C. Owen Lovejoy

The Origin of Man

Five characters separate man from other hominoids-a large neocortex, bipedality, reduced anterior dentition with molar dominance, material culture, and unique sexual and reproductive behavior. Evidence provided by the fossil record, primate behavior, and demographic analysis shows that the traditional view that early human evolution was a direct consequence of brain expansion and material culture is incorrect, and that the unique sexual and reproductive behavior of man may be the sine qua non of human origin.

Ardipithecus ramidus and the Paleobiology of Early Hominids

Hominid fossils predating the emergence of Australopithecus have been sparse and fragmentary. The evolution of our lineage after the last common ancestor we shared with chimpanzees has therefore remained unclear. Ardipithecus ramidus, recovered in ecologically and temporally resolved contexts in Ethiopia’s Afar Rift, now illuminates earlier hominid paleobiology and aspects of extant African ape evolution. More than 110 specimens recovered from 4.4-million-year-old sediments include a partial skeleton with much of the skull, hands, feet, limbs, and pelvis. This hominid combined arboreal palmigrade clambering and careful climbing with a form of terrestrial bipedality more primitive than that of Australopithecus. Ar. ramidus had a reduced canine/premolar complex and a little-derived cranial morphology and consumed a predominantly C3 plant–based diet (plants using the C3 photosynthetic pathway). Its ecological habitat appears to have been largely woodland-focused. Ar. ramidus lacks any characters typical of suspension, vertical climbing, or knuckle-walking. Ar. ramidus indicates that despite the genetic similarities of living humans and chimpanzees, the ancestor we last shared probably differed substantially from any extant African ape. Hominids and extant African apes have each become highly specialized through very different evolutionary pathways. This evidence also illuminates the origins of orthogrady, bipedality, ecology, diet, and social behavior in earliest Hominidae and helps to define the basal hominid adaptation, thereby accentuating the derived nature of Australopithecus.

Reexamining Human Origins in Light of Ardipithecus ramidus

Referential models based on extant African apes have dominated reconstructions of early human evolution since Darwin’s time. These models visualize fundamental human behaviors as intensifications of behaviors observed in living chimpanzees and/or gorillas (for instance, upright feeding, male dominance displays, tool use, culture, hunting, and warfare). Ardipithecus essentially falsifies such models, because extant apes are highly derived relative to our last common ancestors. Moreover, uniquely derived hominid characters, especially those of locomotion and canine reduction, appear to have emerged shortly after the hominid/chimpanzee divergence. Hence, Ardipithecus provides a new window through which to view our clade’s earliest evolution and its ecological context. Early hominids and extant apes are remarkably divergent in many cardinal characters. We can no longer rely on homologies with African apes for accounts of our origins and must turn instead to general evolutionary theory. A proposed adaptive suite for the emergence of Ardipithecus from the last common ancestor that we shared with chimpanzees accounts for these principal ape/human differences, as well as the marked demographic success and cognitive efflorescence of later Plio-Pleistocene hominids.

Asa issie, aramis and the origin of Australopithecus

The origin of Australopithecus, the genus widely interpreted as ancestral to Homo, is a central problem in human evolutionary studies. Australopithecus species differ markedly from extant African apes and candidate ancestral hominids such as Ardipithecus, Orrorin and Sahelanthropus. The earliest described Australopithecus species is Au. anamensis, the probable chronospecies ancestor of Au. afarensis. Here we describe newly discovered fossils from the Middle Awash study area that extend the known Au. anamensis range into northeastern Ethiopia. The new fossils are from chronometrically controlled stratigraphic sequences and date to about 4.1–4.2 million years ago. They include diagnostic craniodental remains, the largest hominid canine yet recovered, and the earliest Australopithecus femur. These new fossils are sampled from a woodland context. Temporal and anatomical intermediacy between Ar. ramidus and Au. afarensis suggest a relatively rapid shift from Ardipithecus to Australopithecus in this region of Africa, involving either replacement or accelerated phyletic evolution.

The obstetric pelvis of A.L. 288-1 (Lucy)

Due to the fragmentary condition of most specimens, there have been few studies on the anatomy of the early hominid bony birth canal. However, recovery of an innominate and sacrum from one individual (A.L. 288-1, Australopithecus afarensis) allows reconstruction of the complete pelvis. Although A.L. 288-1 is considered to have been a female, several morphologies of its true pelvis resemble those of human males, such as sacral angulation, ischiopubic ramus and, principally, funnelling of the pelvic cavity. The implication is that some of the pelvic dimorphisms characteristic of modern Homo sapiens developed subsequent to the emergence of bipedalism. The shape of A.L. 288-ls true pelvis is compared with that of female H. sapiens and Pan troglodytes. A.L. 288-ls pelvis is platypelloid, unlike Homo and Pan. The obstetric consequence of the difference in pelvic shape would have been a unique mechanism of birth in A.L. 288-1, with the fetus being born along the transverse axis of the outlet. Rotation of the fetal cranium within the pelvic canal, a characteristic of human birth, would not have occurred in A.L. 288-1. The platypelloid (false and true) pelvis of A.L. 288-1 is related to the requirements of locomotion and visceral accommodation and support. Although the obstetric analysis indicates that birth might have been slow and difficult in A.L. 288-1, we do not consider there to have been selection for the australopithecine fetus to have been born in a more altricial state than that in pongids. However, exactly when secondary altriciality, which is a characteristic of modern humans, emerged is a current subject of debate.

