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The earliest stages in the evolution of the human body (2–6 Ma) involved a gradual adaptation to terrestrial bipedality through various structural modifications. Body size in these early ancestors was smaller, but sexual dimorphism larger than in modern humans. With the evolution of Homo erectus about 1.5–2.0 Ma, essentially modern body proportions and levels of sexual dimorphism were established. Body size reached a peak between about 50 000 and 500 000 years ago and has declined on average since then. Body shape variation among both earlier Homo and modern humans is strongly conditioned by climatic adaptation.
- The Bipedal Transformation
- Variation in Body Size
- Variation in Body Shape
- Body Size as a Baseline for Other Comparisons
Major changes in body form have taken place during human evolution, related to the initial adoption and increasing commitment to bipedal locomotion and continuing adaptations to climatic, dietary, and other environmental factors. Although all modern humans share a similar body plan, there is still substantial variability in body size and body shape within and between populations that is in part a reflection of environmental adaptation. This research paper reviews the evidence for both the evolution and modern distribution of body form in humans and its behavioral and ecological significance.
The Bipedal Transformation
It is widely agreed that the transition from quadrupedal to bipedal locomotion was one of the most important innovations in human evolution; indeed, bipedality defines our lineage better than any other single characteristic (Clark, 1964; Aiello and Dean, 1990; Fleagle, 1999). Efficient bipedal locomotion requires changes in a number of anatomical characteristics, including restructuring of the lower limb and vertebral column, and other alterations. Recent evidence indicates that some form of bipedality may have been practiced by hominins as early as 6 Ma (Senut et al., 2001; Richmond and Jungers, 2008), and possibly even 7 Ma (Zollikofer et al., 2005), very close to the time when humans split off from African apes.
However, it is also becoming increasingly clear that the transition to a fully modern form of terrestrial bipedality occurred much later in hominin evolution, perhaps only with the appearance of Homo erectus about 1.5–1.8 Ma (Walker and Leakey, 1993; Ruff, 2008). Hominins dating from about 2.0 to 4.5 Ma, including Ardipithecus ramidus, Australopithecus anamensis, Australopithecus africanus, Australopithecus garhi, and Australopithecus sediba, as well as some extending more recently (between 1.2 and 2.0 Ma), including Paranthropus robustus, Paranthropus boisei, and Homo habilis, exhibit a mosaic of locomotor characteristics that indicate either an altered form of terrestrial bipedality and/or retained arboreal (tree climbing) capabilities. These include, in some taxa, a foot better adapted for climbing than in modern humans (Clarke and Tobias, 1995; Lovejoy et al., 2009a; Zipfel et al., 2011; DeSilva et al., 2012; Haile-Selassie et al., 2012), long and robust upper limbs and a more superiorly oriented shoulder joint, again for climbing (Clark et al., 1984; Heinrich et al., 1993; White et al., 1993; Kimbel et al., 1994; Leakey et al., 1995; McHenry and Berger, 1998; Lovejoy et al., 2009b; Ruff, 2009; Green and Alemseged, 2012), and variations in foot, hip, and pelvic structure that indicate some differences in posture during walking compared to modern humans (Ruff, 1998; Lovejoy et al., 2009c; Zipfel et al., 2011; Ruff and Higgins, 2013).
Thus, the adoption of terrestrial bipedality was gradual, and probably involved increasing reliance on this form of locomotion while still retaining the ability to use trees, e.g., for escape from predators or for food procurement. There are many theories on why and how human bipedalism evolved, including ecological and dietary shifts, and changes in social organization (for a review, see Fleagle, 2013). While distinctively human proportions of the body, such as forelimb to hindlimb size, were established by 1.5 Ma in H. erectus (Walker and Leakey, 1993; Ruff, 2008), within this basic body plan variations in other aspects of body size and shape are apparent throughout the more recent fossil record and among living humans.
