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The emergence of modern humans in the Late Pleistocene was characterized by …
Past research on Late Pleistocene human appendicular robusticity (1, 4, 17, 18) has provided conflicting assessments of the degree of contrast between these samples. Consequently, the distributions of measures of diaphyseal robusticity were statistically evaluated with an Ho of similarity. These degrees of similarity between the samples were assessed by using standard residuals from the pooled Late Pleistocene sample. They were compared between the late archaic and pooled early modern human samples by using unpaired Wilcoxon rank-sum tests and across combinations of multiple samples with Kruskal-Wallis tests.
Patterns of Humeral Robusticity
Assessments of Pleistocene diaphyseal robusticity frequently focus on percent cortical area. It sometimes is incorrectly referred to as a narrowing of the medullary canal (medullary stenosis), but it is usually a product of reduced endosteal resorption during development combined with greater subperiosteal deposition (3). The distribution of cortical area relative to total subperiosteal area for Late Pleistocene mid-distal humeri (Fig. 1), however, shows no separation of the samples. There is only a nonsignificant tendency (P = 0.104) for the late archaic humans to have greater relative cortical area. The three early modern human samples are indistinguishable (P = 0.209).
Associated with this similarity in relative cortical area within the cross section, the late archaic human sample exhibits significantly greater cortical areas and polar moments of area relative to humeral length (P = 0.038 and 0.042, respectively) (Fig. 1). Given that humeral length can be taken as a surrogate measurement for overall body size and closely approximates upper arm beam length, these data indicate generally greater loading of the upper limb during manipulative activities among these late archaic humans.
Despite major differences in the associated technologies (Middle Paleolithic vs. Early Upper Paleolithic vs. Middle Upper Paleolithic), there is no significant difference across the early modern human samples in relative cortical area (P = 0.409) and only a tendency toward a significant difference (P = 0.067) in relative polar moment of area. Interestingly, however, it is the Levantine Middle Paleolithic-associated early modern humans who exhibit the most gracile humeri. Within the earlier Upper Paleolithic, the few Early Upper Paleolithic humeri cluster in the zone of overlap between the late archaic humans and the Middle Upper Paleolithic sample.
Consequently, it is possible to see a frequency shift in humeral diaphyseal robusticity from late archaic humans to Early Upper Paleolithic ones to Middle Upper Paleolithic ones, a pattern that follows the increasing sophistication and mechanical effectiveness of the associated technologies. The Middle Paleolithic Levantine sample, however, is anomalous with respect to both its time horizon (alternating through oxygen isotope stage 5 with late archaic humans) and its technological associations.
Patterns of Femoral Robusticity
As with the humerus, femoral midshaft cortical area vs. total area is a poor discriminator of Late Pleistocene human samples (Fig. 2), with an overall P = 0.099, an archaic/modern P = 0.104, and an early modern human P = 0.173. There is a visual tendency for the late archaic and Early Upper Paleolithic humans to have the relatively higher values and the Middle Upper Paleolithic ones to have some of the lower values, but the degree of overlap makes any inferences tentative.
When midshaft cortical area and polar moment of area are plotted against femoral length (Fig. 2), there is a highly significant difference between the late archaic and early modern human samples (both P across the early modern human samples (P = 0.312 and 0.422 respectively). These patterns hold despite considerable within sample variation in both relative cortical area and relative polar moment of area, complete overlap between the early modern human samples, and some degree of overlap between the late archaic human remains and each of the early modern human ones.
However, as mentioned above, there are ecogeographically patterned contrasts in body proportions between these samples. In this regard, the late archaic humans exhibit hyperarctic body proportions (especially wide bi-iliac breadths), the Levantine early modern human sample has linear equatorial body proportions, and European earlier Upper Paleolithic human populations exhibit warm temperate to tropical proportions (9, 10, 14-16). Only some of the later ( exhibit proportions similar to modern populations in the same regions (10).
