Sexual dimorphism in Australopithecus afarensis revisited: How strong is the case for a human-like pattern of dimorphism?

Sexual dimorphism in the enamel-dentine junction (EDJ) of permanent canines of European modern humans

OBJECTIVES

Dental anthropological investigations into sexual dimorphism have conventionally concentrated on evaluating the dimensions and configuration of the enamel cap of canines. However, the morphology of the crown dentine surface can be closely linked to that of the enamel surface. This link can facilitate examination of crown morphology even when the enamel surface is slightly worn. Here, we determine if the morphology of the enamel-dentine junction (EDJ) differs within (maxillary vs. mandibular) and between a sample of male (n = 26) and female (n = 21) contemporary human permanent canines from Europe.

METHODS

The morphological data of the EDJ were gathered employing a template comprising 96 landmarks and sliding semilandmarks. Subsequently, the data underwent analysis through form space principal component analysis following Procrustes registration, utilizing standard 3D geometric morphometric techniques.

RESULTS

Significant differences in the morphology of the EDJ were observed between the sexes, particularly concerning the overall shape of the crown, the symmetry of the mesial and distal edges, and the development of the distal accessory ridge.

CONCLUSIONS

Sex differences in the morphology of the EDJ could relate in part to retention of the canine-premolar honing complex in males. Our results indicate that analyses of the permanent canine EDJ may potentially provide a novel method for estimating the sex of adult and nonadult skeletons.

Sex-biased sampling may influence Homo naledi tooth size variation

A frequent source of debate in paleoanthropology concerns the taxonomic unity of fossil assemblages, with many hominin samples exhibiting elevated levels of variation that can be interpreted as indicating the presence of multiple species. By contrast, the large assemblage of hominin fossils from the Rising Star cave system, assigned to Homo naledi, exhibits a remarkably low degree of variation for most skeletal elements. Many factors can contribute to low sample variation, including genetic drift, strong natural selection, biased sex ratios, and sampling of closely related individuals. In this study, we tested for potential sex-biased sampling in the Rising Star dental sample. We compared coefficients of variation for the H. naledi teeth to those for eight extant hominoid samples. We used a resampling procedure that generated samples from the extant taxa that matched the sample size of the fossil sample for each possible Rising Star dental sex ratio. We found that variation at four H. naledi tooth positions-I2, M1, P4, M1-is so low that the possibility that one sex is represented by few or no individuals in the sample cannot be excluded. Additional evidence is needed to corroborate this inference, such as ancient DNA or enamel proteome data, and our study design does not address other potential factors that would account for low sample variation. Nevertheless, our results highlight the importance of considering the taphonomic history of a hominin assemblage and suggest that sex-biased sampling is a plausible explanation for the low level of phenotypic variation found in some aspects of the current H. naledi assemblage.

Specimens as individuals: Four interventions and recommendations for great ape skeletal collections research and curation

Extensive discourse surrounds the ethics of human skeletal research and curation, but there has yet to be a similar discussion of the treatment of great ape skeletal remains, despite the clear interest in their ethical treatment when alive. Here we trace the history of apes who were killed and collected for natural history museums during the early 20th century and showcase how the guiding research questions of the colonial era continue to influence scholarship. We discuss best practices for improving industry and academic standards of research on, and the curation of, ape remains. The suggested interventions involve restoring individual identity and narrative to great apes while engaging with contextual reflexivity and decolonial theory. The resulting recommendations include contextualizing the individual, piecing individuals back together, challenging/questioning the captive-wild dichotomy, and collaborative international conversations. Our objective is to encourage a conversation regarding ethical and theoretical considerations in great ape skeletal remains research.

Romantic love evolved by co-opting mother-infant bonding

For 25 years, the predominant evolutionary theory of romantic love has been Fisher's theory of independent emotion systems. That theory suggests that sex drive, romantic attraction (romantic love), and attachment are associated with distinct neurobiological and endocrinological systems which evolved independently of each other. Psychological and neurobiological evidence, however, suggest that a competing theory requires attention. A theory of co-opting mother-infant bonding sometime in the recent evolutionary history of humans may partially account for the evolution of romantic love. I present a case for this theory and a new approach to the science of romantic love drawing on human psychological, neurobiological, and (neuro)endocrinological studies as well as animal studies. The hope is that this theoretical review, along with other publications, will generate debate in the literature about the merits of the theory of co-opting mother-infant bonding and a new evolutionary approach to the science of romantic love.

