Behavioral and phylogenetic correlates of limb length proportions in extant apes and monkeys: Implications for interpreting hominin fossils.

The body proportions of extant animals help inform inferences about the behaviors of their extinct relatives, but relationships between body proportions, behavior, and phylogeny in extant primates remain unclear. Advances in behavioral data, molecular phylogenies, and multivariate analytical tools make it an opportune time to perform comprehensive comparative analyses of primate traditional limb length proportions (e.g., intermembral, humerofemoral, brachial, and crural indices), body size-adjusted long bone proportions, and principal components. In this study we used a mix of newly-collected and published data to investigate whether and how the limb length proportions of a diverse sample of primates, including monkeys, apes, and modern humans, are influenced by behavior and phylogeny. We reconfirm that the intermembral index, followed by the first principal component of traditional limb length proportions, is the single most effective variable distinguishing hominoids and other anthropoids. Combined limb length proportions and positional behaviors are strongly correlated in extant anthropoid groups, but phylogeny is a better predictor of limb length proportion variation than of behavior. We confirm convergences between members of the Atelidae and extant apes (especially Pan), members of the Hylobatidae and Pongo, and a potential divergence of Presbytis limb proportions from some other cercopithecoids, which correlate with adaptations for forelimb-dominated behaviors in some colobines. Collectively, these results substantiate hypotheses indicating that extinct hominins and other hominoid taxa can be distinguished by analyzing combinations of their limb length proportions at different taxonomic levels. From these results, we hypothesize that fossil skeletons characterized by notably disparate limb length proportions are unlikely to have exhibited similar behavioral patterns.

The estimation and evolution of hominin body mass.

Body mass is a critical variable in many hominin evolutionary studies, with implications for reconstructing relative brain size, diet, locomotion, subsistence strategy, and social organization. We review methods that have been proposed for estimating body mass from true and trace fossils, consider their applicability in different contexts, and the appropriateness of different modern reference samples. Recently developed techniques based on a wider range of modern populations hold promise for providing more accurate estimates in earlier hominins, although uncertainties remain, particularly in non-Homo taxa. When these methods are applied to almost 300 Late Miocene through Late Pleistocene specimens, the resulting body mass estimates fall within a 25-60 kg range for early non-Homo taxa, increase in early Homo to about 50-90 kg, then remain constant until the Terminal Pleistocene, when they decline.

Hominin fossils from Kromdraai and Drimolen inform Paranthropus robustus craniofacial ontogeny.

Ontogeny provides critical information about the evolutionary history of early hominin adult morphology. We describe fossils from the southern African sites of Kromdraai and Drimolen that provide insights into early craniofacial development in the Pleistocene robust australopith Paranthropus robustus. We show that while most distinctive robust craniofacial features appear relatively late in ontogeny, a few do not. We also find unexpected evidence of independence in the growth of the premaxillary and maxillary regions. Differential growth results in a proportionately larger and more postero-inferiorly rotated cerebral fossa in P. robustus infants than in the developmentally older Australopithecus africanus juvenile from Taung. The accumulated evidence from these fossils suggests that the iconic SK 54 juvenile calvaria is more likely early Homo than Paranthropus. It is also consistent with the hypothesis that P. robustus is more closely related to Homo than to A. africanus.

Underappreciated pioneers.

This contribution focuses on a 1928 multiauthor paper reporting the discovery of a child's skull at Devil's Tower cave on the Rock of Gibraltar. It was ground-breaking. Two of the lead authors, Dorothy Garrod and Dorothea Bate, were women, and it was one of the earliest reports of a fossil hominin to incorporate and integrate detailed information about its stratigraphic and environmental context.

Distinguishing primate taxa with enamel incremental variables.

