The thumb of Miocene apes: new insights from Castell de Barberà (Catalonia, Spain)

An ape partial postcranial skeleton (KNM-NP 64631) from the Middle Miocene of Napudet, northern Kenya

An ape partial postcranial skeleton (KNM-NP 64631) was recovered during the 2015-2021 field seasons at Napudet, a Middle Miocene (∼13 Ma) locality in northern Kenya. Bony elements representing the shoulder, elbow, hip, and ankle joints, thoracic and lumbar vertebral column, and hands and feet, offer valuable new information about the body plan and positional behaviors of Middle Miocene apes. Body mass estimates from femoral head dimensions suggest that the KNM-NP 64631 individual was smaller-bodied (c. 13-17 kg) than some Miocene taxa from eastern Africa, including Ekembo nyanzae, and probably Equatorius africanus or Kenyapithecus wickeri, and was more comparable to smaller-bodied male Nacholapithecus kerioi individuals. Similar to many Miocene apes, the KNM-NP 64631 individual had hip and hallucal tarsometatarsal joints reflecting habitual hindlimb loading in a variety of postures, a distal tibia with a large medial malleolus, an inflated humeral capitulum, probably a long lumbar spine, and a long pollical proximal phalanx relative to femoral head dimensions. The KNM-NP 64631 individual departs from most Early Miocene apes in its possession of a more steeply beveled radial head and deeper humeral zona conoidea, reflecting enhanced supinating-pronating abilities at the humeroradial joint. The KNM-NP 64631 individual also differs from Early Miocene Ekembo heseloni in having a larger elbow joint (inferred from radial head size) relative to the mediolateral width of the lumbar vertebral bodies and a more asymmetrical talar trochlea, and in these ways recalls inferred joint proportions for, and talocrural morphology of, N. kerioi. Compared to most Early Miocene apes, the KNM-NP 64631 individual likely relied on more forelimb-dominated arboreal behaviors, perhaps including vertical climbing (e.g., extended elbow, hoisting). Moreover, the Napudet ape partial postcranial skeleton suggests that an arboreally adapted body plan characterized by relatively large (here, based on joint size) forelimbs, but lacking orthograde suspensory adaptations, may not have been 'unusual' among Middle Miocene apes.

Cortical bone architecture of hominid intermediate phalanges reveals functional signals of locomotion and manipulation

OBJECTIVES

Reconstruction of fossil hominin manual behaviors often relies on comparative analyses of extant hominid hands to understand the relationship between hand use and skeletal morphology. In this context, the intermediate phalanges remain understudied. Thus, here we investigate cortical bone morphology of the intermediate phalanges of extant hominids and compare it to the cortical structure of the proximal phalanges, to investigate the relationship between cortical bone structure and inferred loading during manual behaviors.

MATERIALS AND METHODS

Using micro-CT data, we analyze cortical bone structure of the intermediate phalangeal shaft of digits 2-5 in Pongo pygmaeus (n = 6 individuals), Gorilla gorilla (n = 22), Pan spp. (n = 23), and Homo sapiens (n = 23). The R package morphomap is used to study cortical bone distribution, cortical thickness and cross-sectional properties within and across taxa.

RESULTS

Non-human great apes generally have thick cortical bone on the palmar shaft, with Pongo only having thick cortex on the peaks of the flexor sheath ridges, while African apes have thick cortex along the entire flexor sheath ridge and proximal to the trochlea. Humans are distinct in having thicker dorsal shaft cortex as well as thick cortex at the disto-palmar region of the shaft.

DISCUSSION

Variation in cortical bone distribution and properties of the intermediate phalanges is consistent with differences in locomotor and manipulative behaviors in extant great apes. Comparisons between the intermediate and proximal phalanges reveals similar patterns of cortical bone distribution within each taxon but with potentially greater load experienced by the proximal phalanges, even in knuckle-walking African apes. This study provides a comparative context for the reconstruction of habitual hand use in fossil hominins and hominids.

