Sexual dimorphism in hominins

Sexual dimorphism—systematic differences between males and females of a species in body size, shape, or ornamentation—is among the few anatomical traits preserved in the fossil record that can be linked directly to social behavior. In living primates, the magnitude of sexual dimorphism correlates broadly with mating system and the intensity of male-male competition, making it a key variable for reconstructing the social lives of extinct hominins.^{1, 2} The hominin lineage displays a striking evolutionary trajectory: from early species whose body size dimorphism may have rivaled or exceeded that of living gorillas, through a progressive reduction culminating in the comparatively modest differences between modern men and women. Understanding this trajectory illuminates not only the mating strategies and social organization of our ancestors but also the interplay between sexual selection, natural selection, and ecological context that shaped the human body plan.

At the same time, canine tooth dimorphism—a reliable proxy for male weaponry and intrasexual aggression in anthropoid primates—declined to near-human levels remarkably early, perhaps by the time of Ardipithecus ramidus around 4.4 million years ago.⁷ This decoupling of canine reduction from body size reduction has generated decades of debate about what, exactly, dimorphism patterns can tell us about hominin behavior. Competing hypotheses invoke everything from pair bonding and male provisioning to multi-male group defense and female choice, and the small, fragmentary nature of the fossil record ensures that the debate remains far from settled.^{1, 9}

Defining and measuring dimorphism

Sexual dimorphism in the broad sense refers to any phenotypic difference between males and females of a species, encompassing body mass, skeletal proportions, canine tooth size, pelage coloration, and many other traits. In paleoanthropology, however, the term most often refers to two measurable dimensions: body mass dimorphism and canine size dimorphism.¹ Body mass dimorphism is typically expressed as a ratio of average male mass to average female mass. A ratio of 1.0 indicates monomorphism; modern humans average roughly 1.15 (males about 15% heavier), chimpanzees about 1.3, and gorillas approximately 2.0.^{1, 15} Canine dimorphism is measured analogously, often using mesiodistal or buccolingual crown dimensions or canine crown height.

Body mass dimorphism is considered the best single predictor of mating system among primates, though the relationship is probabilistic rather than deterministic.² Species with intense male-male contest competition tend to exhibit high dimorphism because larger males gain disproportionate mating success, driving selection for increased male body size. Canine crown height dimorphism provides the best discrimination between taxa characterized by high levels of male-male competition and those that are not, because enlarged male canines function as weapons in agonistic encounters.^{2, 9}

The relationship between dimorphism and behavior is not, however, a simple one-to-one correspondence. Dimorphism reflects the independent action of selective pressures on males and females, and the same degree of dimorphism can arise through different combinations of changes in each sex.¹⁰ A species might become more dimorphic either because males increase in size or because females decrease, and a reduction in dimorphism might reflect female size increase rather than male size decrease. Plavcan has emphasized that a complete understanding of dimorphism requires tracking the independent trajectories of male and female traits, an approach that becomes extremely difficult when working with fragmentary fossils of uncertain sex.¹⁰

Dimorphism in living great apes

The living great apes provide the comparative framework within which hominin dimorphism must be interpreted. Gorillas (Gorilla gorilla and G. beringei) are the most dimorphic of the extant apes, with males averaging roughly twice the body mass of females. Adult male gorillas commonly weigh 160–180 kg while females average 70–80 kg, producing a dimorphism ratio near 2.0. This extreme dimorphism is associated with a single-male (or age-graded multi-male) harem social system in which dominant silverback males monopolize access to a group of females. Males compete intensely for tenure over female groups, and male canines are substantially larger than those of females.^{1, 2}

Orangutans (Pongo spp.) exhibit similarly high body mass dimorphism, with males roughly twice the mass of females, associated with a dispersed social system in which flanged males compete aggressively for mating access. Chimpanzees (Pan troglodytes) show moderate dimorphism, with a body mass ratio of approximately 1.3, consistent with their multi-male, multi-female social groups in which male-male competition takes the form of coalitionary politics and dominance hierarchies rather than outright physical contests for exclusive mating access. Bonobos (Pan paniscus) have slightly lower dimorphism than chimpanzees, in keeping with their generally lower levels of male aggression.¹