Enamel has long been of interest for its functional and phylogenetic significance among fossil hominins and other primates. Previous studies demonstrated that enamel incremental features distinguish among hominin fossil taxa, suggesting utility for highlighting taxonomy. However, not all features appear to be useful in mixed samples of fossils, living humans, and apes. Here we tested enamel incremental data from closely related primate taxa to determine which features, if any, distinguish among them. Enamel incremental variables were measured from the M2 of 40 living primate taxa, and we tested our variables using discriminant function analysis at the taxonomic ranks of parvorder, family, tribe, and genus. We then included enamel incremental data from Australopithecus afarensis, Australopithecus africanus, Paranthropus aethiopicus, Paranthropus boisei, and Paranthropus robustus to determine if these features distinguished fossil taxa from living humans and apes. Our initial results show that enamel incremental variables distinguish among primate taxa, but with low classification rates. Further testing with jackknifing methods shows overlap between groups at all taxonomic ranks, suggesting enamel incremental variables are unreliable for taxonomy. The addition of many common enamel incremental growth variables also resulted in multicollinearity in our multivariate analysis. As the dentition and isolated teeth remain a significant portion of the hominin fossil record, verifying enamel incremental features as a useful taxonomic tool is fundamentally important for hominin paleobiology.

Estimating origination times from the early hominin fossil record.

The age of the earliest recovered fossil evidence of a hominin taxon is all too often equated with that taxon's origination. However, the earliest known fossil record nearly always postdates, sometimes by a substantial period of time, the true origination of a taxon. Here we evaluate the first appearance records of the earliest potential hominins (Sahelanthropus, Ardipithecus, Orrorin), as well as of the genera Australopithecus, Homo, and Paranthropus, to illustrate the considerable uncertainty regarding the actual timing of origin of these taxa. By placing confidence intervals on the first appearance records of early hominin taxa, we can better evaluate patterns of hominin diversity, turnover, and potential correlations with climatic and environmental changes.

Birth of Australopithecus.

The announcement of a fossilized child's skull discovered in a quarry in 1924 sub-Saharan Africa might not have seemed destined to be a classic paper. This contribution focuses on anatomist Raymond Dart's 1925 paper in which he designated the Taungs skull the type specimen of Australopithecus africanus. We combine an account of Dart's training and experience, with a telling of the fossil's discovery, analysis, the initial response of a mostly skeptical community, and a review of subsequent discoveries that consolidated the case Dart made for a hitherto unknown human close relative. Dart's paper presented evidence that confirmed the prescience of Charles Darwin's prediction that Africa was the birthplace of modern humans. The Taungs skull's unique mix of great ape and human attributes eventually led to a paradigm shift in our understanding of human evolution.

Rethinking the ecological drivers of hominin evolution.

A central goal of paleoanthropology is understanding the role of ecological change in hominin evolution. Over the past several decades researchers have expanded the hominin fossil record and assembled detailed late Cenozoic paleoclimatic, paleoenvironmental, and paleoecological archives. However, effective use of these data is precluded by the limitations of pattern-matching strategies for inferring causal relationships between ecological and evolutionary change. We examine several obstacles that have hindered progress, and highlight recent research that is addressing them by (i) confronting an incomplete fossil record, (ii) contending with datasets spanning varied spatiotemporal scales, and (iii) using theoretical frameworks to build stronger inferences. Expanding on this work promises to transform challenges into opportunities and set the stage for a new phase of paleoanthropological research.

Nature and relationships of Sahelanthropus tchadensis.

A partial left femur (TM 266-01-063) was recovered in July 2001 at Toros-Menalla, Chad, at the same fossiliferous location as the late Miocene holotype of Sahelanthropus tchadensis (the cranium TM 266-01-060-1). It was recognized as a probable primate femur in 2004 when one of the authors was undertaking a taphonomic survey of the fossil assemblages from Toros-Menalla. We are confident the TM 266 femoral shaft belongs to a hominid. It could sample a hominid hitherto unrepresented at Toros-Menalla, but a more parsimonious working hypothesis is that it belongs to S. tchadensis. The differences between TM 266 and the late Miocene Orrorin tugenensis partial femur BAR 1002'00, from Kenya, are consistent with maintaining at least a species-level distinction between S. tchadensis and O. tugenensis. The results of our preliminary functional analysis suggest the TM 266 femoral shaft belongs to an individual that was not habitually bipedal, something that should be taken into account when considering the relationships of S. tchadensis. The circumstances of its discovery should encourage researchers to check to see whether there is more postcranial evidence of S. tchadensis among the fossils recovered from Toros-Menalla.

