Human evolution

The common ancestor of modern humans and the great apes is estimated to have lived between 5 and 8 Myrs ago, but the earliest evidence in the human, or hominid, fossil record is Ardipithecus ramidus, from a 4.5 Myr Ethiopian site. This genus was succeeded by Australopithecus, within which four species are presently recognised. All combine a relatively primitive postcranial skeleton, a dentition with expanded chewing teeth and a small brain. The most primitive species in our own genus, Homo habilis and Homo rudolfensis, are little advanced over the australopithecines and with hindsight their inclusion in Homo may not be appropriate. The first species to share a substantial number of features with later Homo is Homo ergaster, or ‘early African Homo erectus’, which appears in the fossil record around 2.0 Myr. Outside Africa, fossil hominids appear as Homo erectus-like hominids, in mainland Asia and in Indonesia close to 2 Myr ago; the earliest good evidence of ‘archaic Homo’ in Europe is dated at between 600–700 Kyr before the present. Anatomically modern human, or Homo sapiens, fossils are seen first in the fossil record in Africa around 150 Kyr ago. Taken together with molecular evidence on the extent of DNA variation, this suggests that the transition from ‘archiac’ to ‘modern’ Homo may have taken place in Africa.     

A reappraisal of variation in hominid mandibular corpus dimensions

The relationship between breadth and height of the mandibular corpus has been investigated in a sample of 77 hominid mandibles. An interspecific allometric increase in robusticity with size occurs between four taxonomic subgroups of Australopithecus, but subgroups of Homo vary in robusticity while differing little in size. Within taxonomic subgroups, variation in breadth is not significantly related to variation in height among the “gracile” australapithecines; however, it is isometrically related to height in the “robust” australopithecines and bears an allometric relationship to height in Homo. Thus, robusticity, in conjunction with size, may provide a useful indicator of the taxonomic affinities of hominid mandibles.     

Climatic adaptation and hominid evolution: The thermoregulatory imperative

Anthropologists have long recognized the existence among modern humans of geographical variations in body form that parallel climatic gradients, part of more general zoological phenomena commonly referred to as Bergmann's or Allen's “Rules”. These observations have rarely been applied to earlier hominids, in part because fossil skeletons usually are so incomplete that it is difficult to reconstruct body morphology accurately. However, within the past two decades two early hominids have been discovered that preserve enough of the skeleton to allow confident assessment of their body size and shape. Comparison of these specimens—the Australopithecus afarensis A.L. 288-1 (“Lucy”) and the Homo erectus KNM-WT 15000—with others that are less complete make it evident that the evolution of Homo erectus was accompanied by not only a marked increase in body size, but also a similarly dramatic increase in the linearity of body form. That is, relative to their heights, small australopithecines had very broad bodies, whereas large early Homo had narrow bodies. This difference in body form cannot be explained on the basis of obstetric or biomechanical factors, but is consistent with thermoregulatory constraints on body shape. Specifically, to maintain the same ratio of body surface area to body mass, which is an important thermoregulatory mechanism, increases in height should be accompanied by no change in body breadth, which is exactly what is seen in comparisons of A.L. 288-1 and KNM-WT 15000. Conversely, Neandertals living in colder climates had much wider bodies, which are adaptive for heat retention. Differences in limb length proportions between fossil hominids are also consistent with thermoregulatory principles and the geographic variation observed among modern humans. Climatic adaptation during hominid evolution may have wide-ranging implications, not only with regard to interpreting body morphology, but also in relation to ecological scenarios, population movements, and the evolution of the brain.     

How big were early hominids?