Modern humans present an unusual profile within this comparative landscape. The body mass dimorphism ratio of about 1.15 places Homo sapiens well below chimpanzees, yet modern human societies exhibit a wide range of mating arrangements from social monogamy to extensive polygyny.^{9, 18} This disconnect between relatively low skeletal dimorphism and highly variable mating behavior has prompted some researchers to argue that human dimorphism may be maintained by forces other than direct male-male contest competition, including female mate choice, male-male coalitional competition, or natural selection on females for increased body size related to the demands of gestation and parturition.^{15, 9}

Canine reduction in early hominins

One of the most consistent and earliest-appearing derived features of the hominin lineage is the reduction of canine teeth, particularly in males, which sharply decreases canine sexual dimorphism relative to the condition seen in all living great apes. In chimpanzees, male upper canines are large, projecting, and sexually dimorphic, functioning as display weapons in dominance interactions. In contrast, all known hominins from Ardipithecus onward possess relatively small, non-projecting canines with reduced dimorphism.^{7, 8}

A landmark study by Suwa and colleagues examined canine sexual dimorphism in Ardipithecus ramidus, the 4.4-million-year-old hominin from the Middle Awash region of Ethiopia. Using a probability-based method applied to a sample of over 300 fossil canines spanning six million years, they found that male-to-female canine size ratios in Ar. ramidus averaged 1.06 for upper canines and 1.13 for lower canines—values within the range of variation seen in modern human populations and significantly weaker than in bonobos, the least dimorphic and least aggressive of the living great apes.⁷ This finding implies that the reduction of male canine weaponry occurred very early in hominin evolution, broadly coinciding with the adoption of bipedality, and suggests a fundamental shift in social behavior that minimized direct male-male aggression.

The behavioral interpretation of early canine reduction is debated. Lovejoy argued that reduced canine dimorphism in Ardipithecus signals a shift toward pair bonding and male provisioning, in which males competed not through physical combat but by providing food resources to female partners and their offspring.⁸ On this model, canine reduction reflects the abandonment of intrasexual combat weapons as males adopted a cooperative reproductive strategy. Others have noted, however, that reduced canines could also reflect a dietary shift toward foods processed with the hands or tools, or a change in the nature of agonistic interactions rather than their elimination.^{1, 19} What is clear is that the loss of large, projecting male canines is a hominin synapomorphy—shared across the entire lineage—and occurred millions of years before body size dimorphism declined to modern human levels.

The Australopithecus afarensis debate

No hominin species has generated more controversy over the magnitude of sexual dimorphism than Australopithecus afarensis, the species that includes "Lucy" (AL 288-1) and the "First Family" assemblage at Hadar (AL 333). The disagreement has persisted for over two decades and centers on whether A. afarensis was as dimorphic as gorillas or no more dimorphic than modern humans.

Early assessments by McHenry and others, using body mass estimates derived from joint dimensions, concluded that A. afarensis exhibited strong body size dimorphism, with males substantially larger than females—a pattern consistent with a polygynous social system involving intense male-male competition.¹¹ The very small body of Lucy contrasted starkly with larger specimens from the AL 333 locality, reinforcing the impression of marked size variation. Lockwood's analyses of craniofacial dimorphism in the closely related Australopithecus africanus similarly suggested moderately high dimorphism, with the craniofacial size range exceeding that of chimpanzees or modern humans.¹²

In 2003, Reno and colleagues challenged this consensus with an analysis of femoral head diameter in A. afarensis, optimizing data from both the Lucy skeleton and the AL 333 assemblage. Their simulations, using modern humans, chimpanzees, and gorillas as reference populations, concluded that skeletal size dimorphism in A. afarensis was most similar to that of contemporary Homo sapiens.³ This finding led them to argue that A. afarensis was principally monogamous and was not characterized by substantial sexual bimaturation. Reno and colleagues subsequently expanded their postcranial sample and maintained their position, arguing in 2010 that an enlarged dataset confirmed moderate, human-like dimorphism.²⁰

Plavcan and van Schaik responded sharply, identifying several methodological concerns.^{4, 19} They argued that the AL 333 assemblage may represent a biased sample that over-represents males, inflating apparent dimorphism; that single skeletal dimensions such as femoral head diameter cannot be relied upon to estimate body mass dimorphism accurately; and that temporal variation within the species cannot fully account for the high dimorphism estimates obtained from other studies. They further cautioned that even if A. afarensis showed human-like skeletal dimorphism, this would not necessarily imply monogamy, given the weak relationship between dimorphism and mating system in modern humans themselves.^{4, 19}

The debate took another turn with Gordon's 2025 study, which applied geometric-mean body size estimates derived from multiple postcranial elements—including the humerus, femur, and tibia—and used resampling techniques to compare fossil hominins against modern primates while accounting for the incomplete and uneven nature of real fossil samples. Gordon found that both A. afarensis and A. africanus were significantly more dimorphic than chimpanzees and modern humans, and that A. afarensis in particular was significantly more dimorphic than A. africanus, with males possibly larger relative to females than in any living great ape.⁵ If confirmed by future discoveries, this result would imply that early hominins lived in social systems that were far more hierarchical and competitive than previously assumed.