The adjustment of femoral length for body laterality, and hence for much of the variation in body proportions (see Methods), provides a rather different pattern (Fig. 3). With respect to cortical area vs. adjusted femoral length, there is only a suggestion of an overall difference between the samples and of the late archaic humans remaining more robust (P = 0.107 and 0.089, respectively). However, if the differences between only the three older samples (excluding the Middle Upper Paleolithic human remains) are evaluated, any tendency toward differentiation disappears (P = 0.758). In the mechanically more comprehensive polar moment of area, which combines both the quantity and distribution of bone in the cross sections, there is little difference across the samples (P = 0.380). Moreover, deletion of the Middle Upper Paleolithic sample reveals nearly identical proportions across the earlier three samples (P = 0.999). Consequently, once body proportions are included within the analysis, there is no change in lower limb diaphyseal hypertrophy across the two Middle Paleolithic samples (the late archaic human and Levantine early modern human ones) and the European Early Upper Paleolithic sample. It is only with the Middle Upper Paleolithic that one perceives a shift toward lower limb gracilization.
At the same time, there is a clear shift between these late archaic and early modern humans in femoral diaphyseal cross-sectional shape (Fig. 4). The late archaic sample exhibits the pattern characteristic of all archaic Homo since 1.8 millions of years B.P. of having subcircular femoral midshafts with variable development of the linea aspera but lacking a pilaster (17, 19-21). In contrast, the majority of the early modern humans (all except some of the Middle Upper Paleolithic specimens) have femoral midshafts with prominent pilasters and the antero-posterior diameter greater than the medio-lateral one (17, 22). Even though details of the cross-sectional shapes may indicate contrasting developmental baselines for these two hominid grades, the differences in shape also may have biomechanical implications for lower limb loading patterns (as opposed to merely levels of habitual loading). It therefore is potentially profitable to evaluate the distributions of the antero-posterior (Iap) and medio-lateral (Iml) second moments of area of these femoral midshafts.
The plot of Iap vs. Iml (Fig. 5) reflects the subperiosteal pattern previously observed, namely the late archaic humans have highly significantly rounder femoral midshafts than the early modern human ones (P samples remain insignificantly different from each other (P = 0.683). However, if the individual second moments of area are plotted against femoral length adjusted for patterns of body laterality (Fig. 5), the pattern changes. In the plot of Iap vs. adjusted femoral length, the samples are not significantly different (P = 0.535), even though the late archaic humans are on average slightly less structurally reinforced in this plane than are the early modern human ones.
In contrast to both the polar moment of area and Iap vs. adjusted femoral length, Iml vs. adjusted femoral length provides a highly significant difference between the late archaic and early modern human samples (P the latter samples (P = 0.669). These data indicate residual increased medio-lateral reinforcement of the femoral midshaft among the late archaic humans, even after adjustment for body laterality.
Humeral Robusticity and Manipulation.
The pattern of changes in humeral diaphyseal robusticity is in good correlation with several other human biological indicators of the degree to which the human anatomy was directly used in manipulative behavior during the Late Pleistocene, despite contrasts with current interpretations of the associated Paleolithic archeological record.
Between late archaic humans and all of the considered early modern humans, these biological reflections of manipulative behavior include: marked decrease in the use of the anterior dentition for holding and processing objects, decrease in the robusticity of the more caudal cervical vertebral spinous processes, decrease in the size and rugosity of the proximal humeral insertions of the thoraco-humeral musculature, changes in orientation of the radial tuberosity reflecting biceps brachii moment arms, decrease in radial diaphyseal curvature related to pronation, marked reduction of palmar carpal tuberosities related to the size and mechanical advantages of both extrinsic and intrinsic hand musculature, changes in pollical phalangeal proportions related to pollical flexor muscle mechanical advantages, reduction of the crests for the pollical and digiti minimi opponens muscles, and decrease in the breadth of the pollical and middle phalangeal apical tufts (22-27). Although the degree of developmental plasticity of many of these shifts remains uncertain, all of them can be related biomechanically or attritionally to a reduction in the habitual load levels experienced by the upper limb and dentition for manipulative behavior. Humeral diaphyseal robusticity, which is developmentally highly plastic, patterns closely with these other paleontological indicators.
At the same time, there are archeological reflections of significant shifts of manipulative behavior, all related to technology, that correspond generally to these changes between the late archaic human and two Upper Paleolithic samples. These shifts involve increasing leptolithic exploitation of lithic raw materials, combined with the elaboration of the use of organic raw materials (especially bone and antler) into more effective composite technologies. This behavior is seen to some extent in the Early Upper Paleolithic, and it became far more standardized and elaborated through the Middle Upper Paleolithic (28-30). In addition, the Middle Upper Paleolithic saw the introduction of both ceramic and textile technologies (31, 32).
