Susan G. Larson

The primitive wrist of Homo floresiensis and its implications for hominin evolution

Whether the Late Pleistocene hominin fossils from Flores, Indonesia, represent a new species, Homo floresiensis, or pathological modern humans has been debated. Analysis of three wrist bones from the holotype specimen (LB1) shows that it retains wrist morphology that is primitive for the African ape-human clade. In contrast, Neandertals and modern humans share derived wrist morphology that forms during embryogenesis, which diminishes the probability that pathology could result in the normal primitive state. This evidence indicates that LB1 is not a modern human with an undiagnosed pathology or growth defect; rather, it represents a species descended from a hominin ancestor that branched off before the origin of the clade that includes modern humans, Neandertals, and their last common ancestor.

Patterns of strain in the macaque tibia during functional activity

The strain environment of the tibial midshaft of two female macaques was evaluated through in vivo bone strain experiments using three rosette gauges around the circumference of the bones. Strains were collected for a total of 123 walking and galloping steps as well as several climbing cycles. Principal strains and the angle of the maximum (tensile) principal strain with the long axis of the bone were calculated for each gauge site. In addition, the normal strain distribution throughout the cross section was determined from the longitudinal normal strains (strains in the direction of the long axis of the bone) at each of the three gauge sites, and at the corresponding cross-sectional geometry of the bone. This strain distribution was compared with the cross-sectional properties (area moments) of the midshaft. For both animals, the predominant loading regime was found to be bending about an oblique axis running from anterolateral to posteromedial. The anterior and part of the medial cortex are in tension; the posterior and part of the lateral cortex are in compression. The axis of bending does not coincide with the maximum principal axis of the cross section, which runs mediolaterally. The bones are not especially buttressed in the plane of bending, but offer the greatest strength anteroposteriorly. The cross-sectional geometry therefore does not minimize strain or bone tissue. Peak tibial strains are slightly higher than the peak ulnar strains reported earlier for the same animals (Demes et al. [1998] Am J Phys Anthropol 106:87-100). Peak strains for both the tibia and the ulna are moderate in comparison to strains recorded during walking and galloping activities in nonprimate mammals.

The foot of Homo floresiensis

Homo floresiensis is an endemic hominin species that occupied Liang Bua, a limestone cave on Flores in eastern Indonesia, during the Late Pleistocene epoch. The skeleton of the type specimen (LB1) of H. floresiensis includes a relatively complete left foot and parts of the right foot. These feet provide insights into the evolution of bipedalism and, together with the rest of the skeleton, have implications for hominin dispersal events into Asia. Here we show that LB1’s foot is exceptionally long relative to the femur and tibia, proportions never before documented in hominins but seen in some African apes. Although the metatarsal robusticity sequence is human-like and the hallux is fully adducted, other intrinsic proportions and pedal features are more ape-like. The postcranial anatomy of H. floresiensis is that of a biped, but the unique lower-limb proportions and surprising combination of derived and primitive pedal morphologies suggest kinematic and biomechanical differences from modern human gait. Therefore, LB1 offers the most complete glimpse of a bipedal hominin foot that lacks the full suite of derived features characteristic of modern humans and whose mosaic design may be primitive for the genus Homo. These new findings raise the possibility that the ancestor of H. floresiensis was not Homo erectus but instead some other, more primitive, hominin whose dispersal into southeast Asia is still undocumented.

Uniqueness of Primate Forelimb Posture During Quadrupedal Locomotion

Among the characteristics that are thought to set primate quadrupedal locomotion apart from that of nonprimate mammals are a more protracted limb posture and larger limb angular excursion. However, kinematic aspects of primate or nonprimate quadrupedal locomotion have been documented in only a handful of species, and more widely for the hind than the forelimb. This study presents data on arm (humerus) and forelimb posture during walking for 102 species of mammals, including 53 nonhuman primates and 49 nonprimate mammals. The results demonstrate that primates uniformly display a more protracted arm and forelimb at hand touchdown of a step than nearly all other mammals. Although primates tend to end a step with a less retracted humerus, their total humeral or forelimb angular excursion exceeds that of other mammals. It is suggested that these features are components of functional adaptations to locomotion in an arboreal habitat, using clawless, grasping extremities.

