Rebecca Rogers Ackermann

Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa

Homo naledi is a previously-unknown species of extinct hominin discovered within the Dinaledi Chamber of the Rising Star cave system, Cradle of Humankind, South Africa. This species is characterized by body mass and stature similar to small-bodied human populations but a small endocranial volume similar to australopiths. Cranial morphology of H. naledi is unique, but most similar to early Homo species including Homo erectus, Homo habilis or Homo rudolfensis. While primitive, the dentition is generally small and simple in occlusal morphology. H. naledi has humanlike manipulatory adaptations of the hand and wrist. It also exhibits a humanlike foot and lower limb. These humanlike aspects are contrasted in the postcrania with a more primitive or australopith-like trunk, shoulder, pelvis and proximal femur. Representing at least 15 individuals with most skeletal elements repeated multiple times, this is the largest assemblage of a single species of hominins yet discovered in Africa. DOI: http://dx.doi.org/10.7554/eLife.09560.001

Detecting genetic drift versus selection in human evolution

Recent paleoanthropological discoveries reveal a diverse, potentially speciose human fossil record. Such extensive morphological diversity results from the action of divergent evolutionary forces on an evolving lineage. Here, we apply quantitative evolutionary theory to test whether random evolutionary processes alone can explain the morphological diversity seen among fossil australopith and early Homo crania from the Plio–Pleistocene. We show that although selection may have played an important role in diversifying hominin facial morphology in the late Pliocene, this is not the case during the early evolution of the genus Homo, where genetic drift was probably the primary force responsible for facial diversification.

Common patterns of facial ontogeny in the hominid lineage

Recent evaluation of Neanderthal and modern human ontogeny suggests that taxon‐specific features arose very early in development in both lineages, with early, possibly prenatal, morphological divergence followed by parallel postnatal developmental patterns. Here we use morphometric techniques to compare hominoid facial growth patterns, and show that this developmental phenomenon is, in fact, not unique to comparisons between Neanderthals and modern humans but extends to Australopithecus africanus and to the hominoid lineage more broadly. This finding suggests that a common pattern of juvenile facial development may be more widespread and that the roots of ontogenetically early developmental differentiation are deep—perhaps predating the ape/human split of 6+ million years ago. Anat Rec (New Anat) 269:142–147, 2002.

MORPHOLOGICAL AND MOLECULAR EVIDENCE REVEALS RECENT HYBRIDIZATION BETWEEN GORILLA TAXA

Molecular studies have demonstrated a deep lineage split between the two gorilla species, as well as divisions within these taxa; estimates place this divergence in the mid‐Pleistocene, with gene flow continuing until approximately 80,000 years ago. Here, we present analyses of skeletal data indicating the presence of substantial recent gene flow among gorillas at all taxonomic levels: between populations, subspecies, and species. Complementary analyses of DNA sequence variation suggest that low‐level migration occurred primarily in a westerly‐to‐easterly direction. In western gorillas, the locations of hybrid phenotypes map closely to expectations based on population refugia and riverine barrier hypotheses, supporting the presence of significant vicariance‐driven structuring and occasional admixture within this taxon. In eastern lowland gorillas, the high frequency of hybrid phenotypes is surprising, suggesting that this region represents a zone of introgression between eastern gorillas and migrants from the west, and underscoring the conservation priority of this critically endangered group. These results highlight the complex nature of evolutionary divergence in this genus, indicate that historical gene flow has played a major role in structuring gorilla diversity, and demonstrate that our understanding of the evolutionary processes responsible for shaping biodiversity can benefit immensely from consideration of morphological and molecular data in conjunction.

Phenotypic traits of primate hybrids: Recognizing admixture in the fossil record

For many years, the likelihood that hybridization occurred in human evolution has been debated. Tattersall and Schwartz pointed out one of the core problems with resolving this debate, namely “that nobody has any idea what a Neanderthal/modern human hybrid might look like in theory, and few have dared to suggest in practice that any particular known fossil represents such a hybrid.”1:7117 Moreover, while molecular data is proving increasingly useful for characterising hybrid zones, the utility of the phenotype for this purpose is not clear.2 Here I address these issues, discussing both theoretical and empirically‐derived expectations for what hybrid morphology looks like, with an emphasis on the skeleton of hybrid primates, and consideration of the hominin fossil record.

Mandibular Remains Support Taxonomic Validity of Australopithecus sediba

Since the announcement of the species Australopithecus sediba, questions have been raised over whether the Malapa fossils represent a valid taxon or whether inadequate allowance was made for intraspecific variation, in particular with reference to the temporally and geographically proximate species Au. africanus. The morphology of mandibular remains of Au. sediba, including newly recovered material discussed here, shows that it is not merely a late-surviving morph of Au. africanus. Rather—as is seen elsewhere in the cranium, dentition, and postcranial skeleton—these mandibular remains share similarities with other australopiths but can be differentiated from the hypodigm of Au. africanus in both size and shape as well as in their ontogenetic growth trajectory.