Body mass dimorphism ratio estimates across hominin and extant primate taxa^{1, 5, 6, 11}

Paranthropus and the robust australopithecines

The robust australopithecines—Paranthropus robustus, P. boisei, and P. aethiopicus—present their own dimorphism patterns. Like other australopithecines, Paranthropus species were sexually dimorphic, with males notably larger than females. Estimates for P. boisei place average male mass at roughly 50 kg and female mass at around 34 kg, yielding a dimorphism ratio of approximately 1.47. For P. robustus, the corresponding estimates are roughly 40 kg for males and 32 kg for females, giving a ratio near 1.25.^{6, 11}

Cranial dimorphism in Paranthropus is especially pronounced. Males possessed more prominent sagittal crests—bony ridges running along the top of the skull that served as attachment sites for the powerful temporalis muscles used in mastication. These crests are less developed or absent in females, creating a striking cranial dimorphism that goes beyond simple body size differences. Whether this cranial dimorphism reflects sexual selection (male display or combat) or ecological niche partitioning between the sexes remains uncertain, though most researchers attribute it primarily to allometric scaling: larger male bodies required larger jaw muscles and correspondingly larger attachment areas.¹¹

The overall pattern in Paranthropus resembles that of the gracile australopithecines in exhibiting moderate to high body mass dimorphism combined with reduced canine dimorphism. This combination—consistent across the entire australopithecine grade—suggests that whatever social and behavioral changes eliminated the need for large male canine weapons occurred before the australopithecine radiation, while the selective pressures maintaining body size dimorphism persisted through the Plio-Pleistocene.^{1, 9}

Dimorphism in Homo erectus and later Homo

The emergence of Homo erectus around 1.9 million years ago marked a significant shift in hominin body size, with average body mass increasing substantially over earlier hominins. McHenry documented a rapid increase to essentially modern body size between 2.0 and 1.7 Ma, coinciding with the appearance of H. erectus in East Africa.¹¹ The question of whether this increase was accompanied by a reduction in sexual dimorphism has proven difficult to resolve.

The H. erectus fossil record provides clear evidence of a large range of skeletal size variation, at least equivalent to that observed in living human populations, but it does not provide conclusive evidence that males were systematically larger than females to a substantially greater extent than they are today.¹³ Plavcan's comprehensive review of body size variation in early Homo concluded that general levels of dimorphism have likely remained more or less the same for most of the evolution of Homo—that is, for most of the last two million years. However, he cautioned that the sample sizes are small, sex assignation is uncertain, and there are likely multiple taxa conflated in the H. habilis/rudolfensis sample, meaning that conclusions could change with the addition of a single specimen.¹³

Footprint evidence has offered an alternative window into dimorphism in H. erectus. Villmoare, Hatala, and Jungers analyzed 97 hominin footprints from 1.5-million-year-old deposits near Ileret, Kenya, produced by at least 20 different individuals. Their results indicated that East African H. erectus was more dimorphic than modern Homo sapiens, although less so than highly dimorphic apes such as gorillas.¹⁴ Footprint assemblages have the advantage of sampling a population over a very brief time span, avoiding the conflation of temporal variation with sexual variation that plagues skeletal assemblages accumulated over thousands or millions of years. However, the relationship between foot size and body mass introduces its own uncertainties.

By the Middle Pleistocene, Homo populations appear to have achieved broadly modern levels of dimorphism. Fossils from sites such as Sima de los Huesos in Spain, attributed to the Homo heidelbergensis lineage, show cranial and dental dimorphism within the range of modern humans. The Neanderthal (Homo neanderthalensis) fossil record similarly suggests dimorphism comparable to or slightly above that of modern humans, though sample sizes remain limiting.⁶ Overall, the available evidence is consistent with a model in which the transition from australopithecine-grade dimorphism to modern human levels occurred gradually across the genus Homo, though the precise timing and rate of change remain poorly constrained.