Reappraising the palaeobiology of Australopithecus

The naming of Australopithecus africanus in 1925, based on the Taung Child, heralded a new era in human evolutionary studies and turned the attention of the then Eurasian-centric palaeoanthropologists to Africa, albeit with reluctance. Almost one hundred years later, Africa is recognized as the cradle of humanity, where the entire evolutionary history of our lineage prior to two million years ago took place-after the Homo-Pan split. This Review examines data from diverse sources and offers a revised depiction of the genus and characterizes its role in human evolution. For a long time, our knowledge of Australopithecus came from both A. africanus and Australopithecus afarensis, and the members of this genus were portrayed as bipedal creatures that did not use stone tools, with a largely chimpanzee-like cranium, a prognathic face and a brain slightly larger than that of chimpanzees. Subsequent field and laboratory discoveries, however, have altered this portrayal, showing that Australopithecus species were habitual bipeds but also practised arboreality; that they occasionally used stone tools to supplement their diet with animal resources; and that their infants probably depended on adults to a greater extent than what is seen in apes. The genus gave rise to several taxa, including Homo, but its direct ancestor remains elusive. In sum, Australopithecus had a pivotal bridging role in our evolutionary history owing to its morphological, behavioural and temporal placement between the earliest archaic putative hominins and later hominins-including the genus Homo.

Substantial but Misunderstood Human Sexual Dimorphism Results Mainly From Sexual Selection on Males and Natural Selection on Females

Human sexual dimorphism has been widely misunderstood. A large literature has underestimated the effect of differences in body composition and the role of male contest competition for mates. It is often assumed that sexually dimorphic traits reflect a history of sexual selection, but natural selection frequently builds different phenotypes in males and females. The relatively small sex difference in stature (∼7%) and its decrease during human evolution have been widely presumed to indicate decreased male contest competition for mates. However, females likely increased in stature relative to males in order to successfully deliver large-brained neonates through a bipedally-adapted pelvis. Despite the relatively small differences in stature and body mass (∼16%), there are marked sex differences in body composition. Across multiple samples from groups with different nutrition, males typically have 36% more lean body mass, 65% more muscle mass, and 72% more arm muscle than women, yielding parallel sex differences in strength. These sex differences in muscle and strength are comparable to those seen in primates where sexual selection, arising from aggressive male mating competition, has produced high levels of dimorphism. Body fat percentage shows a reverse pattern, with females having ∼1.6 times more than males and depositing that fat in different body regions than males. We argue that these sex differences in adipose arise mainly from natural selection on women to accumulate neurodevelopmental resources.

New Pliocene hominin remains from the Leado Dido'a area of Woranso-Mille, Ethiopia

Fossiliferous deposits at Woranso-Mille span the period when Australopithecus anamensis gave rise to Australopithecus afarensis (3.8-3.6 Ma) and encompass the core of the A. afarensis range (ca. 3.5-3.2 Ma). Within the latter period, fossils described to date include the intriguing but taxonomically unattributed Burtele foot, dentognathic fossils attributed to Australopithecus deyiremeda, and one specimen securely attributed to A. afarensis (the Nefuraytu mandible). These fossils suggest that at least one additional hominin lineage lived alongside A. afarensis in the Afar Depression. Here we describe a collection of hominin fossils from a new locality in the Leado Dido'a area of Woranso-Mille (LDD-VP-1). The strata in this area are correlated to the same chron as those in the Burtele area (C2An.3n; 3.59-3.33 Ma), and similar in age to the Maka Sands and the Basal through lower Sidi Hakoma Members of the Hadar Formation. We attribute all but one of the LDD hominin specimens to A. afarensis, based on diagnostic morphology of the mandible, maxilla, canines, and premolars. The LDD specimens generally fall within the range of variation previously documented for A. afarensis but increase the frequency of some rare morphological variants. However, one isolated M₃ is extremely small, and its taxonomic affinity is currently unknown. The new observations support previous work on temporal trends in A. afarensis and demonstrate that the large range of variation accepted for this species is present even within a limited spatiotemporal range. The value added with this sample lies in its contribution to controlling for spatiotemporal differences among site samples in the A. afarensis hypodigm and its contemporaneity with non-A. afarensis specimens at Woranso-Mille.