Comparing the Accuracy and Precision of Three Techniques Used for Estimating Missing Landmarks when Reconstructing Fossil Hominin Crania

Various methodological approaches have been used for reconstructing fossil hominin remains in order to increase sample sizes and to better understand morphological variation. Among these, morphometric quantitative techniques for reconstruction are increasingly common. Here we compare the accuracy of three approaches--mean substitution, thin plate splines, and multiple linear regression--for estimating missing landmarks of damaged fossil specimens. Comparisons are made varying the number of missing landmarks, sample sizes, and the reference species of the population used to perform the estimation. The testing is performed on landmark data from individuals of Homo sapiens, Pan troglodytes and Gorilla gorilla, and nine hominin fossil specimens. Results suggest that when a small, same-species fossil reference sample is available to guide reconstructions, thin plate spline approaches perform best. However, if no such sample is available (or if the species of the damaged individual is uncertain), estimates of missing morphology based on a single individual (or even a small sample) of close taxonomic affinity are less accurate than those based on a large sample of individuals drawn from more distantly related extant populations using a technique (such as a regression method) able to leverage the information (e.g., variation/covariation patterning) contained in this large sample. Thin plate splines also show an unexpectedly large amount of error in estimating landmarks, especially over large areas. Recommendations are made for estimating missing landmarks under various scenarios.

Mitochondrial DNA analysis reveals Plio-Pleistocene diversification within the chacma baboon

Modern baboons evolved as a distinct lineage prior to 2.5 Mya. Previous scenarios of diversification within this lineage have assessed the phylogenetic position of the chacma baboon of southern Africa relative to other baboons, but have not examined variation within this taxon. Here we provide a phylogenetic analysis of lineage diversity across the range of the chacma baboon, and show that: (1) chacma baboons diverged as a separate lineage at approximately 1.84 Mya; (2) the chacma lineage is characterised by a deep lineage split dividing chacmas into northeastern (1.52 Mya) and southwestern (1.22 Mya) clades; (3) ruacana baboons of Namibia form their own distinct monophyletic group within the southwestern clade, emerging approximately 0.68 Mya. These patterns likely result from a complex interplay of genetic drift and gene flow as the chacma lineage diversified across a broad geographic landscape during the climatically variable Plio-Pleistocene.

Morphological Integration and the Interpretation of Fossil Hominin Diversity

The fossil record of primate and human evolution cannot provide accurate estimates of within species variation and integration. This means that we cannot directly observe how patterns of integration have evolved over time in this lineage. And yet, our interpretations of fossil diversity are awash with assumptions about variation patterning in precisely these fossil taxa. Most commonly, researchers rely on extant models of variation for interpreting past diversity, by assuming equality of variation (and occasionally covariation) among extant and fossil populations. Yet one of the things we know from studies of integration in primates is that patterns of morphological covariation can differ among even closely related taxa, indicating that they have diverged over evolutionary time, either in response to selection or as the result of neutral evolution. At the same time, overall patterns of integration remain remarkably similar, meaning that in many respects they are highly conserved evolutionarily. Taken together, these seemingly contradictory observations offer an important conceptual framework for interpreting patterns that we observe in the fossil past. This framework dictates that while we can use patterns of covariation in extant taxa as proxies for extinct diversity, and indeed their conserved nature makes them superior to approaches that rely on variation alone, we also need to account for the fact that such patterns change over time, and incorporate that into our models. Here I provide examples using covariation patterns estimated from modern humans and African great apes to demonstrate the extent to which divergence in covariance structure might affect our interpretations of hominin diversity.

Characterizing the Evolutionary Path(s) to Early Homo

Numerous studies suggest that the transition from Australopithecus to Homo was characterized by evolutionary innovation, resulting in the emergence and coexistence of a diversity of forms. However, the evolutionary processes necessary to drive such a transition have not been examined. Here, we apply statistical tests developed from quantitative evolutionary theory to assess whether morphological differences among late australopith and early Homo species in Africa have been shaped by natural selection. Where selection is demonstrated, we identify aspects of morphology that were most likely under selective pressure, and determine the nature (type, rate) of that selection. Results demonstrate that selection must be invoked to explain an Au. africanus—Au. sediba—Homo transition, while transitions from late australopiths to various early Homo species that exclude Au. sediba can be achieved through drift alone. Rate tests indicate that selection is largely directional, acting to rapidly differentiate these taxa. Reconstructions of patterns of directional selection needed to drive the Au. africanus—Au. sediba—Homo transition suggest that selection would have affected all regions of the skull. These results may indicate that an evolutionary path to Homo without Au. sediba is the simpler path and/or provide evidence that this pathway involved more reliance on cultural adaptations to cope with environmental change.