The recent discovery of new postcranial fossils, particularly associated body parts, of several Plio-Pleistocene hominids provides a new opportunity to assess body size in human evolution.¹ Body size plays a central role in the biology of animals because of its relationship to brain size, feeding behavior, habitat preference, social behavior, and much more. Unfortunately, the prediction of body weight from fossils is inherently inaccurate because skeletal size does not reflect body size exactly and because the fossils are from species having body proportions for which there are no analogues among modern species. The approach here is to find the relationship between body size and skeletal size in ape and human specimens of known body weight at death and to apply this knowledge to the hominid fossils, using a variety of statistical methods, knowledge of the associated partial skeletons of the of early hominids, formulae derived from a modern human sample, and, finally, common sense. The following modal weights for males and females emerge: Australopithecus afarensis, 45 and 29 kg; A. africanus, 41 and 30 kg; A. robustus, 40 and 32 kg; A. boisei, 49 and 34 kg; H. habilis, 52 and 32 kg. The best known African early H. erectus were much larger with weights ranging from 55 kg on up. These estimates imply that (1) in the earliest hominid species and the “robust” australopithecines body sizes remained small relative to modern standards, but between 2.0 and 1.7 m.y.a. there was a rapid increase to essentially modern body size with the appearance of Homo erectus; (2) the earliest species had a degree of body size sexual dimorphism well above that seen in modern humans but below that seen in modern gorillas and orangs which implies (along with other evidence) a social organization characterized by kin-related, multi-male groups with females who were not kin-related; (3) relative brain sizes increased through time; (4) there were two divergent trends in relative cheek-tooth size—a steady increase through time from A. afarensis to A. africanus to the “robust” australopithecines, and a decrease beginning with H. habilis to H. erectus to H. sapiens.     

Cranial capacity in hominid evolution

We present an analysis of cranial capacity of 118 hominid crania available from the literature. The crania belong to both the genusAustralopithecus andHomo and provide a clear outline of hominid cranial evolution starting at more than 3 million years ago. Beginning withA. afarensis there is a clear increase in both absolute and relative brain size with every successive time period.H.s. neandertal has an absolutely and relatively smaller brain size (1412cc, E.Q.=5.6) than fossil modernH.s. sapiens (1487cc, E.Q.=5.9). Three evolutionary models of hominid brain evolution were tested: gradualism, punctuated equilibrium, and a mixed model using both gradualism and punctuated equilibrium. Both parametric and non-parametric analyses show a clear trend toward increasing brain size withH. erectus and a possible relationship within archaicH. sapiens. An evolutionary stasis in cranial capacity could not be refuted for all other taxa. Consequently, the mixed model appears to more fully explain hominid cranial capacity evolution. However, taxonomic decisions could directly compromise the possibility of testing the evolutionary mechanisms hypothesized to be operating in hominid brain expansion.     

Dental microwear texture analysis and diet in the Dmanisi hominins

Reconstructions of foraging behavior and diet are central to our understanding of fossil hominin ecology and evolution. Current hypotheses for the evolution of the genus Homo invoke a change in foraging behavior to include higher quality foods. Recent microwear texture analyses of fossil hominin teeth have suggested that the evolution of Homo erectus may have been marked by a transition to a more variable diet. In this study, we used microwear texture analysis to examine the occlusal surface of 2 molars from Dmanisi, a 1.8 million year old fossil hominin site in the Republic of Georgia. The Dmanisi molars were characterized by a moderate degree of surface complexity (Asfc), low textural fill volume (Tfv), and a relatively low scale of maximum complexity (Smc), similar to specimens of early African H. erectus. While caution must be used in drawing conclusions from this small sample (n = 2), these results are consistent with continuity in diet as H. erectus expanded into Eurasia.     

