data from unaffiliated cranial and postcranial specimens (Figure 3). This analysis is aimed at assessing morphological overlap rather than quantifying differences.
Fossil individual LB1 (H. floresiensis) plots closely in brain:body size morphospace with individuals assigned to several Australopithecus taxa, including Au. sediba, Au. africanus, and Au. afarensis, as well as H. habilis. There is also considerable overlap between fossil H. sapiens and H. neanderthalensis (plotted as paired brain and body size estimates from the same individuals; Figure 3). Using body size estimations derived from the orbits, some individual humans fall outside the range of other fossil taxa, with smaller body size than would be expected for their brain size, but brain:body size proportions for humans are broadly shared with H. neanderthalensis and are similar to observed values for H. heidelbergensis and H. erectus.
Two of the three individuals representing the robust hominid Au. boisei fall well below the regression line that characterizes other hominid taxa (Figure 3). These two individuals have estimated body masses (derived from orbital measurements; Kappelman, 1996) that are larger than would be expected for their brain size, whereas the third individual assigned to Au. boisei falls on the regression line and aligns closely with H. habilis.

4. Discussion

4.1. Brain and Body Size Are Correlated in Genus Homo

Brain and body size are correlated across extant primates as well as across the broad range of body types that characterize fossil hominids, from the diminutive Homo floresiensis to the largest Homo neanderthalensis. Even including the small-brained Homo floresiensis and Homo naledi, we find that brain and body size scale along a positively allometric trajectory in genus Homo, with a slope of 1.27, comparable to previously reported slopes for fossil hominids (1.10; Smaers et al., 2021). Brain:body size scaling patterns are also significantly different between genus Homo and earlier hominids, although there is considerable overlap between the smaller species of both Homo and Australopithecus.
Taxa from similar geologic time periods fall at both ends of the Homo scaling trajectory (e.g., H. floresiensis and H. neanderthalensis), evidence that variation in brain size is likely tightly linked to changes in body size (Henneberg, 1998; Venditti et al., 2024). There is also a considerable degree of body-size variation within the human species (i.e., Homo sapiens), and many individuals sampled here have a smaller body mass than expected based on their brain size, a typical phenotype of people from populations that have been under selection for short stature (Bozzola et al., 2009).
Our work expands beyond previous studies in several ways. Crucially, this is one of the most comprehensive analyses to date, and one of the first to include data for the more recently discovered small-bodied taxa, H. floresiensis and H. naledi. These small-bodied taxa do not have brain:body size proportions that distinguish them from larger-bodied taxa in genus Homo—they share the same allometric scaling relationship as other Pleistocene hominids, with the expected ECV for their body size. This finding adds to our understanding of evolutionary trajectories under the selective pressures of insular nanism (particularly in the case of H. floresiensis; Kaifu et al., 2024; Monson et al., 2025).

4.2. Evolution of Encephalization

The human lineage is characterized by greater encephalization than other primates. Given these brain and body size data for primates, including the apes, parsimony supports that this encephalization is derived in fossil Homo, and that the last common ancestor of humans and chimps likely had a brain:body size scaling relationship similar to other extant apes. The earliest hominids, Ardipithecus and Australopithecus anamensis were characterized by brain:body size proportions nearly identical to the scaling trajectory seen in non-human extant primates, including the other apes, and distinct from the proportions that characterized later hominids. Fossil data provide evidence for a fundamental evolutionary shift in allometric scaling of brain:body size relationships at a time between the presence of early hominids (Ardipithecus and Au. anamensis) and later hominids, with Au. afarensis potentially representing a transitional form.
Ardipithecus ramidus and Au. anamensis are securely dated to older than 3.8 Ma and are known from eastern Africa (Haile-Selassie et al., 2019). Fossilized remains attributed to Au. afarensis have also been found exclusively in eastern Africa, but this taxon is dated to approximately 3.8 to 3 Ma (White et al., 2009). Au. afarensis has been interpreted as an intermediate taxon between Ardipithecus/Au. anamensis and later taxa, and likely part of an evolving lineage of early hominids that preceded the evolution of genus Homo (Kimbel & Delezene, 2009). The encephalization inflection point proposed in this study aligns with previous work using both phylogenetic and non-phylogenetic methods (Buckner & Krienen, 2013; Du et al., 2018; Gingerich, 2022; Miller et al., 2019; Pilbeam & Gould, 1974; T. A. Püschel et al., 2024; Venditti et al., 2024), but future studies that incorporate additional fossils and evolutionary modeling will be required to more securely estimate the timing of this shift in encephalization trajectory.
The earliest hominids with brain:body size scaling proportions that align with genus Homo are Australopithecus aethiopicus, Au. africanus and Au. garhi. Based on key differences in brain:body size scaling between the earliest hominids and later taxa (later Australopithecus and genus Homo), we hypothesize that a critical change in encephalization evolved approximately 3 Ma in hominids, resulting in a derived allometric scaling relationship of brain and body size that characterizes all later hominids. We propose Australopithecus aethiopicus, Au. africanus and Au. garhi as candidate taxa for investigating the evolution of key changes in hominid encephalization. The recovery of additional fossil evidence will be critical to these future investigations, particularly in the case of Australopithecus garhi, for which very few fossils have been recovered so far.