Unique Aspects of Quadrupedal Locomotion in Nonhuman Primates

Quadrupedal walking and running are certainly not the first things that come to mind when one considers unique aspects of primate locomotion. However, there is a growing body of information about how the form of quadrupedalism displayed by primates differs from that of nonprimate mammals (see Vilensky, 1987, 1989). One of the most distinctive characteristics of primate quadrupedalism is that they typically utilize a diagonal sequence/diagonal couplets walking gait pattern (i.e., foot falls in sequence: left hind, right fore, right hind, left fore, with diagonal limbs moving as a pair), in contrast to the almost universally employed lateral sequence walking gait (left hind, left fore, right hind, right fore) of nonprimate mammals (Howell, 1944; Prost, 1965, 1969; Hildebrand, 1967; Rollinson and Martin, 1981; Vilensky, 1989; Vilensky and Larson, 1989). This difference in gait pattern is not trivial, since a diagonal sequence/diagonal couplet walking gait creates a strong potential for interference between the ipsilateral hind and forelimbs (Figure 1). The potential for hind/forelimb interference is exacerbated in primates by their long limbs (due to their relatively longer limb bones, Alexander et al., 1979), and by their propensity to use relatively longer stride lengths than nonprimate quadrupeds (Vilensky, 1980; Alexander and Maloiy, 1984; Reynolds, 1987). As a result, many primate quadrupeds must regularly “overstride” during walking, that is, touch down with their hind foot ahead of their ipsilateral hand by passing it either “inside” or “outside” of the forelimb (Hildebrand, 1967; Reynolds, 1985b; Larson and Stern, 1987; see Figure 1). Another distinctive aspect of primate gait utilization is the infrequent use of a running trot (defined as diagonal limbs moving synchronously with relative stance duration of each limb less than 50%; see Hildebrand, 1967).

Descriptions of the upper limb skeleton of Homo floresiensis

Several bones of the upper extremity were recovered during excavations of Late Pleistocene deposits at Liang Bua, Flores, and these have been attributed to Homo floresiensis. At present, these upper limb remains have been assigned to six different individuals - LB1, LB2, LB3, LB4, LB5, and LB6. Several of these bones are complete or nearly so, but some are quite fragmentary. All skeletal remains recovered from Liang Bua were extremely fragile, but have now been stabilized and hardened in the laboratory in Jakarta. They are now curated in museum-quality containers at the National Research and Development Centre for Archaeology in Jakarta, Indonesia. These skeletal remains are described and illustrated photographically. The upper limb presents a unique mosaic of derived (human-like) and primitive morphologies, the combination of which is never found in either healthy or pathological modern humans.

Limb excursion during quadrupedal walking: how do primates compare to other mammals?

Primate quadrupeds are said to use relatively large limb excursions for mammals of their body size. Until recently, this claim was based on a comparison of hindlimb excursion data derived from small samples of primates and non-primates. Using video recordings collected at zoos and primate research centres, the present study documents this contrast on much wider samples of quadrupedal mammals. The results indicate that while on average hindlimb excursion is relatively larger in quadrupedal primates, this contrast is somewhat less dramatic than first reports suggested. Comparisons between the data reported here and previously collected forelimb excursion data reveal a surprising asymmetry between the fore- and hind excursions for most mammalian species. Most commonly, forelimb excursion exceeds that of the hindlimb. We suggest that this is related to a complementary asymmetry in limb length (forelimbs shorter than hind) for the purpose of achieving equal step lengths for both pairs of limbs. Relatively large hindlimb excursions in primates have been related to a mechanism that reduces stresses on the forelimbs and then recovers mechanical energy during gait. We suggest that large excursions of both the fore- and hindlimbs are linked to other alterations in gait parameters, such as step length, contact time, and limb compliance, that have been adopted in quadrupedal primates to facilitate locomotion along slender arboreal substrates.