Sexual dimorphism in Homo erectus inferred from 1.5 Ma footprints near Ileret, Kenya

Sexual dimorphism can be one of the most important indicators of social behavior in fossil species, but the effects of time averaging, geographic variation, and differential preservation can complicate attempts to determine this measure from preserved skeletal anatomy. Here we present an alternative, using footprints from near Ileret, Kenya, to assess the sexual dimorphism of presumptive African Homo erectus at 1.5 Ma. Footprint sites have several unique advantages not typically available to fossils: a single surface can sample a population over a very brief time (in this case likely not more than a single day), and the data are geographically constrained. Further, in many cases, the samples can be much larger than those from skeletal fossil assemblages. Our results indicate that East African Homo erectus was more dimorphic than modern Homo sapiens, although less so than highly dimorphic apes, suggesting that the Ileret footprints offer a unique window into an important transitional period in hominin social behavior.

Three-dimensional analysis of sexual dimorphism in ribcage kinematics of modern humans

OBJECTIVES

Sexual dimorphism is an important biological factor underlying morphological variation in the human skeleton. Previous research found sex-related differences in the static ribcage, with males having more horizontally oriented ribs and a wider lower ribcage than females. Furthermore, a recent study found sex-related differences in the kinematics of the human lungs, with cranio-caudal movements of the caudal part of the lungs accounting for most of the differences between sexes. However, these movements cannot be quantified in the skeletal ribcage, so we do not know if the differences observed in the lungs are also reflected in sex differences in the motion of the skeletal thorax.

MATERIALS AND METHODS

To address this issue, we quantified the morphological variation of 42 contemporary human ribcages (sex-balanced) in both maximal inspiration and expiration using 526 landmarks and semilandmarks. Thoracic centroid size differences between sexes were assessed using a t test, and shape differences were assessed using Procrustes shape coordinates, through mean comparisons and dummy regressions of shape on kinematic status. A principal components analysis was used to explore the full range of morphological variation.

RESULTS

Our results show significant size differences between males and females both in inspiration and expiration (p < .01) as well as significant shape differences, with males deforming more than females during inspiration, especially in the mediolateral dimension of the lower ribcage. Finally, dummy regressions of shape on kinematic status showed a small but statistically significant difference in vectors of breathing kinematics between males and females (14.78°; p < .01).

DISCUSSION

We support that sex-related differences in skeletal ribcage kinematics are discernible, even when soft tissues are not analyzed. We hypothesize that this differential breathing pattern is primarily a result of more pronounced diaphragmatic breathing in males, which might relate to differences in body composition, metabolism, and ultimately greater oxygen demand in males compared to females. Future research should further explore the links between ribcage morphological variation and basal metabolic rate.

Lower limb articular scaling and body mass estimation in Pliocene and Pleistocene hominins

Previous attempts to estimate body mass in pre-Holocene hominins have relied on prediction equations derived from relatively limited extant samples. Here we derive new equations to predict body mass from femoral head breadth and proximal tibial plateau breadth based on a large and diverse sample of modern humans (avoiding the problems associated with using diaphyseal dimensions and/or cadaveric reference samples). In addition, an adjustment for the relatively small femoral heads of non-Homo taxa is developed based on observed differences in hip to knee joint scaling. Body mass is then estimated for 214 terminal Miocene through Pleistocene hominin specimens. Mean body masses for non-Homo taxa range between 39 and 49 kg (39-45 kg if sex-specific means are averaged), with no consistent temporal trend (6-1.85 Ma). Mean body mass increases in early Homo (2.04-1.77 Ma) to 55-59 kg, and then again dramatically in Homo erectus and later archaic middle Pleistocene Homo, to about 70 kg. The same average body mass is maintained in late Pleistocene archaic Homo and early anatomically modern humans through the early/middle Upper Paleolithic (0.024 Ma), only declining in the late Upper Paleolithic, with regional variation. Sexual dimorphism in body mass is greatest in Australopithecus afarensis (log[male/female] = 1.54), declines in Australopithecus africanus and Paranthropus robustus (log ratio 1.36), and then again in early Homo and middle and late Pleistocene archaic Homo (log ratio 1.20-1.27), although it remains somewhat elevated above that of living and middle/late Pleistocene anatomically modern humans (log ratio about 1.15).

