such as "Introduction", "Conclusion"..etc
The middle of the Late Pleistocene, between approximately 100,000 and 30,000 years B.P., saw the emergence and establishment of a novel constellation of human biological characteristics. This evolutionary process, known as the "origins of modern humans," led to the presence across the Old World by ca. 30 thousands of years (kyr) B.P. of a new biological complex to the exclusion of the one which, with evolutionary modification, had been present throughout archaic Homo for the previous 1.7+ million years. With minor evolutionary changes through time and space, this biological pattern remains in place in the present world.
Our comprehension of the evolutionary emergence of modern humans rests primarily on our ability to decipher from the paleoanthropological record the patterns and processes of change, whether adaptive or stochastic, regional or global, which enabled one biological pattern to replace a previously highly successful one in a relatively short period of geological time. This paleobiological and paleoanthropological problem, although dependent on neontological uniformitarian patterns for explanatory reference, can be resolved only through the analysis of the prehistoric record, both paleontological and archeological.
With this problem in mind, paleoanthropological research has included attempts to decipher the patterns and degrees of change of functionally relevant aspects of human biology during the Late Pleistocene. Given the nature of the hominid fossil record with its abundance of fossils from Europe and western Asia and the dearth of reasonably complete remains from elsewhere in the Old World, this research has been focused on the paleoanthropological record of the northwestern Old World. However, sufficient remains are now known from less well represented areas to indicate that, once normal stochastic and ecogeographical patterns of interregional variation within species of large-bodied mobile terrestrial mammals are taken into account, the northwestern Old World is generally representative of the more global patterns of human biology.
With this information in mind, we have been investigating patterns of Late Pleistocene hominid diaphyseal appendicular robusticity by using cross-sectional geometry (1-5). Given the high degree of plasticity of the mammalian diaphyseal cortical bone, especially during development (2, 6, 7), this approach provides a paleobiological window on the habitual activity levels of extinct hominid populations. Moreover, potentially contrasting patterns of upper vs. lower limb diaphyseal cortical hypertrophy allow insights into manipulative vs. locomotor activity levels, thus shedding light on two of the most important aspects of hominid behavioral evolution.
A Genealogical Digression
At the same time, the majority of the research on Late Pleistocene hominid evolution beyond philatelic concerns has been focused on the phylogenetic relationships of geographical groups of late archaic and early modern humans. And despite a century of debate on this issue with the progressive introduction of more diverse and higher-quality data and analytical techniques, combined with more global approaches to the problem, there is little consensus. Indeed, the current and ongoing debate on the phylogenetic aspects of modern human emergence appears to be more concerned with hypothesis confirmation than with hypothesis testing.
The past decade has seen the increasing application of human molecular data to issues of modern human origins. However, with one exception (8), the molecular data that have been brought to bear on the issue have no empirical time depth, only probabilistic inferential time depth dependent on both the nature of the data and the layered analytical assumptions behind the various quantitative techniques used to process those data. Moreover, all of these analyses assume a highly uniform stochastic accumulation of genetic change (i.e., a molecular clock that keeps accurate time throughout the last half-million years) and/or geographically uniform human demographic stability throughout the Middle and Late Pleistocene. These assumptions are simply untenable. Any reasonable assessment of molecular data, analytical techniques, and processes makes it highly unlikely that the standard errors of estimates of divergence times are sufficiently small to be useful. The geographical and demographic fluctuations of Pleistocene hominid populations, given both their foraging adaptive patterns and susceptibility to major Pleistocene climatic fluctuations, make any assumptions of uniform population size and distribution implausible, even for short periods of the last half-million years.
The fossil data as it pertains to strictly phylogenetic issues are not much better except in Atlantic Europe, a peripheral cul-de-sac where the transition was very late, relatively abrupt, and probably unrepresentative of more global patterns. Elsewhere across the Old World the human paleontological evidence is sufficiently ambiguous to be interpretable as indicating varying degrees of population continuity, replacement, and/or gene flow. Moreover, the biological bases and hence phylogenetic usefulness of most of the morphological traits commonly used are simply unknown, making it uncertain what is being analyzed.
