Human evolution. Evolution of early Homo: an integrated biological perspective.

Integration of evidence over the past decade has revised understandings about the major adaptations underlying the origin and early evolution of the genus Homo. Many features associated with Homo sapiens, including our large linear bodies, elongated hind limbs, large energy-expensive brains, reduced sexual dimorphism, increased carnivory, and unique life history traits, were once thought to have evolved near the origin of the genus in response to heightened aridity and open habitats in Africa. However, recent analyses of fossil, archaeological, and environmental data indicate that such traits did not arise as a single package. Instead, some arose substantially earlier and some later than previously thought. From ~2.5 to 1.5 million years ago, three lineages of early Homo evolved in a context of habitat instability and fragmentation on seasonal, intergenerational, and evolutionary time scales. These contexts gave a selective advantage to traits, such as dietary flexibility and larger body size, that facilitated survival in shifting environments.

Tooth wear, Neanderthal facial morphology and the anterior dental loading hypothesis.

The Anterior Dental Loading Hypothesis states that the unique Neanderthal facial and dental anatomy was an adaptive response to the regular application of heavy forces resulting from both the masticatory and cultural use of the anterior teeth. Heavy anterior tooth wear frequently observed in Neanderthal specimens is cited as a main source of evidence for heavy forces being applied to these teeth. From this, it might be predicted that the wear shown on the anterior teeth of Neanderthals would greatly exceed that of the posterior teeth and that this differential would be greater than in other hominins with different facial morphologies. In this paper, a new method of examining tooth wear patterns is used to test these predictions in a large assemblage of Late Pleistocene hominins and a group of recent hunter-gatherers from Igloolik, Canada. The results show that all Late Pleistocene hominins, including Neanderthals, had heavily worn anterior teeth relative to their posterior teeth but, contrary to expectations, this was more pronounced in the modern humans than in the Neanderthals. The Igloolik Inuit showed heavier anterior tooth wear relative to their posterior teeth than any Late Pleistocene hominins. There was, however, a characteristic Neanderthal pattern in which wear was more evenly spread between anterior teeth than in modern humans. Overall, the evidence presented here suggests that all Late Pleistocene hominins habitually applied heavy forces between their anterior teeth and that Neanderthals were not exceptional in this regard. These results therefore does not support the Anterior Dental Loading Hypothesis.

Five years of Homo floresiensis.

Since Homo floresiensis was first described in October 2004 there has been a lively debate over its status. Is it a late surviving species of early Homo or merely a modern individual afflicted with disordered growth and one of the many syndromes resulting in microchephaly? Recently the discovery team has published a series of articles providing detailed descriptions of the hominin material, its geomorphological context, and the associated archaeology and faunal material (Morwood and Jungers: J Hum Evol 57 (2009) 437-648). In addition, other researchers have put forward new hypotheses for possible pathologies including Laron's Syndrome and Myxoedematous Endemic (ME) Cretinism. Here I review this new information and conclude that the evidence supports the hypothesis that Homo floresiensis is a late-surviving species of early Homo with its closest morphological affinities to early African pre-erectus/ergaster hominins. Although this hypothesis requires fundamental paradigm changes in our understanding of human evolution, it provides a more economical explanation for H. floresiensis than do the alternatives. None of the current explanations for microcephaly and disordered growth account for the range of features observed in H. floresiensis. Neither do they provide explanations for why a pathological condition in modern humans would mimic so closely the morphology observed in earlier hominins. This conclusion is based on the current evidence for H. floresiensis and on the particular pathological explanations that have appeared in the literature. There is no doubt that controversy over H. floresiensis will continue until new and conclusive evidence is available to settle the debate one way or another.

Technical note: A new method for measuring long bone curvature using 3D landmarks and semi-landmarks.

Here we describe and evaluate a new method for quantifying long bone curvature using geometric morphometric and semi-landmark analysis of the human femur. The technique is compared with traditional ways of measuring subtense and point of maximum curvature using either coordinate calipers or projection onto graph paper. Of the traditional methods the graph paper method is more reliable than using coordinate calipers. Measurement error is consistently lower for measuring point of maximum curvature than for measuring subtense. The results warrant caution when comparing data collected by the different traditional methods. Landmark data collection proves reliable and has a low measurement error. However, measurement error increases with the number of semi-landmarks included in the analysis of curvature. Measurements of subtense can be estimated more reliably using 3D landmarks along the curve than using traditional techniques. We use equidistant semi-landmarks to quantify the curve because sliding the semi-landmarks masks the curvature signal. Principal components analysis of these equidistant semi-landmarks provides the added benefit of describing the shape of the curve. These results are promising for functional and forensic analysis of long bone curvature in modern human populations and in the fossil record.

Fetal and infant head circumference sexual dimorphism in primates.