Variation in Body Size
Body mass (weight) for fossil specimens has been estimated using various parts of the skeleton, including craniodental dimensions, but it is clear that postcranial dimensions provide the least biased and potentially most accurate means for reconstructing body size, especially in an evolutionary lineage where changes in relative tooth and cranial size have been dramatic (Pilbeam and Gould, 1974). Two basic types of methods can be distinguished: a mechanical approach, which uses features that are related to the mechanical load-bearing function of the skeleton, such as hip joint (femoral head) size, and a morphometric approach, in which features that actually reflect the size and shape of the postcranial skeleton are used (Auerbach and Ruff, 2004). For the latter, it has been shown that stature, estimated from limb bone length, and body breadth, measured across the pelvis, together provide a good estimate of body mass (Ruff, 2000; Auerbach and Ruff, 2004; Ruff et al., 2005). Correlations between body masses determined using both techniques in the same specimens are good, indicating little bias in results (Ruff et al., 1997; Auerbach and Ruff, 2004).
Changes in estimated body mass over 6 My of human evolution are shown in Figure 1, along with a set of values for modern living humans for comparison. A total of 148 premodern individual specimens are included. In most of these, body mass was estimated from hip joint size (femoral head breadth measured directly or in a few cases estimated from acetabular breadth), using equations derived from modern humans (Ruff, 2010). In 26 cases, it was also possible to estimate body mass from stature and pelvic breadth, again using equations derived from modern humans (Ruff et al., 2005). In these specimens, an average of the two estimates was used (as in previous studies, the two techniques produced very similar results). Unlike in some previous studies (e.g., Ruff et al., 1997), no pelvic breadths were estimated, due to recent evidence that the patterning of relative pelvic breadth among fossil hominins was more complex than previously modeled (Trinkaus, 2009; Ruff, 2010). Because of the much higher density of data available for more recent periods, the data are plotted on a logarithmic temporal scale. Specimens are divided into Homo (Homo sp., H. erectus, and Homo sapiens, including Neandertals) and non- Homo (see earlier text for taxa). The modern living data are 53 sex-specific sample means (Ruff, 2010). Both modern and earlier data are from the Old World (except for two modern Inupiat samples) and include samples or individuals from both higher and lower latitudes as well as both sexes; all are adults.
Figure 1. Temporal changes in body mass (weight) over the past 6 My of hominin evolution (note logarithmic time scale). Body masses in fossil specimens determined primarily from femoral head size (see text). Filled triangles: non-Homo species; open triangle: Gona specimen, of uncertain taxonomy (see Ruff, 2010); open circles: Homo; x’s: modern humans (worldwide sample of male and female population means). Quadratic equation used to fit regression line through premodern Homo. Dotted line indicates modern median.
Several trends are apparent in Figure 1. First, body size was relatively small (21–53 kg) in the earliest period of human evolution and markedly increased with the appearance of H. erectus at about 2 Ma. This likely coincided with a number of other biological and behavioral changes, including a complete commitment to terrestrial bipedality (also see earlier text), increased diurnal travel through open country, and possibly an increased reliance on hunting (or at least foraging range) (Wheeler, 1992; McHenry, 1994; Ruff, 2009). Note that one specimen of uncertain attribution – the pelvis from Gona, Ethiopia (Simpson et al., 2008) – falls well under the known body size range of Homo, further supporting its assignment to a non-Homo species, most likely Australopithecus boisei (Ruff, 2010).
The second general observation apparent from Figure 1 is that body size in premodern Homo was generally above the average for modern (living) human populations. The median body mass for the modern humans is 57 kg; 87% of the earlier Homo specimens are heavier than this, with an overall median of 68 kg (19% heavier). Part of this may be attributable to a male bias in the fossil record: of the 87 out of 113 earlier Homo specimens that can be reliably sexed, 63% are male. However, this pattern is evident even within sex: earlier Homo males average 20% heavier and females 13% heavier than modern males and females. Another possible factor is latitude: as discussed later in the text, among modern humans (and many other mammalian species) there is an ecogeographic cline of increased body mass in colder climates, and a majority of the premodern specimens in Figure 2 (especially among the later ones) are from higher latitudes. Figure 2 is a plot of temporal trends in body mass over the past 35 000 years among only higher-latitude males (mainly European), compared to modern higher-latitude males. The linear regression on time (not including the modern data) is negative and near-significant (p ¼ .09). Higher-latitude males in the Early- Middle Upper Paleolithic (16 000–35 000 years ago) average about 9% heavier than modern higher-latitude males. Median body mass in males from the Late Upper Paleolithic (10 000–15 000 years ago) is very similar to that of the living sample.