Dimorphism, mating systems, and social organization

The relationship between sexual dimorphism and mating system has been central to primate socioecology since the comparative work of Clutton-Brock and others in the 1970s and 1980s. Among nonhuman anthropoid primates, male agonistic contest competition is the only factor that has consistently received support from comparative analyses in explaining why males are larger and have larger canine teeth than females.^{1, 2} Species in which males compete aggressively for exclusive access to groups of females (polygynous species) tend to show the highest dimorphism, while species with reduced male competition (pair-bonded or multi-male/multi-female species) tend to be less dimorphic.

Applying this framework to the hominin fossil record has proven more difficult than early enthusiasm suggested. Plavcan and van Schaik argued in 2003 that advances in understanding the behavioral and ecological correlates of dimorphism in living primates have not improved the ability to reconstruct social systems in extinct species on the basis of dimorphism alone, beyond the general inference that high dimorphism is associated with polygyny or intense male-male competition.¹⁹ The unique hominin pattern—high body size dimorphism combined with low canine dimorphism—has no exact analogue among living primates, which makes behavioral inference particularly hazardous.

Lovejoy's provisioning model, developed initially in the 1980s and elaborated in his 2009 analysis of Ardipithecus ramidus, represents the most ambitious attempt to link hominin dimorphism patterns to a specific social system.⁸ Lovejoy argued that reduced canine dimorphism and the adoption of bipedalism in Ardipithecus signal a shift toward monogamous pair bonding, in which males provisioned females and their offspring by carrying food bipedally over long distances. On this model, the selective advantage shifted from male combat success (rewarding large canines and large body size) to male provisioning ability (rewarding bipedality and reduced aggression). The reduction of canine weaponry thus reflects the replacement of intrasexual combat with a cooperative reproductive strategy mediated by female choice for reliable provisioners.

Critics of the provisioning model note several problems. High body size dimorphism in australopithecines, if confirmed by studies such as Gordon's, is inconsistent with a monogamous system, which typically produces low dimorphism.^{5, 19} The model also relies on assumptions about concealed ovulation and exclusive pair bonds that may not apply to early hominins, and the ethnographic record shows that male provisioning is highly variable across human foraging societies. Chapais proposed an alternative evolutionary sequence in which hominins transitioned from promiscuity (as in chimpanzees) through polygyny (as in gorillas) to the pair bonding characteristic of modern human family structures, with each stage characterized by different selective pressures on dimorphism.¹⁷

Modern human sexual dimorphism complicates these models further. Despite a body mass ratio of only about 1.15, human societies range from predominantly monogamous to highly polygynous, and cross-cultural analyses have found that stature dimorphism does not correlate significantly with marriage system across populations.¹⁸ Gaulin and Boster, in their analysis of 155 societies, found that the more males and females measured in a given population, the more its stature dimorphism converged on a ratio of 1.073, suggesting that much of the apparent cross-cultural variation in dimorphism reflects sampling artifact rather than genuine differences in sexual selection intensity.¹⁸ These findings suggest that the modern human level of dimorphism may be maintained by a combination of sexual selection on males and natural selection on females rather than by a single behavioral mechanism.

Rensch's rule and allometric patterns

Rensch's rule, formulated by the evolutionary biologist Bernhard Rensch in 1950, describes an allometric pattern observed across many animal lineages: in taxa where males are the larger sex, sexual size dimorphism increases with increasing average body size, while in taxa where females are larger, dimorphism decreases with increasing body size. The rule has been documented in a wide range of taxa, including mites, lizards, birds, and primates.¹

Whether Rensch's rule holds within the order Primates has been debated. Some phylogenetic analyses have found a positive allometric relationship between male and female body size consistent with the rule, while others have found no significant relationship. Plavcan and van Schaik found that body weight dimorphism increases with body size in anthropoid primates, consistent with Rensch's rule, but other studies using different phylogenetic methods have reached conflicting conclusions.² The proposed mechanism is that in larger species, male-male competition intensifies because larger body size confers greater benefits in physical contests, and because larger species tend to live at lower population densities with more dispersed female groups, favoring male monopolization.