Long-term patterns of body mass and stature evolution within the hominin lineage

Body size is a central determinant of a species' biology and adaptive strategy, but the number of reliable estimates of hominin body mass and stature have been insufficient to determine long-term patterns and subtle interactions in these size components within our lineage. Here, we analyse 254 body mass and 204 stature estimates from a total of 311 hominin specimens dating from 4.4 Ma to the Holocene using multi-level chronological and taxonomic analytical categories. The results demonstrate complex temporal patterns of body size variation with phases of relative stasis intermitted by periods of rapid increases. The observed trajectories could result from punctuated increases at speciation events, but also differential proliferation of large-bodied taxa or the extinction of small-bodied populations. Combined taxonomic and temporal analyses show that in relation to australopithecines, early Homo is characterized by significantly larger average body mass and stature but retains considerable diversity, including small body sizes. Within later Homo, stature and body mass evolution follow different trajectories: average modern stature is maintained from ca 1.6 Ma, while consistently higher body masses are not established until the Middle Pleistocene at ca 0.5-0.4 Ma, likely caused by directional selection related to colonizing higher latitudes. Selection against small-bodied individuals (less than 40 kg; less than 140 cm) after 1.4 Ma is associated with a decrease in relative size variability in later Homo species compared with earlier Homo and australopithecines. The isolated small-bodied individuals of Homo naledi (ca 0.3 Ma) and Homo floresiensis (ca 100-60 ka) constitute important exceptions to these general patterns, adding further layers of complexity to the evolution of body size within the genus Homo. At the end of the Late Pleistocene and Holocene, body size in Homo sapiens declines on average, but also extends to lower limits not seen in comparable frequency since early Homo.

New footprints from Laetoli (Tanzania) provide evidence for marked body size variation in early hominins

Laetoli is a well-known palaeontological locality in northern Tanzania whose outstanding record includes the earliest hominin footprints in the world (3.66 million years old), discovered in 1978 at Site G and attributed to Australopithecus afarensis. Here, we report hominin tracks unearthed in the new Site S at Laetoli and referred to two bipedal individuals (S1 and S2) moving on the same palaeosurface and in the same direction as the three hominins documented at Site G. The stature estimates for S1 greatly exceed those previously reconstructed for Au. afarensis from both skeletal material and footprint data. In combination with a comparative reappraisal of the Site G footprints, the evidence collected here embodies very important additions to the Pliocene record of hominin behaviour and morphology. Our results are consistent with considerable body size variation and, probably, degree of sexual dimorphism within a single species of bipedal hominins as early as 3.66 million years ago.

DOI:http://dx.doi.org/10.7554/eLife.19568.001

Fossil footprints are extremely useful tools in the palaeontological record. Their physical features can help to identify their makers, but can also be used to infer biological information. How did the track-maker move? How large was it? How fast was it going?

Footprints of hominins (namely the group to which humans and our ancestors belong) are pretty rare. Nearly all of the hominin footprints discovered so far are attributed to species of the genus Homo, to which modern humans belong. The only exceptions are the footprints that were discovered in the 1970s at Laetoli (in Tanzania) on a cemented ash layer produced by a volcanic eruption. These are thought to have been made by three members of the hominin species Australopithecus afarensis – the same species as the famous “Lucy” from Ethiopia – around 3.66 million years ago.

The extent to which body shape and size varied between different members of Au. afarensis – for example, between males and females – has been the subject of a long debate among researchers. Based on the skeletal remains found so far in East Africa, some scholars believe that individuals only varied moderately, as in modern humans, while others state that it was pronounced, as in some modern apes like gorillas.