Body size and body shape in early hominins - implications of the Gona Pelvis

Discovery of the first complete Early Pleistocene hominin pelvis, Gona BSN49/P27, attributed to Homo erectus, raises a number of issues regarding early hominin body size and shape variation. Here, acetabular breadth, femoral head breadth, and body mass calculated from femoral head breadth are compared in 37 early hominin (6.0-0.26 Ma) specimens, including BSN49/P27. Acetabular and estimated femoral head sizes in the Gona specimen fall close to the means for non-Homo specimens (Orrorin tugenesis, Australopithecus africanus, Paranthropus robustus), and well below the ranges of all previously described Early and Middle Pleistocene Homo specimens. The Gona specimen has an estimated body mass of 33.2 kg, close to the mean for the non-Homo sample (34.1 kg, range 24-51.5 kg, n = 19) and far outside the range for any previously known Homo specimen (mean = 70.5 kg; range 52-82 kg, n = 17). Inclusion of the Gona specimen within H. erectus increases inferred sexual dimorphism in body mass in this taxon to a level greater than that observed here for any other hominin taxon, and increases variation in body mass within H. erectus females to a level much greater than that observed for any living primate species. This raises questions regarding the taxonomic attribution of the Gona specimen. When considered within the context of overall variation in body breadth among early hominins, the mediolaterally very wide Gona pelvis fits within the distribution of other lower latitude Early and Middle Pleistocene specimens, and below that of higher latitude specimens. Thus, ecogeographic variation in body breadth was present among earlier hominins as it is in living humans. The increased M-L pelvic breadth in all earlier hominins relative to modern humans is related to an increase in ellipticity of the birth canal, possibly as a result of a non-rotational birth mechanism that was common to both australopithecines and archaic Homo.     

Allometric patterns of cranial bone thickness in fossil hominids

The interspecific allometry of five measures of total cranial bone thickness is examined in 10 extant catarrhine genera and two fossil hominid samples representing A. africanus and Asian H. erectus. Analysis of the modern sample shows that most interspecific variation in vault thickness can be accounted for by variation in body size. Correlation values are moderate to high (r = 0.75–0.98), and all variables exhibit positive allometry. The bone thickness:body mass relationship of modern humans broadly conforms with that of other primates. However, in the distribution of relative thickness throughout the skull, H. sapiens is distinguished by relative thickening of the parietal and extreme relative thinning of the temporal squama. The bone thickness:body mass relationship in the two early hominid species is examined using published mean body weight estimates generated from post-cranial predictor variables. A. africanus exhibits great similarity to modern humans in its relation to the catarrhine regression data and in the distribution of relative thickness throughout the skull. H. erectus also shows a modern human-like pattern in the distribution of its relative thickness; however, its bone thickness:body mass relationship is dissimilar to that displayed by all other taxa, including the other hominid species. On the basis of these results, it is suggested that the published body weight estimate assigned to H. erectus greatly underestimates actual mean body size for Asian members of this species. © 1996 Wiley-Liss, Inc.     

Cladometric analysis of Pliocene hominids

Quantification of individual crown features allows maximization of information retrieval from isolated hominid molars. The Lukeino specimen demonstrates phenetic affinity to Pan; the Lothagam fossil appears closer to a hypothetical ancestral hominid morphotype than the Laetolil specimens. Consideration of 41 metric features in a cladistic framework establishes Australopithecus afarensis as the sister taxon of Homo and of later australopithecines.     

Estimating hominid life history: the critical interbirth interval

Unlike any great apes, humans have expanded into a wide variety of habitats during the course of evolution, beginning with the transition by australopithecines from forest to savanna habitation. Novel environments are likely to have imposed hominids a demographic challenge due to such factors as higher predation risk and scarcer food resources. In fact, recent studies have found a paucity of older relative to younger adults in hominid fossil remains, indicating considerably high adult mortality in australopithecines, early Homo, and Neanderthals. It is not clear to date why only human ancestors among all hominoid species could survive in these harsh environments. In this paper, we explore the possibility that hominids had shorter interbirth intervals to enhance fertility than the extant apes. To infer interbirth intervals in fossil hominids, we introduce the notion of the critical interbirth interval, or the threshold length of birth spacing above which a population is expected to go to extinction. We develop a new method to obtain the critical interbirth intervals of hominids based on the observed ratios of older adults to all adults in fossil samples. Our analysis suggests that the critical interbirth intervals of australopithecines, early Homo, and Neanderthals are significantly shorter than the observed interbirth intervals of extant great apes. We also discuss possible factors that may have caused the evolutionary divergence of hominid life history traits from those of great apes.     