Systematics of Miocene apes: State of the art of a neverending controversy

Hominoids diverged from cercopithecoids during the Oligocene in Afro-Arabia, initially radiating in that continent and subsequently dispersing into Eurasia. From the Late Miocene onward, the geographic range of hominoids progressively shrank, except for hominins, which dispersed out of Africa during the Pleistocene. Although the overall picture of hominoid evolution is clear based on available fossil evidence, many uncertainties persist regarding the phylogeny and paleobiogeography of Miocene apes (nonhominin hominoids), owing to their sparse record, pervasive homoplasy, and the decimated current diversity of this group. We review Miocene ape systematics and evolution by focusing on the most parsimonious cladograms published during the last decade. First, we provide a historical account of the progress made in Miocene ape phylogeny and paleobiogeography, report an updated classification of Miocene apes, and provide a list of Miocene ape species-locality occurrences together with an analysis of their paleobiodiversity dynamics. Second, we discuss various critical issues of Miocene ape phylogeny and paleobiogeography (hylobatid and crown hominid origins, plus the relationships of Oreopithecus) in the light of the highly divergent results obtained from cladistic analyses of craniodental and postcranial characters separately. We conclude that cladistic efforts to disentangle Miocene ape phylogeny are potentially biased by a long-branch attraction problem caused by the numerous postcranial similarities shared between hylobatids and hominids-despite the increasingly held view that they are likely homoplastic to a large extent, as illustrated by Sivapithecus and Pierolapithecus-and further aggravated by abundant missing data owing to incomplete preservation. Finally, we argue that-besides the recovery of additional fossils, the retrieval of paleoproteomic data, and a better integration between cladistics and geometric morphometrics-Miocene ape phylogenetics should take advantage of total-evidence (tip-dating) Bayesian methods of phylogenetic inference combining morphologic, molecular, and chronostratigraphic data. This would hopefully help ascertain whether hylobatid divergence was more basal than currently supported.

A revised (earliest Vallesian) age for the hominoid-bearing locality of Can Mata 1 based on new magnetostratigraphic and biostratigraphic data from Abocador de Can Mata (Vallès-Penedès Basin, NE Iberian Peninsula)

The Abocador de Can Mata (ACM) composite stratigraphic sequence (els Hostalets de Pierola, Vallès-Penedès Basin, NE Iberian Peninsula) has yielded a diverse primate assemblage from the late Aragonian (Middle to Late Miocene). Detailed litho-, bio-, and magnetostratigraphic control has enabled an accurate dating of these fossil remains. Comparable data, however, were lacking for the nearby locality of Can Mata 1 (CM1), which yielded a dryopithecine canine of a female individual. Given the lack of hipparionin equids and giraffids, CM1 has been correlated to the latest Aragonian (Mammal Neogene [MN] zone MN7+8). Here we revise the age of CM1 based on fieldwork and associated paleomagnetic samplings undertaken in 2018-2021. Our results extend the ACM composite sequence upward and indicate that CM1 correlates to the earliest Vallesian (MN9). The updated ACM sequence has a thickness of ∼300 m and comprises 12 magnetozones correlated to subchrons C5Ar.1r to C5n.2n (∼12.6-11.1 Ma; latest MN6 to earliest MN9, late Aragonian to earliest Vallesian). CM1 is correlated to C5r.1r (11.146-11.056 Ma), with an interpolated age of 11.11 Ma, thus postdating the dispersal of hipparionin horses into the Vallès-Penedès Basin-which is correlated to the previous subchron C5r.1n, with an interpolated age of 11.18 Ma, and by definition marks the beginning of the Vallesian. CM1 also minimally postdates the earliest record of giraffids at ACM-representing their earliest well-dated occurrence in the basin-being correlated to C5r.1n with an interpolated age of 11.11 Ma. We conclude that CM1 has an earliest Vallesian (MN9) age of ∼11.1 Ma, intermediate between the Aragonian dryopithecins and the Vallesian hispanopithecins. Ongoing paleontological surveillance at ACM thus offers the prospect to yield additional earliest Vallesian ape remains, which are essential to clarify their taxonomic allocation as well as to confirm whether hispanopithecins evolved locally from dryopithecins rather than immigrating from elsewhere during MN9.