Part Two: The role of constitutional factors, diet, and infectious disease in the etiology of porotic hyperostosis and periosteal reactions in prehistoric infants and children

Abstract A model of analysis incorporating methodological improvements and epidemiological refinements has been employed to investigate the etiology of porotic hyperostosis and periosteal reactions in infants and children from the Libben Site, a Late Woodland ossuary and occupation site from Ottawa County, Ohio. Results of the age-specific intrapopulational analysis of porotic hyperostosis demonstrate that the skeletal lesion strongly fits the age-specific distribution of hypochromic microcytic iron-deficiency anemia in infants and children. The data indicate that the lesion is a response to nutritional stress. Similarly, our findings show that the age-specific distribution of periosteal reactions strongly coincides with, and appears to be a response to, infectious disease as it occurs in infants and children. More importantly, survivorship and growth data indicate that porotic hyperostosis and periosteal reactions are strongly associated with patterns of infant and child morbidity and mortality, and therefore appear to play an important role in selection and fitness at Libben. Based upon the age-specific patterns and associations observed for porotic hyperostosis and periosteal reactions in the Libben infants and children, it is suggested that: 1. the current methodological procedure of partitioning skeletal populations into broad age categories can significantly distort important age-specific pathophysiological relationships; 2. skeletal lesions should be classified according to their physical quality (i.e., remodeled and unremodeled) to provide an estimate of both the morbidity and mortality associated with the age-specific distribution of a particular skeletal lesion; 3. porotic hyperostosis may be a valuable indicator of nutritional stress which can be employed to evaluate the nutritional status of prehistoric human populations; 4. the pathogenesis of porotic hyperostosis can best be understood in terms of the synergistic interactions between constitutional factors, diet, and infectious disease.

The Pelvis and Femur of Ardipithecus ramidus: The Emergence of Upright Walking

The femur and pelvis of Ardipithecus ramidus have characters indicative of both upright bipedal walking and movement in trees. Consequently, bipedality in Ar. ramidus was more primitive than in later Australopithecus. Compared with monkeys and Early Miocene apes such as Proconsul, the ilium in Ar. ramidus is mediolaterally expanded, and its sacroiliac joint is located more posteriorly. These changes are shared with some Middle and Late Miocene apes as well as with African apes and later hominids. However, in contrast to extant apes, bipedality in Ar. ramidus was facilitated by craniocaudal shortening of the ilium and enhanced lordotic recurvature of the lower spine. Given the predominant absence of derived traits in other skeletal regions of Ar. ramidus, including the forelimb, these adaptations were probably acquired shortly after divergence from our last common ancestor with chimpanzees. They therefore bear little or no functional relationship to the highly derived suspension, vertical climbing, knuckle-walking, and facultative bipedality of extant African apes.

The Great Divides: Ardipithecus ramidus Reveals the Postcrania of Our Last Common Ancestors with African Apes

Genomic comparisons have established the chimpanzee and bonobo as our closest living relatives. However, the intricacies of gene regulation and expression caution against the use of these extant apes in deducing the anatomical structure of the last common ancestor that we shared with them. Evidence for this structure must therefore be sought from the fossil record. Until now, that record has provided few relevant data because available fossils were too recent or too incomplete. Evidence from Ardipithecus ramidus now suggests that the last common ancestor lacked the hand, foot, pelvic, vertebral, and limb structures and proportions specialized for suspension, vertical climbing, and knuckle-walking among extant African apes. If this hypothesis is correct, each extant African ape genus must have independently acquired these specializations from more generalized ancestors who still practiced careful arboreal climbing and bridging. African apes and hominids acquired advanced orthogrady in parallel. Hominoid spinal invagination is an embryogenetic mechanism that reoriented the shoulder girdle more laterally. It was unaccompanied by substantial lumbar spine abbreviation, an adaptation restricted to vertical climbing and/or suspension. The specialized locomotor anatomies and behaviors of chimpanzees and gorillas therefore constitute poor models for the origin and evolution of human bipedality.

Sexual Dimorphism in Australopithecus afarensis Was Similar to That of Modern Humans

The substantial fossil record for Australopithecus afarensis includes both an adult partial skeleton [Afar Locality (A.L.) 288-1, “Lucy”] and a large simultaneous death assemblage (A.L. 333). Here we optimize data derived from both to more accurately estimate skeletal size dimorphism. Postcranial ratios derived from A.L. 288-1 enable a significant increase in sample size compared with previous studies. Extensive simulations using modern humans, chimpanzees, and gorillas confirm that this technique is accurate and that skeletal size dimorphism in A. afarensis was most similar to that of contemporary Homo sapiens. These data eliminate some apparent discrepancies between the canine and skeletal size dimorphism in hominoids, imply that the species was not characterized by substantial sexual bimaturation, and greatly increase the probability that the reproductive strategy of A. afarensis was principally monogamy.

Combining Prehension and Propulsion: The Foot of Ardipithecus ramidus

Several elements of the Ardipithecus ramidus foot are preserved, primarily in the ARA-VP-6/500 partial skeleton. The foot has a widely abducent hallux, which was not propulsive during terrestrial bipedality. However, it lacks the highly derived tarsometatarsal laxity and inversion in extant African apes that provide maximum conformity to substrates during vertical climbing. Instead, it exhibits primitive characters that maintain plantar rigidity from foot-flat through toe-off, reminiscent of some Miocene apes and Old World monkeys. Moreover, the action of the fibularis longus muscle was more like its homolog in Old World monkeys than in African apes. Phalangeal lengths were most similar to those of Gorilla. The Ardipithecus gait pattern would thus have been unique among known primates. The last common ancestor of hominids and chimpanzees was therefore a careful climber that retained adaptations to above-branch plantigrady.