For the hominin lineage specifically, Rensch's rule predicts that as average body size increased during the transition from australopithecines to Homo, dimorphism should have increased rather than decreased—the opposite of what the fossil record appears to show. This mismatch suggests that the reduction of dimorphism in Homo was driven by behavioral or social changes that overrode the allometric tendency, such as a reduction in the intensity of male-male contest competition, an increase in female body size related to obstetric demands, or both.^{10, 15}

Pelvic dimorphism and obstetric constraints

The human pelvis is the most sexually dimorphic region of the skeleton, and this dimorphism has deep evolutionary roots tied to the demands of bipedalism and parturition. The classic formulation of the "obstetrical dilemma," coined by Sherwood Washburn in 1960, proposed that bipedal locomotion selected for a narrow pelvis to improve gait efficiency, while the increasing brain size of hominin neonates selected for a wider birth canal, creating antagonistic selective pressures that differed between the sexes.¹⁶

The resulting dimorphism is substantial. In modern humans, women have relatively wider pelves, larger pelvic inlets, and more circular birth canals than men, adaptations that facilitate the passage of large-brained neonates. The subpubic angle is markedly wider in females, and the sacrum is shorter and wider. These differences are so consistent that pelvic morphology is the single most reliable skeletal indicator of biological sex in human osteology.¹⁶

Recent research has complicated Washburn's original formulation. Biomechanical and kinematic studies suggest that pelvic width does not substantially compromise bipedal locomotor efficiency, undermining the locomotor constraint side of the dilemma. Grunstra and colleagues argued in 2023 that the birth canal may be constrained not primarily by locomotion but by the need for pelvic floor support of the abdominal viscera and the heavy human fetus during gestation.¹⁶ Nonetheless, the fundamental observation that female pelves are adapted for parturition in ways that male pelves are not remains robust, and this sex-specific selection has likely influenced the evolution of overall body size dimorphism. Lassek and Gaulin argued that female stature likely increased relative to males during human evolution precisely to accommodate the obstetric demands of delivering large-brained neonates through a bipedally-adapted pelvis, partially offsetting what might otherwise have been a greater degree of body size dimorphism.¹⁵

Pelvic dimorphism appears in the fossil record at least by the time of Homo erectus. The Gona pelvis from Ethiopia, dated to approximately 0.9–1.4 Ma and attributed to H. erectus, has a remarkably wide birth canal, suggesting that obstetric constraints were already shaping female pelvic morphology by this time. When pelvic dimorphism first appeared in hominin evolution is difficult to determine because australopithecine pelvic fossils are rare and often fragmentary, but the known specimens from A. afarensis (e.g., AL 288-1) show a pelvis already adapted for bipedalism with features distinct from the male morphotype.

Male competition, female choice, and body composition

Human sexual dimorphism extends well beyond stature and skeletal mass. Lassek and Gaulin drew attention to the substantial but frequently overlooked sex differences in body composition that characterize modern humans.¹⁵ Despite a relatively modest difference in stature (approximately 7%) and total body mass (approximately 16%), males typically have 36% more lean body mass, 65% more total muscle mass, and 72% more arm muscle than females, producing correspondingly large sex differences in upper body strength. These differences are far greater than would be expected from the stature difference alone and strongly suggest a history of sexual selection favoring male fighting ability.

Sex differences in modern human body composition¹⁵

The reverse pattern holds for adipose tissue. Women have approximately 1.6 times the body fat percentage of men and deposit fat preferentially in the gluteofemoral region. Lassek and Gaulin argued that this female-specific fat pattern reflects natural selection for the accumulation of long-chain polyunsaturated fatty acids (particularly DHA) critical for fetal brain development, rather than sexual selection per se.¹⁵ On their view, the relatively small stature difference between human males and females masks a much larger underlying dimorphism in functional anatomy: male bodies are optimized for combat and physical competition, while female bodies are optimized for the energetic demands of gestation, lactation, and provisioning of large-brained offspring.

The role of female choice in shaping hominin dimorphism has received increasing attention. Plavcan noted that the causes of human sexual size dimorphism are uncertain and could involve several non-mutually-exclusive mechanisms, including mate competition, resource competition, intergroup violence, and female choice.⁹ A phylogenetic reconstruction of the evolution of dimorphism including fossil hominins suggested that the modern human condition is derived rather than ancestral, implying that at least some behavioral similarities between humans and chimpanzees associated with dimorphism may have arisen independently rather than being inherited from a common ancestor.⁹