A proximal radius of Barberapithecus huerzeleri from Castell de Barberà: Implications for locomotor diversity among pliopithecoids

Pliopithecoids are a diverse group of Miocene catarrhine primates from Eurasia. Their positional behavior is still unknown, and many species are known exclusively from dentognathic remains. Here, we describe a proximal radius (IPS66267) from the late Miocene of Castell de Barberà (Vallès-Penedès Basin, NE Iberian Peninsula) that represents the first postcranial specimen of the pliopithecoid Barberapithecus huerzeleri. A body mass estimate based on the radius is compared with dental estimates, and its morphology is compared with that of extant and fossil anthropoids by qualitative means as well as by landmark-based three-dimensional geometric morphometrics. The estimated body mass of ∼5 kg for IPS66267 closely matches the dental estimates for the (female) holotype, thereby discounting an alternative attribution to the large-bodied hominoid recorded at Castell de Barberà. In multiple features (oval and moderately tilted head with a pronounced lateral lip and a restricted articular area for the capitulum; proximodistally expanded proximal radioulnar joint; and short, robust, and anteroposteriorly compressed neck), the specimen differs from hominoids and resembles instead extant nonateline monkeys and stem catarrhines. The results of the morphometric analysis further indicate that the Barberapithecus proximal radius shows closer similarities with nonsuspensory arboreal cercopithecoids and the dendropithecid Simiolus. From a locomotor viewpoint, the radius of Barberapithecus lacks most of the features functionally related to climbing and/or suspensory behaviors and displays instead a proximal radioulnar joint that would have been particularly stable under pronation. On the other hand, the Barberapithecus radius differs from other stem catarrhines in the less anteroposteriorly compressed and less tilted radial head with a deeper capitular fovea, suggesting a somewhat enhanced mobility at the elbow joint. We conclude that pronograde arboreal quadrupedalism was the main component of the locomotor repertoire of Barberapithecus but that, similar to other crouzeliids, it might have displayed better climbing abilities than pliopithecids.

Trabecular bone patterning across the human hand

Hand bone morphology is regularly used to link particular hominin species with behaviors relevant to cognitive/technological progress. Debates about the functional significance of differing hominin hand bone morphologies tend to rely on establishing phylogenetic relationships and/or inferring behavior from epigenetic variation arising from mechanical loading and adaptive bone modeling. Most research focuses on variation in cortical bone structure, but additional information about hand function may be provided through the analysis of internal trabecular structure. While primate hand bone trabecular structure is known to vary in ways that are consistent with expected joint loading differences during manipulation and locomotion, no study exists that has documented this variation across the numerous bones of the hand. We quantify the trabecular structure in 22 bones of the human hand (early/extant modern Homo sapiens) and compare structural variation between two groups associated with post-agricultural/industrial (post-Neolithic) and foraging/hunter-gatherer (forager) subsistence strategies. We (1) establish trabecular bone volume fraction (BV/TV), modulus (E), degree of anisotropy (DA), mean trabecular thickness (Tb.Th) and spacing (Tb.Sp); (2) visualize the average distribution of site-specific BV/TV for each bone; and (3) examine if the variation in trabecular structure is consistent with expected joint loading differences among the regions of the hand and between the groups. Results indicate similar distributions of trabecular bone in both groups, with those of the forager sample presenting higher BV/TV, E, and lower DA, suggesting greater and more variable loading during manipulation. We find indications of higher loading along the ulnar side of the forager sample hand, with high site-specific BV/TV distributions among the carpals that are suggestive of high loading while the wrist moves through the 'dart-thrower's' motion. These results support the use of trabecular structure to infer behavior and have direct implications for refining our understanding of human hand evolution and fossil hominin hand use.