Sexual dimorphism in the postcranial skeleton of New World primates

This study examines sexual dimorphism in 24 dimensions of the postcranial skeleton of four platyrrhine species: Callithrix jacchus, Saguinus nigricollis, Saimiri sciureus, and Cebus albifrons. The two callitrichid species show a relatively small amount of variation in the degree of sexual dimorphism among the different dimensions. Variation is considerably higher in the two cebid species as reflected by a mosaic pattern of sexual dimorphisms with males being significantly larger than females in some dimensions, and females significantly larger than males in others. In dimensions of the pectoral girdle and limb bones, males and females in each of the two cebid species are essentially scaled versions of each other, with males being peramorphic compared to females. This pattern is primarily the result of time hypermorphosis, i.e. an extension of the growth period in time in males. Rate hypermorphosis, i.e. an increase in the rate of growth in time in males, appears to play an additional role, however, in S. sciureus. By contrast, in dimensions of the true pelvis, sex differences in shape are dissociated from those in size. They are interpreted as the result of acceleration, i.e. increase in rate of shape change in females, as an adaptation to obstetrical functions. Interspecific analyses indicate positive allometry of mean degree of postcranial dimorphism with respect to body size. This coincides with previous findings by Leutenegger and Cheverud [1982, 1985] on the scaling of sexual dimorphism in body weight and canine size, and thus supports their model which posits selection on body size as the prime mover for the evolution of sexual dimorphism.

A hominoid proximal humerus from the Early Miocene of Rusinga Island, Kenya

Abstract Recently, an isolated proximal humerus of an early Miocene hominoid was discovered at Rusinga Island, Kenya. The precise taxonomic allocation of this specimen is currently problematic, but the fossil almost certainly belongs to either Dendropithecus macinnesi or Proconsul africanus . The humeral head is expanded above the greater tuberosity in the new fossil suggesting that this early Miocene hominoid possessed a relatively mobile shoulder joint for climbing as well as for postural and feeding activities, and that rapid protraction at the shoulder joint was not important. This proximal humeral morphology contrasts with that of Aegyptopithecus , but is quite similar to that of Pliopithecus . Among extant primates, a similar proximal humeral morphology occurs in several platyrrhines such as Alouatta , but living apes share several apparently derived features of the proximal humerus which are lacking in the Rusinga Island specimen.

The type specimen (LB1) of Homo floresiensis did not have Laron syndrome.

The type specimen (LB1) of Homo floresiensis has been hypothesized to be a pathological human afflicted with Laron Syndrome (LS), a type of primary growth hormone insensitivity (Hershkovitz et al.: Am J Phys Anthropol 134 [2007] 198-208). Comparing measurements, photographs and three-dimensional, computed-tomography reconstructions of LB1 with data and diagnoses from the literature on LS, we critically evaluate numerous skull and postcranial traits that Hershkovitz et al. identified as being shared by LB1 and patients with LS. The statements regarding most of these traits are new to the clinical literature and lack quantitative support. LB1 and patients with LS differ markedly in the size and shape of the cranium; thickness and pneumatization of cranial bones; morphology of the face, mandible, teeth, and chin; form of the shoulder, wrist, and pelvis; and general body proportions including relative foot size. Claims that patients with LS are similar to LB1 in displaying protracted scapulae, short clavicles, low degrees of humeral torsion, flaring ilia, and curved tibiae are not supported by data or corroborating images. Some points of similarity (e.g., femoral neck-shaft angle, femoral bicondylar angle, and estimated stature) can be found in other hominins, and cannot be considered diagnostic. From our review and analysis, we conclude that LB1 did not suffer from LS.