Masao et al. have now unearthed new bipedal footprints from two individuals who were moving on the same surface and in the same direction as the three individuals who made the footprints documented in the 1970s. The estimated height of one of the new individuals (about 1.65 metres) greatly exceeds those previously published for Au. afarensis. This evidence supports the theory that body size varied considerably amongst individuals within the species.

Masao et al. tentatively suggest that the new footprints can be considered as a whole with the 1970s ones. The tall individual may have been the dominant male of a larger group, the others smaller females and juveniles. Thus, considerable differences may have existed between males and females in these remote human ancestors, similar to modern gorillas.

The newly discovered tracks are only 150 metres away from the previously discovered sets of footprints. This leaves open the possibility that additional tracks may be unearthed nearby that will further our knowledge about the variability and behaviour of our extinct ancestors.

DOI:http://dx.doi.org/10.7554/eLife.19568.002

Australopithecus robustus societies – one-male or multimale?

Abstract Determining the sex of individual specimens is important in estimating the degree of sexual dimorphism. Sexual dimorphism, in turn, provides clues for reconstructing the social organisation and mating systems of extinct species. In an article published in Science, Lockwood et al. (Lockwood CA, Menter CG, Moggi-Cecchi J, Keyser AW. Extended male growth in a fossil hominin species. Science. 2007;318:1443–1446.) suggested an uneven sex ratio (in favour of males) for the known individuals of the South African Pleistocene hominid, Australopithecus robustus, and claimed evidence of an extended period of growth (delayed maturity) for the males of this species. They concluded that this finding, combined with estimates of sexual size dimorphism, suggests a polygynous reproductive strategy, and a social system similar to that of silverback gorillas (i.e. one-male harems). On re-examination of these claims, and based on further analysis, I agree with Lockwood et al. that morphologically A. robustus exhibits an increased (almost gorilla-like) level of facial dimorphism, but propose using an alternate (clustering) technique for grouping the specimens of highly dimorphic species into sexes, and argue that their pronouncements regarding a polygynous social structure of these early hominids are inconclusive. I contend instead that the habitat occupied by this species suggests rather that a one-male harem social structure would have been counterproductive.

Forearm articular proportions and the antebrachial index in Homo sapiens, Australopithecus afarensis and the great apes

When hominin bipedality evolved, the forearms were free to adopt nonlocomotor tasks which may have resulted in changes to the articular surfaces of the ulna and the relative lengths of the forearm bones. Similarly, sex differences in forearm proportions may be more likely to emerge in bipeds than in the great apes given the locomotor constraints in Gorilla, Pan and Pongo. To test these assumptions, ulnar articular proportions and the antebrachial index (radius length/ulna length) in Homo sapiens (n=51), Gorilla gorilla (n=88), Pan troglodytes (n=49), Pongo pygmaeus (n=36) and Australopithecus afarensis A.L. 288-1 and A.L. 438-1 are compared. Intercept-adjusted ratios are used to control for size and minimize the effects of allometry. Canonical scores axes show that the proximally broad and elongated trochlear notch with respect to size in H. sapiens and A. afarensis is largely distinct from G. gorilla, P. troglodytes and P. pygmaeus. A cluster analysis of scaled ulnar articular dimensions groups H. sapiens males with A.L. 438-1 ulna length estimates, while one A.L. 288-1 ulna length estimate groups with Pan and another clusters most closely with H. sapiens, G. gorilla and A.L. 438-1. The relatively low antebrachial index characterizing H. sapiens and non-outlier estimates of A.L. 288-1 and A.L. 438-1 differs from those of the great apes. Unique sex differences in H. sapiens suggest a link between bipedality and forearm functional morphology.