Trabecular architecture in the thumb of Pan and Homo: implications for investigating hand use, loading, and hand preference in the fossil record

OBJECTIVES

Humans display an 85-95% cross-cultural right-hand bias in skilled tasks, which is considered a derived behavior because such a high frequency is not reported in wild non-human primates. Handedness is generally considered to be an evolutionary byproduct of selection for manual dexterity and augmented visuo-cognitive capabilities within the context of complex stone tool manufacture/use. Testing this hypothesis requires an understanding of when appreciable levels of right dominant behavior entered the fossil record. Because bone remodels in vivo, skeletal asymmetries are thought to reflect greater mechanical loading on the dominant side, but incomplete preservation of external morphology and ambiguities about past loading environments complicate interpretations. We test if internal trabecular bone is capable of providing additional information by analyzing the thumb of Homo sapiens and Pan.

MATERIALS AND METHODS

We assess trabecular structure at the distal head and proximal base of paired (left/right) first metacarpals using micro-CT scans of Homo sapiens (n = 14) and Pan (n = 9). Throughout each epiphysis we quantify average and local bone volume fraction (BV/TV), degree of anisotropy (DA), and elastic modulus (E) to address bone volume patterning and directional asymmetry.

RESULTS

We find a right directional asymmetry in H. sapiens consistent with population-level handedness, but also report a left directional asymmetry in Pan that may be the result of postural and/or locomotor loading.

CONCLUSION

We conclude that trabecular bone is capable of detecting right/left directional asymmetry, but suggest coupling studies of internal structure with analyses of other skeletal elements and cortical bone prior to applications in the fossil record.

The Middle Miocene ape Pierolapithecus catalaunicus exhibits extant great ape-like morphometric affinities on its patella: inferences on knee function and evolution

The mosaic nature of the Miocene ape postcranium hinders the reconstruction of the positional behavior and locomotion of these taxa based on isolated elements only. The fossil great ape Pierolapithecus catalaunicus (IPS 21350 skeleton; 11.9 Ma) exhibits a relatively wide and shallow thorax with moderate hand length and phalangeal curvature, dorsally-oriented metacarpophalangeal joints, and loss of ulnocarpal articulation. This evidence reveals enhanced orthograde postures without modern ape-like below-branch suspensory adaptations. Therefore, it has been proposed that natural selection enhanced vertical climbing (and not suspension per se) in Pierolapithecus catalaunicus. Although limb long bones are not available for this species, its patella (IPS 21350.37) can potentially provide insights into its knee function and thus on the complexity of its total morphological pattern. Here we provide a detailed description and morphometric analyses of IPS 21350.37, which are based on four external dimensions intended to capture the overall patellar shape. Our results reveal that the patella of Pierolapithecus is similar to that of extant great apes: proximodistally short, mediolaterally broad and anteroposteriorly thin. Previous biomechanical studies of the anthropoid knee based on the same measurements proposed that the modern great ape patella reflects a mobile knee joint while the long, narrow and thick patella of platyrrhine and especially cercopithecoid monkeys would increase the quadriceps moment arm in knee extension during walking, galloping, climbing and leaping. The patella of Pierolapithecus differs not only from that of monkeys and hylobatids, but also from that of basal hominoids (e.g., Proconsul and Nacholapithecus), which display slightly thinner patellae than extant great apes (the previously-inferred plesiomorphic hominoid condition). If patellar shape in Pierolapithecus is related to modern great ape-like knee function, our results suggest that increased knee mobility might have originally evolved in relation to enhanced climbing capabilities in great apes (such as specialized vertical climbing).