4.3. Brain Anatomy and Cognition

The change in allometric trajectory seen in fossil hominids suggests that some genetic or developmental factor(s) may act on either brain size or body size at least semi-independently (Boddy et al., 2012; Grabowski et al., 2016; Montgomery et al., 2010; Montgomery & Mundy, 2012). Some researchers have pointed to particular genes, like ASPM, and those in the DUF1220-encoding gene family, that may have been under selection and contributed to brain size increases during early hominid evolution (Dumas et al., 2012; Gilbert et al., 2005; Grabowski, 2016; Zhang, 2003); the relationship between expression of candidate genes involved in brain:body size proportions and hypotheses of paedomorphy (i.e., the retention of juvenile characteristics throughout life) in human evolution remain under investigation (e.g., Gómez-Robles et al., 2024; Vinicius, 2005).
Small-brained hominids like H. floresiensis also show dramatic cortical reorganization compared to non-human apes (Falk et al., 2005), supporting Holloway’s (1983) hypothesis that reorganization of the brain may not be limited by absolute size. This should not be particularly surprising given the wide range of brain sizes in living humans without any observable effect on cognitive ability (Grabowski et al., 2015; Schoenemann et al., 2000). Montgomery et al. (2010) previously argued that relative decreases in the brain size of Homo floresiensis follow the expectations of the Island Rule (Foster, 1964). They demonstrated that decreases in absolute body mass are actually associated with increases in relative brain size, hinting at an allometric relationship between these traits even in smaller hominids (Montgomery et al., 2010). Our results support Montgomery et al.’s (2010) findings, and we echo them in saying that “we should perhaps not be surprised by the evolution of a small brained, small bodied hominin” (Montgomery et al., 2010, p. 14).
Because humans do not have the largest mammalian brain in an absolute sense, it is frequently assumed that it is our relatively large brains (as compared to our bodies) that make the human species more cognitively capable than other animals. Encephalization is often credited for the evolution of human distinctiveness, including our global dispersal, complex culture, and aptitude for technological innovation. The ubiquitous statement that humans are more encephalized than other Pleistocene hominids remains commonplace in scientific papers (Alba, 2010; Antón et al., 2014; de Sousa et al., 2023; Garvin et al., 2017; Hublin et al., 2015; Profico et al., 2023; H. P. Püschel et al., 2021; Will et al., 2021), as well as in textbooks and popular media, shaping public understanding of human evolution and human distinctiveness by influencing conclusions about behavior and cognitive abilities in later Homo, and especially, Homo sapiens and Neanderthals. This paradigm also weighs powerfully on theoretical frameworks for the evolution of pelvic anatomy and human pregnancy, which rely on encephalization as a selective pressure (Churchill & Vansickle, 2017; Frémondière et al., 2022; Xu et al., 2025). Larger relative brain size has also been linked to a myriad of behavioral, anatomical, and cultural traits, including diet, mating systems, range sizes and activity periods, life history, and group social dynamics (Barton & Capellini, 2011; Clutton-Brock & Harvey, 1980; DeCasien et al., 2017; Dunbar & Shultz, 2017; Heldstab et al., 2016; Schillaci, 2006; Street et al., 2017).
Given the allometric relationship between brain and body size in genus Homo, we suggest that large-bodied hominids should not automatically be considered more capable of complex behaviors than their smaller-bodied ancestors and sister taxa. For example, we have substantial evidence that advanced technologies (beyond the capabilities of living non-human apes) can be produced by small-brained hominids (e.g., H. floresiensis; Falk et al., 2005). Fossil hominids have been associated with complex tool use for at least 2.5 to 3.3 Ma (Harmand et al., 2015; Semaw et al., 1997). The fact that these technologies increase in complexity over time (albeit non-linearly) has typically been attributed to increases in endocranial volume (Antón et al., 2014). Because symbolic art has only been definitively associated with species in genus Homo (Brumm et al., 2021; Tattersall & Schwartz, 2009), and because genus Homo is taxonomically defined in part by a large endocranial volume (Mayr, 1963), it has long been assumed that a large brain is necessary for complex symbolic behaviors.
Together, the data presented here support a distinct pattern of hominid encephalization—reflected in significant correlations between brain and body size—and demonstrate a brain:body size scaling relationship that is distinct from extant primates. Instead of viewing encephalization as a key trait that distinguishes Homo sapiens from other hominids, we should instead consider encephalization as a key trait that distinguishes almost all hominids from other primates. Because all hominids after 3 Ma are encephalized relative to extant primates, perhaps we should also not be surprised to find these extinct taxa associated with complex tool manufacture and use (Semaw et al., 1997), or complex symbolic behavior such as production of art. Cultural innovations in the Pleistocene—including development of language, tools, symbolism, and teaching and learning strategies—may have played more of a role in presumed increases in ‘intelligence’ than changes to brain and body size (e.g., Stibel, 2025; Tattersall, 2023; Tylén et al., 2020).
In contrast, Ar. ramidus and Au. anamensis retain ancestral brain:body size scaling and are not associated with tool manufacture and use. Our results align with archeological evidence and support that at least some complex behaviors likely evolved early in the hominid lineage, after our divergence from the ancestral primate brain:body size relationship and the evolution of increased encephalization. If encephalization does indeed play a key role in complex behaviors, we gently suggest that all hominids after 3 Ma be considered potentially capable of these behaviors. Recovery of new paleontological and archeological evidence will be essential for testing this hypothesis.