Body mass estimates of hominin fossils and the evolution of human body size

Body size directly influences an animal's place in the natural world, including its energy requirements, home range size, relative brain size, locomotion, diet, life history, and behavior. Thus, an understanding of the biology of extinct organisms, including species in our own lineage, requires accurate estimates of body size. Since the last major review of hominin body size based on postcranial morphology over 20 years ago, new fossils have been discovered, species attributions have been clarified, and methods improved. Here, we present the most comprehensive and thoroughly vetted set of individual fossil hominin body mass predictions to date, and estimation equations based on a large (n = 220) sample of modern humans of known body masses. We also present species averages based exclusively on fossils with reliable taxonomic attributions, estimates of species averages by sex, and a metric for levels of sexual dimorphism. Finally, we identify individual traits that appear to be the most reliable for mass estimation for each fossil species, for use when only one measurement is available for a fossil. Our results show that many early hominins were generally smaller-bodied than previously thought, an outcome likely due to larger estimates in previous studies resulting from the use of large-bodied modern human reference samples. Current evidence indicates that modern human-like large size first appeared by at least 3-3.5 Ma in some Australopithecus afarensis individuals. Our results challenge an evolutionary model arguing that body size increased from Australopithecus to early Homo. Instead, we show that there is no reliable evidence that the body size of non-erectus early Homo differed from that of australopiths, and confirm that Homo erectus evolved larger average body size than earlier hominins.

From Lucy to Kadanuumuu: balanced analyses of Australopithecus afarensis assemblages confirm only moderate skeletal dimorphism

Sexual dimorphism in body size is often used as a correlate of social and reproductive behavior in Australopithecus afarensis. In addition to a number of isolated specimens, the sample for this species includes two small associated skeletons (A.L. 288-1 or "Lucy" and A.L. 128/129) and a geologically contemporaneous death assemblage of several larger individuals (A.L. 333). These have driven both perceptions and quantitative analyses concluding that Au. afarensis was markedly dimorphic. The Template Method enables simultaneous evaluation of multiple skeletal sites, thereby greatly expanding sample size, and reveals that A. afarensis dimorphism was similar to that of modern humans. A new very large partial skeleton (KSD-VP-1/1 or "Kadanuumuu") can now also be used, like Lucy, as a template specimen. In addition, the recently developed Geometric Mean Method has been used to argue that Au. afarensis was equally or even more dimorphic than gorillas. However, in its previous application Lucy and A.L. 128/129 accounted for 10 of 11 estimates of female size. Here we directly compare the two methods and demonstrate that including multiple measurements from the same partial skeleton that falls at the margin of the species size range dramatically inflates dimorphism estimates. Prevention of the dominance of a single specimen's contribution to calculations of multiple dimorphism estimates confirms that Au. afarensis was only moderately dimorphic.

The pattern of hominin postcranial evolution reconsidered in light of size-related shape variation of the distal humerus

Previous research suggests that some hominin postcranial features do not follow a linear path of increasing modernization through geological time. With respect to the distal humerus, in particular, the earliest known hominin specimens are reportedly among the most modern in morphology, while some later humeri appear further removed from the average modern human shape. Although Plio-Pleistocene humeri vary widely in size, previous studies have failed to account for size-related shape variation when making morphometric comparisons. This study reexamines hominin postcranial evolution in light of distal humeral allometry. Using two-dimensional landmark data, the relationship between specimen size and shape among modern humans is quantified using multivariate regression and principal components analysis of size-shape space. Fossils are compared with modern human shapes expected at a given size, as well as with the overall average human shape. The null hypothesis of humeral isometry in modern humans is rejected. Subsequently, if one takes allometry into account, the apparent pattern of hominin humeral evolution does not resemble the pattern described above. All 14 of the Plio-Pleistocene hominin fossils examined here share a similar pattern of shape differences from equivalently-sized modern humans, though they vary in the extent to which these differences are expressed. The oldest specimen in the sample (KNM-KP 271; Australopithecus anamensis) exhibits the least human-like elbow morphology. Similarly primitive morphology characterizes all younger species of Australopithecus as well as Paranthropus robustus. After 2 Ma, a subtly more human-like elbow morphology is apparent among specimens attributed to early Homo, as well as among isolated specimens that may represent either Homo or Paranthropus boisei. This study emphasizes the need to consider size-related shape variation when individual fossil specimens are compared with the average shape of a comparative group, particularly when specimens fall near an extreme of the comparative size distribution.