The morphology of Oreopithecus bambolii pollical distal phalanx

Oreopithecus bambolii is a Late Miocene ape from Italy, first described in the late 19th century. Its interpretation is still highly controversial, especially in reference to its hand proportions and thumb morphology. In this study, the authors provide detailed descriptions of the available Oreopithecus pollical distal phalanx (PDP) specimens, as well as bivariate and multivariate morphometric analyses in comparison with humans, extant apes, selected anthropoid monkeys, and available Miocene PDP specimens. The multivariate results reveal two opposite poles on the hominoid PDP shape spectrum: on one side, a mediolaterally broad and dorsopalmarly short human PDP, and on the other side, the narrow and "conical" PDP of chimpanzees and orangutans. The authors contend that Oreopithecus exhibits intermediate PDP proportions that are largely primitive for hominoids because it shares morphological similarities with Proconsul. Furthermore, Oreopithecus displays a mediolaterally wide tuft for a hominoid, as well as a palmarly elevated attachment for a long tendon of a flexor muscle that is associated at its proximal edge with a proximal fossa and at its distal edge with an ungual fossa. These nonmetrical traits have been associated in humans with their capability to oppose and contact the proximal pads of the thumb and fingers, that is, pad-to-pad precision grasping. These traits reinforce previous studies that indicate a human-like thumb-to-hand length ratio compatible with pad-to-pad precision grasping in Oreopithecus. Although specific hand use is still unresolved in Oreopithecus, the results suggest enhanced manipulative skills (unrelated to stone tool-making) in this taxon relative to other (extant or fossil) hominoids.

Tool making, hand morphology and fossil hominins

Was stone tool making a factor in the evolution of human hand morphology? Is it possible to find evidence in fossil hominin hands for this capability? These questions are being addressed with increasingly sophisticated studies that are testing two hypotheses; (i) that humans have unique patterns of grip and hand movement capabilities compatible with effective stone tool making and use of the tools and, if this is the case, (ii) that there exist unique patterns of morphology in human hands that are consistent with these capabilities. Comparative analyses of human stone tool behaviours and chimpanzee feeding behaviours have revealed a distinctive set of forceful pinch grips by humans that are effective in the control of stones by one hand during manufacture and use of the tools. Comparative dissections, kinematic analyses and biomechanical studies indicate that humans do have a unique pattern of muscle architecture and joint surface form and functions consistent with the derived capabilities. A major remaining challenge is to identify skeletal features that reflect the full morphological pattern, and therefore may serve as clues to fossil hominin manipulative capabilities. Hominin fossils are evaluated for evidence of patterns of derived human grip and stress-accommodation features.

A partial skeleton of the fossil great ape Hispanopithecus laietanus from Can Feu and the mosaic evolution of crown-hominoid positional behaviors

The extinct dryopithecine Hispanopithecus (Primates: Hominidae), from the Late Miocene of Europe, is the oldest fossil great ape displaying an orthograde body plan coupled with unambiguous suspensory adaptations. On the basis of hand morphology, Hispanopithecus laietanus has been considered to primitively retain adaptations to above-branch quadrupedalism-thus displaying a locomotor repertoire unknown among extant or fossil hominoids, which has been considered unlikely by some researchers. Here we describe a partial skeleton of H. laietanus from the Vallesian (MN9) locality of Can Feu 1 (Vallès-Penedès Basin, NE Iberian Peninsula), with an estimated age of 10.0-9.7 Ma. It includes dentognathic and postcranial remains of a single, female adult individual, with an estimated body mass of 22-25 kg. The postcranial remains of the rib cage, shoulder girdle and forelimb show a mixture of monkey-like and modern-hominoid-like features. In turn, the proximal morphology of the ulna-most completely preserved in the Can Feu skeleton than among previously-available remains-indicates the possession of an elbow complex suitable for preserving stability along the full range of flexion/extension and enabling a broad range of pronation/supination. Such features, suitable for suspensory behaviors, are however combined with an olecranon morphology that is functionally related to quadrupedalism. Overall, when all the available postcranial evidence for H. laietanus is considered, it emerges that this taxon displayed a locomotor repertoire currently unknown among other apes (extant or extinct alike), uniquely combining suspensory-related features with primitively-retained adaptations to above-branch palmigrady. Despite phylogenetic uncertainties, Hispanopithecus is invariably considered an extinct member of the great-ape-and-human clade. Therefore, the combination of quadrupedal and suspensory adaptations in this Miocene crown hominoid clearly evidences the mosaic nature of locomotor evolution in the Hominoidea, as well as the impossibility to reconstruct the ancestral locomotor repertoires for crown hominoid subclades on the basis of extant taxa alone.