4.4. Brain Size Diversification and Cognition

Ecological pressures shaped hominid diversification throughout the Plio-Pleistocene, and we continue to recover fossils that span broad geographic areas and a range of body sizes (Figure 4). With this diversification in body size and shape, we see concomitant changes in brain size and shape (DeSilva et al., 2023; Stibel, 2021, 2023, 2025). Brain size has been brought further into focus recently with the widespread public and academic interest in the ‘cognitive capabilities’ of small-brained hominids like Homo naledi (Martinón-Torres et al., 2024). Within paleoanthropology, an inordinate amount of attention has been placed on the distinctiveness of the human (Homo sapiens) brain:body size relationship. However, we cannot forget how assumptions of associations between brain size and intelligence played principally in the development of both U.S. anthropology and eugenics (Schroeder et al., 2025). Early studies of brain size were meant to reinforce active power structures through racialization of biology; now, fossil hominids are considered cognitively inferior to humans based on brain size alone (Schroeder et al., 2025). Given this problematic history, and the dramatic expansion of a fossil record that highlights the diversity of hominid biology over the last 5 million years, we advocate for a large-scale reevaluation of encephalization and brain size in hominid evolution that more thoroughly considers impacts of allometric scaling and body size, as well as reproductive life history.
The evolution of encephalization in hominids may have been facilitated by different metabolic strategies, including changes in gut morphology and processing, as well as maternal investment, gestational length, and prenatal growth rates (Barrickman & Lin, 2010; Barton & Capellini, 2011; Gómez-Robles et al., 2024; González-Forero, 2023; Monson et al., 2022). Because adult and neonatal brain size are tightly correlated in primates, including in humans (DeSilva & Lesnik, 2008), we hypothesize that the positively allometric relationship between brain and body size proportions in hominids is related to critical changes in gestation and reproductive life history. Previous research has demonstrated that prenatal growth rates started increasing early in hominid evolution (Monson et al., 2022), which may provide a mechanistic avenue for derived brain:body size proportions in our lineage. Based on estimates of hominid neonatal brain and body mass, it is likely that Australopithecus was already giving birth to relatively large infants compared to other non-human apes like chimpanzees (DeSilva, 2011). These relatively large infants may have required increased metabolic and physiological support both pre- and postnatally, potentially driving selection for alloparental care in early hominids (DeSilva, 2011). As alloparental care is associated with larger brain size, faster weaning, and shorter interbirth intervals (Heldstab et al., 2019; Mitani & Watts, 1997; Nakahashi et al., 2018; Ross, 2003), changes in reproductive life history likely played a significant role in the evolution of human brain:body size scaling.