Biomechanics of the hip and birth in early Homo

A complex of traits in the femur and pelvis of Homo ereclus and early “erectus-like” specimens has been described, but never satisfactorily explained. Here the functional relationships between pelvic and femoral structure in humans are explored using both theoretical biomechanical models and empirical tests within modern samples of diverse body form (Pecos Amerindians, East Africans). Results indicate that a long femoral neck increases mediolateral bending of the femoral diaphysis and decreases gluteal abductor and hip joint reaction forces. Increasing biacetabular breadth along with femoral neck length further increases M-L bending of the femoral shaft and maintains abductor and joint reaction forces at near “normal” levels. When compared to modern humans, Homo erectus and early “erectus-like” specimens are characterized by a long femoral neck and greatly increased M-L relative to A-P bending strength of the femoral shaft, coupled with no decrease in hip joint size and a probable increase in abductor force relative to body size. All of this strongly suggests that biacetabular breadth as well as femoral neck length was relatively large in early Homo. Several features preserved in early Homo partial hip bones also indicate that the true (lower) pelvis was very M-L broad, as well as A-P narrow. This is similar to the lower pelvic shape of australopithecines and suggests that nonrotational birth, in which the newborn's head is oriented transversely through the pelvic outlet, characterized early Homo as well as Australopithecus. Because M-L breadth of the pelvis is constrained by other factors, this may have limited increases in cranial capacity within Homo until rotational birth was established during the late Middle Pleistocene. During or after the transition to rotational birth biacetabular breadth decreased, reducing the body weight moment arm about the hip and allowing femoral neck length (abductor moment arm) to also decrease, both of which reduced M-L bending of the proximal femoral shaft. Variation in femoral structural properties within early Homo and other East African Early Pleistocene specimens has several taxonomic and phylogenetic implications. © 1995 Wiley-Liss, Inc.     

Origin of the Genus <Emphasis Type="Italic">Homo</Emphasis>

The origin of the genus Homo in Africa signals the beginning of the shift from increasingly bipedal apes to primitive, large-brained, stone tool-making, meat-eaters that traveled far and wide. This early part of the human genus is represented by three species: Homo habilis, Homo rudolfensis, and Homo erectus. H. habilis is known for retaining primitive features that link it to australopiths and for being the first stone tool makers. Little is known about H. rudolfensis except that it had a relatively large brain and large teeth compared to H. habilis and that it overlapped in time and space with other early Homo. Our understanding of the paleobiology and evolution of the larger-brained H. erectus is enhanced due to its rich fossil record. H. erectus was the first obligate, fully committed biped, and with a body adapted for modern striding locomotion, it was also the first in the human lineage to disperse outside of Africa. The early members of the genus Homo are the first to tip the scale from the more apish side of our evolutionary history toward the more human one.     

Developmental age and taxonomic affinity of the Mojokerto child,Java, Indonesia

An increasing number of claims place hominids outside Africa and deep in Southeast Asia at about the same time that Homo erectus first appears in Africa. The most complete of the early specimens is the partial child's calvaria from Mojokerto (Perning I), Java, Indonesia. Discovered in 1936, the child has been assigned to Australopithecus and multiple species of Homo, including H. modjokertensis, and given developmental ages ranging from 1–8 years. This study systematically assesses Mojokerto relative to modern human and fossil hominid growth series and relative to adult fossil hominids. Cranial base and vault comparisons between Mojokerto and H. sapiens sapiens (Hss) (n = 56), Neandertal (n = 4), and H. erectus (n = 4) juveniles suggest a developmental age range between 4 and 6 years. This range is based in part on new standards for assessing the relative development of the glenoid fossa. Regression analyses of vault arcs and chords indicate that H. erectus juveniles have more rounded frontals and less angulated occipitals than their adult counterparts, whereas Hss juveniles do not show these differences relative to adults. The growth of the cranial superstructures and face appear critical to creating differences in vault contours between H. erectus and Hss. In comparison with adult H. erectus and early Homo (n = 27) and adult Hss (n = 179), the Mojokerto child is best considered a juvenile H. erectus on the basis of synapomorphies of the cranial vault, particularly a metopic eminence and occipital torus, as well as a suite of characters that describe but do not define H. erectus, including obelion depression, supratoral gutter, postorbital constriction, mastoid fissure, lack of sphenoid contribution to glenoid fossa, and length and breadth ratios of the temporomandibular joint. Mojokerto is similar to other juvenile H. erectus in the degree of development of its cranial superstructures and its vault contours relative to adult Indonesian specimens. The synapomorphies which Mojokerto shares with H. erectus are often considered autapomorphies of Asian H. erectus and confirm the early establishment and long-term continuity of the Asian H. erectus bauplan. This continuity does not, however, necessarily reflect on the pattern of origin of modern humans in the region. Am J Phys Anthropol 102:497–514, 1997. © 1997 Wiley-Liss, Inc.     