The simple accumulation of additional neontological and paleontological data and its analysis by current techniques are insufficient for the resolution of these phylogenetic issues. For these reasons, it may well be scientifically more profitable, once one reasonably can define paleontological samples and their distributions in time and space, to look at changing patterns of biology and behavior, no matter what the original genealogical relationships were between the groups.
Materials and Methods
Given the dearth of associated partial skeletons and largely intact long bones from most of the Late Pleistocene of Africa and eastern Asia, the analysis here is focused on two primary samples, one of late archaic humans from the northwestern Old World and the other of early modern humans (defined on the basis of cranio-facial and nondiaphyseal postcranial morphology) from across Eurasia. Despite minor trends through time in facial gracilization among the mostly Middle Paleolithic-associated late archaic humans, they represent a similar group across this geographical area with apparent stasis in most aspects of postcranial morphology. The early modern human group, however, combines three groups. The first is of Levantine Middle Paleolithic hominids from middle of oxygen isotope stage 5, and it may well represent [based on body proportions (9, 10) and associated fauna (11)] a temporary dispersal into the region from northeastern Africa. The second is a small sample of European Early (pre-30 kyr B.P.) Upper Paleolithic humans, and the third is of east Asian, Near Eastern, and European Middle Upper Paleolithic-associated humans, from kyr B.P. Specimens with pathological lesions that appear to have altered habitual biomechanical load levels (e.g., Neandertal 1 humeri) were not included.
To maximize the accuracy and biomechanical relevance of the analysis, the diaphyses of long bones were compared by using cross-sectional areas and second moments of area (also known as area moments of inertia), analyzed in the context of variation in body proportions. Experience has shown that analyses using only external measures of diaphyseal size and lacking the integration of ecogeographical patterning in body shape can provide misleading results. Appendicular robusticity therefore was assessed by computing cross-sectional geometric parameters of all of the available humeri and femora, here presented for the humeral mid-distal (35%) diaphysis and the femoral midshaft (50%). Given the near universality of right-side upper limb dominance among these extinct hominids, associated with variable levels of humeral asymmetry, only right humeri were considered.
Cross sections were reconstructed noninvasively by using transcribed molds of sub-periosteal contours combined with bi-planar radiography for the parallax-adjusted determination of cortical thicknesses, from which the endosteal contours were interpolated. Cross-sectional parameters (total and cortical area, anatomically oriented and maximum-minimum second moments of area, and the polar moment of area) were computed from digitized cross sections by using a PC version of SLICE (12, 13). In this framework, cortical area represents structural resistance to axial loading, second moments of area indicate resistance to bending in the plane in question, and polar moments of area approximate strength relative to torsional forces. Furthermore, because the polar moment of area is the sum of any two perpendicular second moments of area, it also provides an indication of overall biomechanical structural integrity.
The resultant parameters for the samples are compared graphically, for the humerus and initially for the femur by using lne-lne plots of the resultant values. For the humerus, given its normally non-weight-bearing role in humans, the logarithmic transformation appears to be adequate to adjust for allometric effects, especially of cross-sectional measures vs. bone length. For the femur, however, load levels are dependent on both body mass (weight and momentum) and beam characteristics. To correct for documented variance of ecogeographically patterned Late Pleistocene human body proportions (9, 10, 14-16), bi-iliac breadth was used to represent variance in body laterality, and femoral length was used for both beam length and to represent stature. Given the relative constancy of bi-iliac breadth within ecogeographically defined human groups (14), bi-iliac breadth was estimated from femoral length for specimens lacking sufficiently complete pelvic remains for direct determination of bi-iliac breadth. This calculation was done by using the mean associated bi-iliac breadth (BIB) and femoral length (FL) of the larger relevant sample, such that:
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