5. Conclusions

Diversification of brain size in Plio-Pleistocene hominids follows an allometric scaling relationship that we hypothesize evolved in this family in the Late Pliocene. Homo sapiens, and other taxa in genus Homo, have likely been under selection for increased body and/or brain size, but these traits have co-evolved along a derived allometric scaling trajectory that characterizes variation in the genus. Hominid ECV and body mass increased throughout the Plio-Pleistocene for many taxa, and human brain:body size proportions track with those size changes (Kaifu et al., 2024). Rather than describing humans as more encephalized than other taxa, a more accurate statement would be that humans have larger brains and bodies than earlier hominids—these changes likely reflect increased resource intake and evolutionary changes to reproductive and ontogenetic life history strategies (Antón et al., 2014; Isler & van Schaik, 2012; Powell et al., 2017). Overall, a derived suite of reproductive traits may have been influential in the widespread dispersal and ecological dominance of Homo sapiens and may be tied to the advances in cognitive abilities that are associated with these human characteristics. Continued research on the evolution of reproduction, as well as hominid brain anatomy and complexity (e.g., Balzeau et al., 2012; Bruner & Beaudet, 2023; Kochiyama et al., 2018), promises to refine our understanding of relationships between the brain and complex behaviors.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/humans6020012/s1; Figure S1: Absolute values of extant primate endocranial volume as a function of body mass; Figure S2: Absolute values of endocranial volume as a function of body mass between extant primates and fossil hominids; Table S1: Non-transformed and natural log-transformed values of extant primate endocranial volume and body mass. Data citations: (Grabowski et al., 2016; Powell et al., 2017; Walpole et al., 2012); Table S2: Individual specimens with paired ECV and body mass measurements. Data citations: (Carlson et al., 2011; Grabowski, 2016; Grabowski et al., 2015; Kappelman, 1996; Kubo et al., 2013; Suwa et al., 2009; White et al., 2009); Supplementary Data: Raw data for all fossil included in this study. Data citations: (Arsuaga et al., 1997; Berger et al., 2010; Brown et al., 2004; Carlson et al., 2011; Dominguez-Rodrigo et al., 2013; Garvin et al., 2017; Grabowski, 2016; Grabowski et al., 2015, 2016; Haile-Selassie et al., 2019; Hawks et al., 2017; Holloway et al., 2004; Kappelman, 1996; Kubo et al., 2013; Leakey et al., 1995; Lordkipanidze et al., 2007; McHenry, 1992; Pablos, 2015; Pickering et al., 2025; Pontzer, 2012; Robbins et al., 2021; Robson et al., 2006; Ruff, 2010; Ruff & Burgess, 2015; Ruff & Wood, 2023; Ruff et al., 1997; Steudel-Numbers & Tilkens, 2004; White et al., 2009).

Author Contributions

Conceptualization, T.A.M., A.P.W. and M.F.B.; methodology, T.A.M. and A.P.W.; software, A.P.W.; validation, T.A.M., A.P.W. and M.F.B.; formal analysis, T.A.M. and A.P.W.; investigation, T.A.M. and A.P.W.; resources, T.A.M., A.P.W. and M.F.B.; data curation, T.A.M. and A.P.W.; writing—original draft preparation, T.A.M.; writing—review and editing, T.A.M., A.P.W. and M.F.B.; visualization, A.P.W. and M.F.B.; supervision, T.A.M.; project administration, T.A.M.; funding acquisition, T.A.M. and M.F.B. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Science Foundation (NSF 2235771 to T.A.M. and M.F.B.).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article and the Supplementary Materials. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
ECVEndocranial volume