The australopithecines in review

Over the past 75 years since the discovery of the first australopithecine at Taung in southern Africa there has been a growing realisation that there is no simple, linear ancestor-descendant relationship connecting the australopithecines to laterHomo. There are currently at least ten recognised species of australopithecine, including two species of earlyHomo, that have been recently transferred to the genusAustralopithecus. These known species span the period between about 4.2-1.2 Ma and throughout the majority of this period there are multiple contemporaneous hominin species in eastern and southern Africa. This contribution reviews current knowledge about the australopithecine species and their inferred relationships to each other and to the genusHomo. At present it is impossible to resolve the phylogenetic relationships of the australopithecines with any degree of confidence. There is a growing realisation of the ‘bushy’ nature of hominin evolution throughout the australopithecine period and also of the inevitability that additional early hominin species remain to be discovered. Paper submitted for inclusion in the Proceedings of the International Symposium of the Ramón Areces Foundation “Evolution of the Human Family: State of the Art” held in Madrid on the 11–13 March, 1998     

Heterochronic processes in human evolution: An ontogenetic analysis of the hominid pelvis

Changes in pelvic shape in human ontogeny and hominid phylogeny suggest that the heterochronic processes involved differ greatly from the neotenic process traditionally described in the evolution of the skull. The morphology of 150 juvenile and adult pelves of African apes, 60 juvenile and adult pelves of modern humans, two adult pelves and a juvenile hip bone of australopithecines (Sts 14, AL 288, MLD 7) was studied. Multivariate results, ontogenetic allometries, and growth curves confirm that the pelvic growth pattern in humans differs markedly from those of the African apes. The results permit the following conclusions. First, the appearance of a new feature (acetabulo-cristal buttress and cristal tubercle) at the time of human birth allows the addition of traits, such as the attainment of a proportionally narrower pelvis, with more sagittally positioned iliac blades. Pelvic proportions and orientation change progressively in early childhood as bipedalism is practiced. Other changes in pelvic proportions occur later with the adolescent growth spurt. Second, comparison of juvenile and adult australopithecines to modern humans indicates that 1) some pelvic traits of adult Australopithecus resemble those of neonate Homo; 2) the pelvic growth of Australopithecus was probably closer to that of apes, than to that of humans; and 3) prolonged growth in length of hindlimb and pelvis after sexual maturity seems to be a unique feature of Homo. The position of the acetabulo-cristal buttress and of the cristal tubercle on the ilium are similar in adult Australopithecus and neonate Homo suggesting that this feature may have been displaced later during hominid evolution. Progressive displacement of the acetabulo-cristal buttress on the ilium occurs both during hominid evolution (from Australopithecus to Homo sapiens) and human growth (from neonate to adult). This suggests peramorphic evolution of the pelvic morphology of hominids combining three processes of recapitulation (pre-displacement, acceleration and time hypermorphosis). The results lend credence to the hypothesis that no single heterochronic process accounts for all human evolutionary change; rather this reflects a combination of relative changes in growth rhythm and duration, including other perturbations, such as the appearance of new morphological features. Am J Phys Anthropol 105:441–459, 1998. © 1998 Wiley-Liss, Inc.     