Studies have shown that after controlling for the effects of body size on brain size, the brains of adult humans, rhesus monkeys, and chimpanzees differ in relative size, where males have a greater volume of cerebral tissue than females. We assess whether head circumference sexual dimorphism is present during early development by evaluating sex differences in relative head circumference in living fetuses and infants within the first year of life. Head circumference is used as a proxy for brain size in the fetus and infant. Femur length is used as a proxy for body length in the fetus. Ultrasonography was used to obtain fetal measures, and anthropometry was used to obtain postnatal measures in humans, rhesus monkeys, baboons, and common marmosets. We show that statistically significant but low levels of head circumference sexual dimorphism are present in humans, rhesus monkeys, and baboons in early life. On average, males have head circumferences about 2% larger than females of comparable femur/body length in humans, rhesus monkeys, and baboons. No evidence for head circumference sexual dimorphism in the common marmoset was found. Dimorphism was present across all body size ranges. We suggest that head circumference sexual dimorphism emerges largely postnatally and increases throughout maturation, particularly in humans who reach adult dimorphism values greater than the monkeys. We suggest that brain dimorphism is not likely to impose an additional energetic burden to the gestating or lactating mother. Finally, some of the problems with ascribing functional significance to brain size sexual dimorphism are discussed, and the energetic implications for brain size sexual dimorphism in infancy are assessed.

Early hominin limb proportions.

Recent analyses and new fossil discoveries suggest that the evolution of hominin limb length proportions is complex, with evolutionary reversals and a decoupling of proportions within and between limbs. This study takes into account intraspecific variation to test whether or not the limb proportions of four early hominin associated skeletons (AL 288-1, OH 62, BOU-VP-12/1, and KNM-WT 15000) can be considered to be significantly different from one another. Exact randomization methods were used to compare the differences between pairs of fossil skeletons to the differences observed between all possible pairs of individuals within large samples of Gorilla gorilla, Pan troglodytes, Pongo pygmaeus, and Homo sapiens. Although the difference in humerofemoral proportions between OH 62 and AL 288-1 does not exceed variation in the extant samples, it is rare. When humerofemoral midshaft circumferences are compared, the difference between OH 62 and AL 288-1 is fairly common in extant species. This, in combination with error associated with the limb lengths estimates, suggests that it may be premature to consider H. (or Australopithecus) habilis as having more apelike limb proportions than those in A. afarensis. The humerofemoral index of BOU-VP-12/1 differs significantly from both OH 62 and AL 288-1, but not from KNM-WT 15000. Published length estimates, if correct, suggest that the relative forearm length of BOU-VP-12/1 is unique among hominins, exceeding those of the African apes and resembling the proportions in Pongo. Evidence that A. afarensis exhibited a less apelike upper:lower limb design than A. africanus (and possibly H. habilis) suggests that, if A. afarensis is broadly ancestral to A. africanus, the latter did not simply inherit primitive morphology associated with arboreality, but is derived in this regard. The fact that the limb proportions of OH 62 (and possibly KNM-ER 3735) are no more human like than those of AL 288-1 underscores the primitive body design of H. habilis.

Energetic consequences of being a Homo erectus female.

Body size is one of the most important characteristics of any animal because it affects a range of behavioral, ecological, and physiological traits including energy requirements, choice of food, reproductive strategies, predation risk, range size, and locomotor style. This article focuses on the implications of being large bodied for Homo erectus females, estimated to have been over 50% heavier than average australopithecine females. The energy requirements of these hominins are modeled using data on activity patterns, body mass, and life history from living primates. Particular attention is given to the inferred energetic costs of reproduction for Homo erectus females based on chimpanzee and human reproductive scheduling. Daily energy requirements during gestation and lactation would have been significantly higher for Homo erectus females, as would total energetic cost per offspring if the australopithecines and Homo erectus had similar reproductive schedules (gestation and lactation lengths and interbirth intervals). Shortening the interbirth interval could considerably reduce the costs per offspring to Homo erectus and have the added advantage of increasing reproductive output. The mother would, however, incur additional daily costs of caring for the dependent offspring. If Homo erectus females adopted this reproductive strategy, it would necessarily imply a revolution in the way in which females obtained and utilized energy to support their increased energetic requirements. This transformation is likely to have occurred on several levels involving cooperative economic division of labor, locomotor energetics, menopause, organ size, and other physiological mechanisms for reducing the energetic load on females.

Brains and guts in human evolution: the Expensive Tissue Hypothesis

The brain is a very expensive organ in metabolic terms. Each unit of brain tissue requires over 22 times the amount of metabolic energy as an equivalent unit of muscle tissue. There is no correlation across mammals, however, between the relative size of the brain and the relative basal metabolic rate. The Expensive Tissue Hypothesis explains this apparent paradox by looking at the metabolic cost of the brain in the context of the costs of other metabolically expensive organs in the body. The results show that the increase in brain size in humans is balanced by an equivalent reduction in the size of the gastro-intestinal tract. In other words, the increased energetic demands of a relatively large brain are balanced by the reduced energy demands of a relatively small gastro-intestinal tract. This relationship also seems to be true in non-human primates. The size of the gastro-intestinal tract is dependent on both body size and the quality of the diety. It is argued that humans (and other primates) could not have developed a relatively large brain without also adopting a high quality diet that would have permitted a reduction in the relative size of the gastro-intestinal tract. Dietary change is therefore viewed as a "prime releaser" in brain evolution. It is argued that a high quality diet is necessary for the evolution of a relatively large brain. However, the change to such a high quality diet, which involved an increased proportion of animal based products, need not have been one of the "prime movers" in brain evolution. In this context, and based on the archaeological and palaeoanthropological record, the factors most probably surrounding the evolution of the human brain are discussed.