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Figure 2. Allometric scaling of endocranial volume and body mass in extant primates and Plio-Pleistocene hominids. The black lines represent significant linear regression model fits. The gray shading represents the 95% confidence interval. (a) Endocranial volume and body mass are significantly correlated across fossil taxa of genus Homo (R2 = 0.84, p < 0.01). The slopes of the fossil Homo and extant primate regression lines are significantly different (p < 0.001). (b) Expanded version of (a) that includes all fossil taxa in this study (n = 16). Mean trait values of fossil taxa are plotted with ±1 standard error. Endocranial volume and body mass are significantly correlated across fossil hominids (R2 = 0.48, p < 0.001). The slopes of the extant primate (below) and fossil hominid (above) regression lines are significantly different (p < 0.001). (c) Endocranial volume and body mass are significantly correlated in extant primates (R2 = 0.94, p < 0.001). Absolute values for panels 2b-2c are presented in Figures S1 and S2. Shapes and colors differentiate extant families (see legend for details).
Figure 2. Allometric scaling of endocranial volume and body mass in extant primates and Plio-Pleistocene hominids. The black lines represent significant linear regression model fits. The gray shading represents the 95% confidence interval. (a) Endocranial volume and body mass are significantly correlated across fossil taxa of genus Homo (R2 = 0.84, p < 0.01). The slopes of the fossil Homo and extant primate regression lines are significantly different (p < 0.001). (b) Expanded version of (a) that includes all fossil taxa in this study (n = 16). Mean trait values of fossil taxa are plotted with ±1 standard error. Endocranial volume and body mass are significantly correlated across fossil hominids (R2 = 0.48, p < 0.001). The slopes of the extant primate (below) and fossil hominid (above) regression lines are significantly different (p < 0.001). (c) Endocranial volume and body mass are significantly correlated in extant primates (R2 = 0.94, p < 0.001). Absolute values for panels 2b-2c are presented in Figures S1 and S2. Shapes and colors differentiate extant families (see legend for details).
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Figure 3. Variation in brain:body size proportions across hominids. All data points represent individual fossils (see Supplementary Materials for specimen numbers, data values, and data sources). Colors distinguish genera, and shapes distinguish species. The black line is the linear regression model for all hominids (R2 = 0.48, F(DF) = 42.83, p < 0.01; y = 1.19x − 6.37), and the gray shading is the 95% confidence interval.
Figure 3. Variation in brain:body size proportions across hominids. All data points represent individual fossils (see Supplementary Materials for specimen numbers, data values, and data sources). Colors distinguish genera, and shapes distinguish species. The black line is the linear regression model for all hominids (R2 = 0.48, F(DF) = 42.83, p < 0.01; y = 1.19x − 6.37), and the gray shading is the 95% confidence interval.
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Figure 4. Map with hominid taxa included in this study alongside a timeline of major events in hominid evolutionary history. Taxa are placed loosely according to geographic range; geologic age ranges are approximate. Colors distinguish genera, as well as robust and gracile Australopithecus. Note that some taxa shown here have been pooled in our analyses (see Section 2 for details on taxonomic designations) and others do not have data available. We note that although our H. heidelbergensis sample includes only European fossils, this taxon is presented here as encompassing African and European fossils. We do not agree with this taxonomy; we choose not to treat the African fossils as a separate species in this visualization because of ethical considerations surrounding the most widely used alternative species name that is currently available (see Reed, 2025, for discussion). Major behavioral and environmental changes are indicated in the timeline. Illustrative icons indicate the onset of the Oldowan, Acheulean, Middle Stone Age, and Late Stone Age archeological industries. Additional behavioral changes include obligatory bipedalism, controlled use of fire, and hunting. Leaf icons indicate select environmental factors hypothesized to have influenced hominid evolution (from left to right: ecological shift to increasing proportions of C4 vegetation in Africa; Neanderthals occupying high latitude areas with cold climates; H. floresiensis occupying Flores Island in Indonesia). The brain icon represents the evolution of hominid encephalization as demonstrated in this study.
Figure 4. Map with hominid taxa included in this study alongside a timeline of major events in hominid evolutionary history. Taxa are placed loosely according to geographic range; geologic age ranges are approximate. Colors distinguish genera, as well as robust and gracile Australopithecus. Note that some taxa shown here have been pooled in our analyses (see Section 2 for details on taxonomic designations) and others do not have data available. We note that although our H. heidelbergensis sample includes only European fossils, this taxon is presented here as encompassing African and European fossils. We do not agree with this taxonomy; we choose not to treat the African fossils as a separate species in this visualization because of ethical considerations surrounding the most widely used alternative species name that is currently available (see Reed, 2025, for discussion). Major behavioral and environmental changes are indicated in the timeline. Illustrative icons indicate the onset of the Oldowan, Acheulean, Middle Stone Age, and Late Stone Age archeological industries. Additional behavioral changes include obligatory bipedalism, controlled use of fire, and hunting. Leaf icons indicate select environmental factors hypothesized to have influenced hominid evolution (from left to right: ecological shift to increasing proportions of C4 vegetation in Africa; Neanderthals occupying high latitude areas with cold climates; H. floresiensis occupying Flores Island in Indonesia). The brain icon represents the evolution of hominid encephalization as demonstrated in this study.
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Monson, T.A.; Weitz, A.P.; Brasil, M.F. The Evolution of Brain and Body Size in Genus Homo. Humans 2026, 6, 12. https://doi.org/10.3390/humans6020012

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Monson TA, Weitz AP, Brasil MF. The Evolution of Brain and Body Size in Genus Homo. Humans. 2026; 6(2):12. https://doi.org/10.3390/humans6020012

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Monson, Tesla A., Andrew P. Weitz, and Marianne F. Brasil. 2026. "The Evolution of Brain and Body Size in Genus Homo" Humans 6, no. 2: 12. https://doi.org/10.3390/humans6020012

APA Style

Monson, T. A., Weitz, A. P., & Brasil, M. F. (2026). The Evolution of Brain and Body Size in Genus Homo. Humans, 6(2), 12. https://doi.org/10.3390/humans6020012

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