Rensch's rule, Bergmann's effect and adult sexual dimorphism in wild monogamous owl monkeys (Aotus azarai) of Argentina

Some monogamous primates are characterized by biparental care, territoriality, and a reduced level of physical dimorphism. In others, the relationship between those behavioral patterns and dimorphism is less clear. I tested Bergmann's and Rensch's rules using Aotus spp. body mass data and I characterized the extent of sexual dimorphism in body mass, dental and physical measurements in a socially monogamous owl monkey population (n = 91 adults) from the Argentinean Gran Chaco. A. azarai azarai from the Argentinean Chaco is larger than the more tropical owl monkey species (r = 0.7, N = 6 species), but there is no apparent increase in sexual dimorphism with increased body mass. The body masses of adult male and female A. a. azarai were remarkably similar (Mean = 1.26 kg); there were no marked sex differences in most skeletal measurements, but males had higher and wider upper and lower canines than did females. Body mass and neck circumference were positively and strongly related (r = 0.533, n = 52), and the body mass of adults was not a reliable indicator of their age (r = 0.03, n = 10). The data illustrate the complexities inherent in examining and summarizing within population variation in skeletal and nonskeletal measurements and contribute to a better understanding of the relationships between monogamous behavioral patterns and sexual dimorphism.

Avoidance of overheating and selection for both hair loss and bipedality in hominins

Two frequently debated aspects of hominin evolution are the development of upright bipedal stance and reduction in body hair. It has long been argued, on the basis of heat-balance models, that thermoregulation might have been important in the evolution of both of these traits. Previous models were based on a stationary individual standing in direct sunlight; here we extend this approach to consider a walking hominin, having argued that walking is more thermally challenging than remaining still. Further, stationary activities may be more compatible with shade seeking than activities (such as foraging) involving travel across the landscape. Our model predictions suggest that upright stance probably evolved for nonthermoregulatory reasons. However, the thermoregulatory explanation for hair loss was supported. Specifically, we postulate progressive hair loss being selected and this allowing individuals to be active in hot, open environments initially around dusk and dawn without overheating. Then, as our ancestors' hair loss increased and sweating ability improved over evolutionary time, the fraction of the day when they could remain active in such environments extended. Our model suggests that only when hair loss and sweating ability reach near-modern human levels could hominins have been active in the heat of the day in hot, open environments.

Thermoregulation and endurance running in extinct hominins: Wheeler's models revisited

Thermoregulation is often cited as a potentially important influence on the evolution of hominins, thanks to a highly influential series of papers in the Journal of Human Evolution in the 1980s and 1990s by Peter Wheeler. These papers developed quantitative modeling of heat balance between different potential hominins and their environment. Here, we return to these models, update them in line with new developments and measurements in animal thermal biology, and modify them to represent a running hominin rather than the stationary form considered previously. In particular, we use our modified Wheeler model to investigate thermoregulatory aspects of the evolution of endurance running ability. Our model suggests that for endurance running to be possible, a hominin would need locomotive efficiency, sweating rates, and areas of hairless skin similar to modern humans. We argue that these restrictions suggest that endurance running may have been possible (from a thermoregulatory viewpoint) for Homo erectus, but is unlikely for any earlier hominins.

Inter- and intra-specific scaling of articular surface areas in the hominoid talus

The morphology of postcranial articular surfaces is expected to reflect their weight-bearing properties, as well as the stability and mobility of the articulations to which they contribute. Previous studies have mainly confirmed earlier predictions of isometric scaling between articular surface areas and body mass; the exception to this is 'male-type', convex articular surface areas, which may scale allometrically due to differences in locomotor strategies within the analysed samples. In the present study, we used new surface scanning technology to quantify more accurately articular surface areas and to test those predictions within the talus of hominoid primates, including modern humans. Our results, contrary to predictions, suggest that there are no generalised rules of articular scaling within the talus of hominoids. Instead, we suggest that articular scaling patterns are highly context-specific, depending on the role of each articulation during locomotion, as well as taxon- and sex-specific differences in locomotion and ontogenetic growth trajectories within any given sample. While this may prove problematic for inferring body mass based on articular surface area, it also offers new opportunities of gaining substantial insights into the locomotor patterns of extinct species.

An enlarged postcranial sample confirms Australopithecus afarensis dimorphism was similar to modern humans

In a previous study, we introduced the template method as a means of enlarging the Australopithecus afarensis postcranial sample to more accurately estimate its skeletal dimorphism. Results indicated dimorphism to be largely comparable to that of Homo sapiens. Some have since argued that our results were biased by artificial homogeneity in our Au. afarensis sample. Here we report the results from inclusion of 12 additional, newly reported, specimens. The results are consistent with those of our original study and with the hypothesis that early hominid demographic success derived from a reproductive strategy involving male provisioning of pair-bonded females.