Methodological challenges in the fossil record

Estimating sexual dimorphism from fossil assemblages is fraught with difficulties that directly affect the reliability of behavioral inferences. The first and most fundamental problem is sample size. Most hominin species are represented by a handful of reasonably complete postcranial fossils, and for many species, not a single specimen can be assigned to sex with confidence on the basis of skeletal morphology alone. A dimorphism ratio calculated from two or three specimens of each sex carries enormous uncertainty, and a single new discovery can dramatically shift the estimate.¹³

The second problem is sex assignment. In the absence of DNA preservation or unambiguous pelvic morphology, researchers typically assign sex on the basis of size: larger specimens are assumed to be male and smaller ones female. This procedure is circular when the goal is to estimate dimorphism, because it maximizes the apparent difference between the sexes by definition. Alternative approaches, such as mixture-model analyses that attempt to identify distinct size clusters without prior sex assignment, require sample sizes that are rarely available for hominin species.^{5, 13}

Third, species attribution is often uncertain. The early hominin fossil record contains multiple coexisting species, and individual specimens may be assigned to different taxa by different researchers. If specimens from two species of different body size are combined into a single sample, the resulting size variation will be misinterpreted as sexual dimorphism. This problem is especially acute for early Homo, where the taxonomic boundary between H. habilis and H. rudolfensis remains contested, and both taxa overlap in time and geography with late australopithecines.¹³

Fourth, temporal mixing is a pervasive concern. Fossil assemblages typically accumulate over thousands or tens of thousands of years, during which time a species may have changed in average body size due to climate fluctuations, dietary shifts, or other factors. Size variation within a time-averaged assemblage will include both true sexual dimorphism and temporal variation, and separating the two is extremely difficult. Reno and colleagues attempted to address this by focusing on the AL 333 assemblage, which may represent a single catastrophic death event, but even the contemporaneity of this assemblage has been questioned.^{3, 4}

Finally, the choice of estimation method matters enormously. Body mass can be estimated from joint surfaces (femoral head, tibial plateau, acetabulum), long bone shaft dimensions, or multivariate combinations of measurements, and different methods can yield substantially different results for the same specimen. Grabowski and colleagues developed improved body mass estimation equations based on a large sample of 220 modern humans of known body mass and found that many early hominins were smaller-bodied than previously thought, an outcome likely attributable to the use of large-bodied modern human reference samples in earlier studies.⁶ These methodological advances underscore how sensitive dimorphism estimates are to the analytical choices made by researchers.

Synthesis and current understanding

The evolutionary trajectory of sexual dimorphism in hominins can be summarized, with appropriate caveats, as follows. Canine dimorphism was reduced to near-human levels very early, perhaps by the divergence of hominins from the common ancestor with chimpanzees, and certainly by the time of Ardipithecus ramidus at 4.4 Ma.⁷ Body size dimorphism, in contrast, remained elevated through the australopithecine period, with A. afarensis exhibiting dimorphism that was at minimum moderate and possibly very high by primate standards.⁵ During the transition to the genus Homo, body size dimorphism decreased to approximately modern levels, though the precise timing and rate of this decrease remain poorly constrained.¹³

This pattern—early canine reduction followed by later body size reduction—presents a genuine puzzle. In living primates, canine dimorphism and body size dimorphism are both correlated with male-male competition, and they tend to covary across species. The hominin dissociation between these two dimensions of dimorphism has no exact modern analogue and suggests that unique selective pressures were at work.^{1, 9} The loss of canine weapons may have been driven by a shift in the mode of male-male competition (from physical combat to coalitionary or political strategies), by female preference for less aggressive males, by dietary changes that reduced the functional importance of large canines, or by some combination of these factors. The subsequent reduction in body size dimorphism may reflect a further decline in male competition intensity, an increase in female body size driven by obstetric demands, or the emergence of social structures (such as cooperative breeding or extensive alloparenting) that reduced the payoff for large male body size.^{10, 15}

What is clear is that sexual dimorphism, despite its appeal as a window into ancient behavior, resists simple interpretation. Dimorphism is a complex, multifactorial trait shaped by independent selection on males and females, constrained by genetic correlations between the sexes, and influenced by ecology, diet, and life history as well as mating system.¹⁰ The fossil record provides tantalizing but ambiguous evidence, and advances in analytical methods—from geometric morphometrics to ancient DNA—will be needed to sharpen the picture. For now, the study of sexual dimorphism in hominins remains one of the most productive and contested areas in paleoanthropology, linking anatomical evidence to questions about the origins of the human family, the nature of male-female relationships in deep time, and the evolutionary forces that made us who we are.