Brain endocast asymmetry in pongids and hominids: Some preliminary findings on the paleontology of cerebral dominance

Observations on petalial asymmetry for 190 hominoid endocasts are reported, and their statistical differences assessed. While all taxa of hominoids show asymmetries to various degrees, the patterns or combinations of petalial asymmetries are very different, with fossil hominids and modern Homo sapiens showing an identical pattern of left-occipital, right-frontal petalias, which contrasts with those found normally in pongids. Of the pongids, Gorilla shows the greater degree of asymmetry in left-occipital petalias. Only modern Homo and hominids (Australopithecus, Homo erectus, Neandertals) show a distinct left-occipital, right-frontal petalial pattern. Analysis by x² statistics shows the differences to be highly significant. Due to small sample size and incompleteness of endocasts, small-brained hominids, i.e., Australopithecus, are problematical. To the degree that gross petalial patterns are correlated with cognitive task specialization, we speculate that human cognitive patterns evolved early in hominid evolution and were related to selection pressures operating on both symbolic and spatiovisual integration, and that these faculties are corroborated in the archaeological record.     

Mammal diversity and environment evolution during the Plio-Pleistocene in East Africa

Human evolution began in East Africa four million years ago, with a transition from an arboreal state to a more terrestrial one. This evolution seems to be correlated with a large environmental change in East Africa around 2.5 m.y. due to a major climatic change leading to drier and cooler conditions. Cenogram analysis (a graphical representation of community structure) can be used to reconstruct the vegetation cover at a regional scale, and to infer the changing climatic conditions. Using cenogram sequences of different sites along the Rift Valley, we were able to determine the regional ecological context in which mammals and hominids have evolved in East Africa during the last 3 million years. Between 3.5 and 2 m.y., during a general climatic change, successive faunas of South Tanzania reflect the progressive opening of their environment. In contrast around Lake Turkana a mosaic of isolated dry and wet habitats were present throughout this period. At this time, the Rift seems to have been spatially structured in several basins isolated from one other, and isolated faunas experienced separate speciation events (particularly with the appearance ofHomo genus). After 2 m.y., the disappearance of the isolating barriers on one hand, and a regional increase in aridity, on the other hand, led to more homogenous faunas arising throughout the region. Replacements of mammal species occurred (especiallyHomo erectus replacingHomo habilis) and several others mammal species, including australopithecines, disappeared during this same period.     

Sangiran 5,(“Pithecanthropus dubius”), homo erectus, “Meganthropus,” orPongo?

There are now eleven known mandibular remains from the Lower and Middle Pleistocene of Java, all but one being from the Sangiran site. All of these have been assigned toHomo erectus by most workers, while others have suggested as many as four different hominoid taxa. The author finds that the jaws cannot be a homogeneous sample. Morphologically, they are a mixture of undoubtedH. erectus, “H. meganthropus,” and possibly a pongid. If the jaws are allH. erectus then they have a sexual dimorphism exceeding that of modern gorillas. The case of“Pithecanthropus dubius” (Sangiran 5) is even less certain; even its hominid status is disputed. If it is indeedHomo it must be placed with the other“H. meganthropus” specimens. Its size and morphology are well beyond the known range anyH. erectus.     

Bipedalism in Orrorin tugenensis revealed by its femora

Three fragments of femora of Orrorin tugenensis, a 6 Ma hominid from the Lukeino Formation, Kenya, possesses a suite of derived characters that reveal that the species was habitually bipedal. Detailed anatomical comparisons with modern humans, Australopithecines and Miocene and extant African apes, reveal that Orrorin shares several apomorphic features with Australopithecines and Homo, but none with Pan or Gorilla. Within the Hominidae, the femur of Orrorin is closer morphologically to that of modern humans than it is to those of australopithecines.     

Patterns of evolution in Middle Pleistocene hominids

This paper reviews the chronology and morphological variability of Middle Pleistocene H. erectus. specimens. Functional complexes are delineated within the skull and dentition, and their total morphological patterns quantified using univariate and multivariate statistical analysis. Statistical distances are calculated between H. erectus and other hominid samples for each complex, compared to illustrate patterns of mosaic evolution within the skull and dentition of middle Quaternary hominids, and estimated evolution rates are derived. An attempt is made to relate the observed morphological patterns to ecological shifts by early hominid communities, and to assess their significance for hominid taxonomy.