Figure 2. Change in body mass among higher-latitude males over the past 35 000 years. Body masses and symbols as in Figure 1. Linear regression fit through premodern sample. Dotted line indicates modern median.
This highlights another aspect of temporal trends in body size: maximum body mass was apparently reached in the late Middle and early Late Pleistocene, i.e., between about 500 000 and 50 000 years ago, after which it declined to modern human values by the end of the Pleistocene (10 000 years ago). Thus, a quadratic curve best fits the premodern Homo data in Figure 1 (as determined by the akaike information criterion (AIC)). Increased body size during the late Middle and early Late Pleistocene may be related to foraging strategies such as an increase in big game hunting (e.g., Thieme, 1997) as well as more common occupation of high-latitude environments (see below). The later decrease in body size may be attributable to changing technology, resource exploitation, and demographic shifts in the terminal Pleistocene that no longer selected as strongly for large body size (Frayer, 1984; Richards et al., 2001; Milisauskas, 2002; Lagerlöf, 2007). There is evidence that average body size continued to decline through the Neolithic, at least in Europe (Frayer, 1984; Niskanen et al., 2012), perhaps because of decreased nutritional quality and/ or increased disease transmission associated with the agricultural revolution (Cohen and Armelagos, 1984). Body size has increased dramatically over the past century in many populations, probably due to improvements in the environment (Eveleth and Tanner, 1990; Katzmarzyk and Leonard, 1998), although the opposite trend has been observed in some disadvantaged populations (Tobias, 1985).
Sexual dimorphism (i.e., differences between sexes) in body size is another important characteristic of a species with many potential behavioral and ecological implications (e.g., McHenry, 1994; Plavcan and Van Schaik, 1997). Recent analyses of sexual dimorphism in body mass, using methods similar to those described earlier, indicate elevated levels of dimorphism in hominin taxa prior to the appearance of H. erectus, with an average male/female ratio of about 1.3 or higher (Ruff, 2002, 2010). Species within Homo are characterized by reduced levels of dimorphism, similar to those of modern humans, i.e., an average male/female ratio of about 1.1–1.2. This implies changes in ecology and behavior with the evolution of Homo. It also implies that in at least some aspects of social organization, early Homo may not have differed markedly from modern hunter-gatherers.
Variation in Body Shape
One aspect of the environment that has probably had a profound effect on human body shape is climate. Systematic relationships between climate and body form among homeothermic animals have been recognized for more than a century and have been codified as Bergmann’s and Allen’s ecogeographical ‘Rules’ (Mayr, 1963). Bergmann’s Rule states that within a species or closely related group of species, populations living in colder climates will be larger in body mass than those in warmer climates, while Allen’s Rule states that colder climates will be associated with shorter extremities and warmer climates with longer extremities. Both of these observations are actually consequences of a more general relationship between body mass and body surface area, in which both a larger body mass and relatively shorter extremities decrease the ratio of surface area to body mass, thus conserving heat, and vice versa. The same principle can also be applied to the overall shape of the human trunk: it can be shown that an absolutely wide trunk (and thus wide body) will lead to a decreased surface area/body mass ratio, regardless of body height, while an absolutely narrow trunk will lead to an increase in the ratio, explaining the narrower bodies of tropical populations and wider bodies of arctic populations (Ruff, 1994).
Ecogeographical clines that conform to physiological (climatic) expectations have been demonstrated for many aspects of living human body form, including body mass, body breadth, relative limb length, and direct estimates of surface area to body mass (Roberts, 1978; Ruff, 1994; Cowgill et al., 2012). It is more difficult to evaluate such morphological variability in our fossil ancestors, but recent paleoanthropological discoveries combined with developments in methodology have provided some new insights.