The anomaly remains the Levantine early modern humans. Their associated Middle Paleolithic archeological record is indistinguishable from that of late archaic humans, given normal variation within the Middle Paleolithic (33). The skeletal contrasts are unlikely to be merely stochastic, given both the multiple biological reflections of a contrast in manipulative behavior and the plasticity through the life cycle of several of these reflections, including diaphyseal hypertrophy, muscular attachment areas, and dental attrition. It is more likely that the behavioral dichotomy is not reflected in the currently analyzed aspects of the associated archeological record.
Femoral Robusticity and Locomotion.
Despite some previous assessments inferring a significant decrease in hominid locomotor levels with the emergence of modern humans, these data make it clear that any reduction in lower limb hypertrophy related to habitual adult locomotor levels is after ca. 30 kyr B.P. It therefore follows the establishment of early modern humans across the Old World. Other paleontological reflections of locomotor levels across these groups are more ambiguous in their implications. Gluteal tuberosity breadths change little between late archaic and Early Upper Paleolithic humans, but reduce in both Levantine Middle Paleolithic and Eurasian Middle Upper Paleolithic early modern human samples (23). Quadriceps femoris moment arms reduce uniformly between late archaic and early modern humans, as does pedal phalangeal diaphyseal hypertrophy (23, 34). However, femoral neck shaft angles (inversely reflecting locomotor levels during development) increase markedly in the Levantine early modern sample, less so in the Early Upper Paleolithic sample, but then decrease in the Middle Upper Paleolithic sample (24, 35).
Human foraging locomotor behavior is complex, because the resultant cumulative loads on the lower limb include distances traveled, intensity of activity, and levels and patterns of burden carrying. The archeological reflections of this behavior generally are ambiguous, because there is little necessary correlation between the distances materials have moved and the time, level and/or number of individual locomotor events responsible for the discard patterns on the prehistoric landscape. It is possible that the suggestions of increased social and hence task-related organizational complexity seen in the Middle Upper Paleolithic are functionally correlated with the shift in femoral robusticity, but any connection is likely to be very indirect and difficult to confirm.
The shift between late archaic and early modern humans in medio-lateral diaphyseal reinforcement is difficult to interpret. It is possible that it reflects decreases in pelvic and proximal femoral breadths relative to bone length, less pronounced than those between Early Pleistocene archaic Homo and recent humans (36) but still reflecting proportionately smaller bi-acetabular breadths and femoral biomechanical neck lengths. The alternative interpretation would be that there was a reduction in habitual medio-lateral loading of the lower limb during locomotion, an interpretation that other aspects of lower limb morphology may, but do not necessarily, support (34).
A consideration of the patterns of proximal appendicular diaphyseal cortical bone hypertrophy in Late Pleistocene Homo indicates a mosaic pattern of reduction in habitual load levels. There is a consistent contrast between late archaic and Middle Upper Paleolithic humans indicating a significant reduction in the habitual loads on both the upper and lower limbs. However, in the upper limb the greatest reduction is among the Levantine Middle Paleolithic-associated early modern humans, with the European Early Upper Paleolithic early modern humans exhibiting an intermediate level of hypertrophy. In the lower limb, in contrast, there is no overall difference in structural rigidity among the pre-30 kyr B.P. Late Pleistocene Homo samples. These morphologically plastic reflections of activity levels and patterns correlate well with other appendicular indications of manipulative and locomotor behaviors, but they are more difficult to correlate with the behaviorally often ambiguous Paleolithic archeological record. They nonetheless provide further insight into the nature, if not the genealogical history, of the evolutionary emergence of early modern humans.
* To whom reprint requests should be addressed at: Department of Anthropology, Campus Box 1114, Washington University, St. Louis, MO 63130. e-mail: [email protected]
I am grateful to the many curators who have permitted analysis of human remains in their care, to C.B. Ruff and S.E. Churchill for continued discussions and insight into these evolutionary processes, to T. Kimura, S.E. Churchill, and B. Holt for unpublished data, and to the L.S.B. Leakey Foundation, National Science Foundation, Wenner-Gren Foundation, and Centre National de la Recherche Scientifique for support.
kyr, thousands of years; BIB, bi-iliac breadth; FL, femoral length.
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