Birth of Homo erectus.

Eugène Dubois was the pioneer of human origins research in South-East Asia, specifically on two of the islands, Sumatra and Java, now included in Indonesia. Dubois was a polymath, whose research interests embraced encephalization and hydrology as well as paleoanthropology. His interpretations of the hominin fossil evidence he collected, which he eventually assigned to Pithecanthropus erectus, changed over the years, and he evidently felt defensive about those interpretations, but in his 1894 paper he presents cogent reasons for his decision. The taxon he introduced is still recognized, and while it is no longer seen as "the" link between fossil apes and modern humans, it is currently one of the longest surviving hominin taxa.

Mandibular corpus shape is a taxonomic indicator in extant hominids.

OBJECTIVES: The aim of this study is to understand whether the shape of three sub-regions of the mandibular corpus (the alveolar arch, corpus at M1 and posterior symphysis) are useful for making taxonomic assessments at the genus and species levels in extant hominids. MATERIALS AND METHODS: We use data taken from 3D surface scans of the mandibular corpus of seven extant hominid taxa: Gorilla gorilla gorilla, Gorilla beringei graueri, Homo sapiens, Pan paniscus, Pan troglodytes schweinfurthii, Pongo abelii, and Pongo pygmaeus pygmaeus to generate four shape variables: alveolar arch shape (AAS), corpus shape at M1 (CSM1 ), posterior symphysis shape at the midline (PSSM), and posterior symphysis shape (PSS). To ascertain how reliable each mandibular shape variable is for assessing taxonomy, we ran canonical discriminant and discriminant function analysis, reporting cross-validated results. RESULTS: Using a combination of three mandibular corpus shape variables, 99% of specimens were classified correctly for genus-level analyses. A maximum of 100% of Pan specimens, 94% of Gorilla specimens and 96% of Pongo specimens were classified correctly at the species level when up to three mandibular shape variables were included in the analyses. When mandibular corpus variables were considered in isolation, posterior symphysis shape yielded the highest overall correct classification results. DISCUSSION: The high taxonomic classification rates at both the genus and species level, using 3D surface data and advanced quantification techniques, show that the shape of the alveolar arch, corpus at M1 and symphysis can distinguish extant hominid taxa. These findings have implications for assessing the taxonomy of extinct hominid specimens which preserve these mandibular sub-regions.

Birth of primate comparative anatomy.

In 1698, a creature with a perplexing mix of human and "ape" features died in London. Brought back to England by merchants who had acquired it during a trading mission to West Africa, it attracted the attention of the Royal Society, and after the death of what we now know was a juvenile chimpanzee, Edward Tyson, a distinguished physician/anatomist, was commissioned to undertake its dissection. Tyson, who was assisted by William Cowper, prepared a detailed written and graphic description of their meticulous dissection, and this forms the major part of his 1699 publication Orang-outang sive Homo sylvestris: or The Anatomy of a Pygmie compared with that of a Monkey, an Ape, and a Man. Tyson records the many ways his "pygmie" resembled, and differed from, modern humans, including acute assessments of its brain and pelvic anatomy. Tyson's monograph is a text-book example of the comparative method. He, and it, deserve more recognition.

Statistical estimates of hominin origination and extinction dates: A case study examining the Australopithecus anamensis-afarensis lineage.