All early hominins apparently had relatively wide bodies compared to humans living in the same areas today (Ruff, 2010), which may be the result of an altered birth process in which the neonate’s head did not rotate during birth as it does today (Tague and Lovejoy, 1986; Ruff, 1995, 2010). However, even among earlier human ancestors, there is evidence for latitudinal variation in body breadth similar to that observed among modern humans, with specimens from higher latitudes exhibiting wider bodies than those from lower latitudes (Ruff, 2010). Relative limb length is also affected by locomotion, so this aspect of morphology is more difficult to evaluate in terms of climatic adaptation per se in early hominins. However, among H. erectus and later taxa, where terrestrial bipedality was well established, lower-latitude specimens are characterized by relatively longer extremities, and higher-latitude specimens by shorter extremities, again matching ecogeographic predictions (Trinkaus, 1981; Ruff and Walker, 1993; Ruff, 1994; Holliday, 1997; Rosenberg et al., 2006). In fact, in some ways earlier humans appear to exhibit extreme morphological adaptations to climate, i.e., ‘hyperarctic’ and ‘hypertropical’ body forms (Ruff and Walker, 1993; Ruff, 1994; Holliday, 1997; Weaver, 2003), possibly as a result of less efficient cultural buffering against the environment.
Such observations can also inform interpretations of past population movements and in situ adaptations. Occupants of Western Europe during the Middle and early Late Pleistocene were subjected to periodic episodes of extreme cold, i.e., glaciations; cool conditions prevailed even during some interglacial periods (Trinkaus et al., 1999). Body breadths were correspondingly wide, either absolutely or relative to stature, and limb lengths relatively short (Ruff, 1994; Holliday, 1997; Arsuaga et al., 1999; Ruff, 2010; Walker et al., 2011a,b). About 25 000–45 000 years ago, archaic humans (Neandertals) in Europe were gradually replaced by ‘early anatomically modern humans’ (EAM) (Finlayson et al., 2006; Benazzi et al., 2011; Higham et al., 2011). These EAM populations were much taller, relatively less wide-bodied, and had relatively longer limbs (Ruff, 1994; Holliday, 1997), all evidence of a more tropical origin. Over time, the body proportions of European EAMs became less ‘tropical,’ increasing in relative body breadth and decreasing in relative limb length; until by the Late Upper Paleolithic (10 000–19 000 years ago), they were essentially indistinguishable from those of modern Europeans (Ruff, 1994; Holliday, 1997). This change can be seen, at least in part, as evidence of environmental adaptation to colder climatic conditions following migration from farther south, i.e., northeast African and/or the Middle East (White et al., 2003). Anatomical evidence of previous climatic adaptation to cold conditions followed by a migration over the Bering Sea Land Bridge is also apparent among early inhabitants of the New World, shedding new light on the origins of these populations (Ruff, 1994; Auerbach, 2012).
Body Size as a Baseline for Other Comparisons
In addition to its biological and cultural significance, body size is important in human evolutionary studies because it is often used as a ‘denominator’ against which to evaluate other physical traits. One prime example is brain size, which is commonly expressed as a ratio against body mass, or encephalization quotient (EQ) (e.g., Pilbeam and Gould, 1974). EQ increased in early Homo (1.5–2.0 Ma) relative to earlier taxa, but then remained constant for at least a million years (to 0.5 Ma), after which it increased again, exponentially, to modern values (McHenry, 1994; Ruff et al., 1997). Skeletal robusticity has also been evaluated relative to body size, and shown to follow an inverse trend relative to brain size (Ruff et al., 1993), which may reflect the increasing importance of technology and decreasing selective advantage of physical strength among modern humans.
Finally, the body size of earlier humans can be used as a baseline for contextualizing the health of modern populations. Comparing present-day measures of stature and body mass with those of ancestral populations may give better estimates of potential body size limits and provide guidance in setting nutritional and other health standards (WHO, 1995).
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