Digit ratios predict polygyny in early apes, Ardipithecus, Neanderthals and early modern humans but not in Australopithecus

Social behaviour of fossil hominoid species is notoriously difficult to predict owing to difficulties in estimating body size dimorphism from fragmentary remains and, in hominins, low canine size dimorphism. Recent studies have shown that the second-to-fourth digit ratio (2D : 4D), a putative biomarker for prenatal androgen effects (PAEs), covaries with intra-sexual competition and social systems across haplorrhines; non-pair-bonded polygynous taxa have significantly lower 2D : 4D ratios (high PAE) than pair-bonded monogamous species. Here, we use proximal phalanx ratios of extant and fossil specimens to reconstruct the social systems of extinct hominoids. Pierolapithecus catalaunicus, Hispanopithecus laietanus and Ardipithecus ramidus have ratios consistent with polygynous extant species, whereas the ratio of Australopithecus afarensis is consistent with monogamous extant species. The early anatomically modern human Qafzeh 9 and Neanderthals have lower digit ratios than most contemporary human populations, indicating increased androgenization and possibly higher incidence of polygyny. Although speculative owing to small sample sizes, these results suggest that digit ratios represent a supplementary approach for elucidating the social systems of fossil hominins.

Phylogeny of early Australopithecus: new fossil evidence from the Woranso-Mille (central Afar, Ethiopia)

The earliest evidence of Australopithecus goes back to ca 4.2 Ma with the first recorded appearance of Australopithecus 'anamensis' at Kanapoi, Kenya. Australopithecus afarensis is well documented between 3.6 and 3.0 Ma mainly from deposits at Laetoli (Tanzania) and Hadar (Ethiopia). The phylogenetic relationship of these two 'species' is hypothesized as ancestor-descendant. However, the lack of fossil evidence from the time between 3.6 and 3.9 Ma has been one of its weakest points. Recent fieldwork in the Woranso-Mille study area in the Afar region of Ethiopia has yielded fossil hominids dated between 3.6 and 3.8 Ma. These new fossils play a significant role in testing the proposed relationship between Au. anamensis and Au. afarensis. The Woranso-Mille hominids (3.6-3.8 Ma) show a mosaic of primitive, predominantly Au. anamensis-like, and some derived (Au. afarensis-like) dentognathic features. Furthermore, they show that, as currently known, there are no discrete and functionally significant anatomical differences between Au. anamensis and Au. afarensis. Based on the currently available evidence, it appears that there is no compelling evidence to falsify the hypothesis of 'chronospecies pair' or ancestor-descendant relationship between Au. anamensis and Au. afarensis. Most importantly, however, the temporally and morphologically intermediate Woranso-Mille hominids indicate that the species names Au. afarensis and Au. anamensis do not refer to two real species, but rather to earlier and later representatives of a single phyletically evolving lineage. However, if retaining these two names is necessary for communication purposes, the Woranso-Mille hominids are best referred to as Au. anamensis based on new dentognathic evidence.

Latitudinal variation in cranial dimorphism in Macaca fascicularis

This study examines latitudinal and insular variation in the expression of sexual dimorphism in cranial length in three geographical groupings of Macaca fascicularis. In addition, the relationship between cranial length dimorphism (CLD) and sex-specific size is examined. The results of the study identified a significant relationship between CLD and latitude for only one of the three geographic groupings. Sex-specific relationships between cranial length and CLD were detected. The pattern of these relationships varied by geographic grouping. This study is important because it demonstrates that despite very similar levels of CLD in a single primate species, there exists important geographic variability in the correlates of that dimorphism. I suggest that geographically varying ecological factors may influence sex-specific natural selection and the intensity of CLD in M. fascicularis. Gaining a better understanding of this geographical variability will require that future research examines morphological variation, including CLD, within its corresponding ecological and social contexts. Such research should be comparative, and incorporate multiple geographically separated populations with disparate environmental settings.