Reliable estimates of when hominin taxa originated and went extinct are central to addressing many paleoanthropological questions, including those relating to macroevolutionary patterns. The timing of hominin temporal ranges can be used to test chronological predictions generated from phylogenetic hypotheses. For example, hypotheses of phyletic ancestor-descendant relationships, based on morphological data, predict no temporal range overlap between the two taxa. However, a fossil taxon's observed temporal range is almost certainly underestimated due to the incompleteness of both the fossil record itself and its sampling, and this decreases the likelihood of observing temporal overlap. Here, we focus on a well-known and widely accepted early hominin lineage, Australopithecus anamensis-afarensis, and place 95% confidence intervals (CIs) on its origination and extinction dates. We do so to assess whether its temporal range is consistent with it being a phyletic descendant of Ardipithecus ramidus and/or a direct ancestor to the earliest claimed representative of Homo (i.e., Ledi-Geraru). We find that the last appearance of Ar. ramidus falls within the origination CI of Au. anamensis-afarensis, whereas the claimed first appearance of Homo postdates the extinction CI. These results are consistent with Homo evolving from Au. anamensis-afarensis, but temporal overlap between Ar. ramidus and Au. anamensis-afarensis cannot be rejected at this time. Though additional samples are needed, future research should extend our initial analyses to incorporate the uncertainties surrounding the range endpoints of Ar. ramidus and earliest Homo. Overall, our findings demonstrate the need for quantifying the uncertainty surrounding the appearances and disappearances of hominin taxa in order to better understand the timing of evolutionary events in our clade's history.

Evolution of the modern human brain.

When compared to the brains of our closest living relatives, chimpanzees and bonobos, the brains of modern humans are larger and differently shaped. This chapter reviews what we know about the evolutionary history of these differences. We can make an educated guess about the size and shape of the brains of the hypothetical common ancestor of modern humans and chimpanzees/bonobos, but between ca. 8 million years ago and the present day evidence about the size and shape of the brain comes from either natural endocasts, which are literally brain-shaped rocks, or from individuals for which enough of the brain case is preserved to provide estimates of endocranial volume and/or the relative proportions of the different regions of the cerebral hemispheres and the cerebellum. The tempo and mode of brain size increase in the hominin clade has been the subject of spirited debate, but we suggest that some of this controversy is the combination of an overreliance on frequentist statistical tests and researchers addressing these issues at different taxonomic scales. The existence and significance of shape changes are also controversial topics, made more so by the dearth of reliable evidence.

Comparative isotopic evidence from East Turkana supports a dietary shift within the genus Homo.

It has been suggested that a shift in diet is one of the key adaptations that distinguishes the genus Homo from earlier hominins, but recent stable isotopic analyses of fossils attributed to Homo in the Turkana Basin show an increase in the consumption of C4 resources circa 1.65 million years ago, significantly after the earliest evidence for Homo in the eastern African fossil record. These data are consistent with ingesting more C4 plants, more animal tissues of C4 herbivores, or both, but it is also possible that this change reflects factors unrelated to changes in the palaeobiology of the genus Homo. Here we use new and published carbon and oxygen isotopic data (n = 999) taken from large-bodied fossil mammals, and pedogenic carbonates in fossil soils, from East Turkana in northern Kenya to investigate the context of this change in the isotope signal within Homo. By targeting taxa and temporal intervals unrepresented or undersampled in previous analyses, we were able to conduct the first comprehensive analysis of the ecological context of hominin diet at East Turkana during a period crucial for detecting any dietary and related behavioural differences between early Homo (H. habilis and/or H. rudolfensis) and Homo erectus. Our analyses suggest that the genus Homo underwent a dietary shift (as indicated by δ13Cena and δ18Oena values) that is (1) unrelated to changes in the East Turkana vegetation community and (2) unlike patterns found in other East Turkana large mammals, including Paranthropus and Theropithecus. These data suggest that within the Turkana Basin a dietary shift occurred well after we see the first evidence of early Homo in the region.

The evolutionary history of the human face.

The face is the most distinctive feature used to identify others. Modern humans have a short, retracted face beneath a large globular braincase that is distinctively different from that of our closest living relatives. The face is a skeletal complex formed by 14 individual bones that houses parts of the digestive, respiratory, visual and olfactory systems. A key to understanding the origin and evolution of the human face is analysis of the faces of extinct taxa in the hominin clade over the last 6 million years. Yet, as new fossils are recovered and the number of hominin species grows, the question of how and when the modern human face originated remains unclear. By examining key features of the facial skeleton, here we evaluate the evolutionary history of the modern human face in the context of its development, morphology and function, and suggest that its appearance is the result of a combination of biomechanical, physiological and social influences.