Bipedalism and human birth: The obstetrical dilemma revisited

…adaptation to bipedal locomotion decreased the size of the bony birth-canal at the same time that the exigencies of tool use selected for larger brains. This obstetrical dilemma was solved by delivery of the fetus at a much earlier stage of development. (Washburn¹) …there can be no doubt that many of the obstetrical problems of Mrs. H. Sapiens are due to the combination of a narrower pelvis and a bigger head in the species. (Krogman²)     

Life-History Inference in the Early Hominins Australopithecus and Paranthropus

The life histories of early hominins are commonly characterized as being like those of great apes. However, the life histories of the extant great apes differ considerably from one another. Moreover, the extent to which their life histories correlate with the two aspects of morphology used to infer the life histories of fossil species, brain size and dental development, has remained subject to debate. Increased knowledge of great ape life histories and, more recently, dental development —in particular ages at first molar emergence— now make it clearer that the latter is strongly associated with important life-history attributes, whereas brain size, as reflected by cranial capacity, is less informative. Here we estimate ages at M1 emergence in several infant/juvenile individuals of Australopithecus and Paranthropus based on previous estimates of ages at death, determined through dental histology. These are uniformly earlier than would be predicted either by adult cranial capacity or by comparison to ages at M1 emergence in free-living extant great apes. This suggests that either, 1) the life histories of the early hominins were faster than those of all extant great apes; 2) there was selection for rapid initial dental development and presumably early weaning, but that early hominin life histories were otherwise more prolonged and consistent with adult cranial capacities; or 3) the ages at death have been systematically underestimated, resulting in underestimates of the ages at M1 emergence. We investigate the implications of each of these alternatives and, where possible, explore evidence that might support one over the others.     

Dental Ontogeny in Pliocene and Early Pleistocene Hominins

Until recently, our understanding of the evolution of human growth and development derived from studies of fossil juveniles that employed extant populations for both age determination and comparison. This circular approach has led to considerable debate about the human-like and ape-like affinities of fossil hominins. Teeth are invaluable for understanding maturation as age at death can be directly assessed from dental microstructure, and dental development has been shown to correlate with life history across primates broadly. We employ non-destructive synchrotron imaging to characterize incremental development, molar emergence, and age at death in more than 20 Australopithecus anamensis, Australopithecus africanus, Paranthropus robustus and South African early Homo juveniles. Long-period line periodicities range from at least 6–12 days (possibly 5–13 days), and do not support the hypothesis that australopiths have lower mean values than extant or fossil Homo. Crown formation times of australopith and early Homo postcanine teeth fall below or at the low end of extant human values; Paranthropus robustus dentitions have the shortest formation times. Pliocene and early Pleistocene hominins show remarkable variation, and previous reports of age at death that employ a narrow range of estimated long-period line periodicities, cuspal enamel thicknesses, or initiation ages are likely to be in error. New chronological ages for SK 62 and StW 151 are several months younger than previous histological estimates, while Sts 24 is more than one year older. Extant human standards overestimate age at death in hominins predating Homo sapiens, and should not be applied to other fossil taxa. We urge caution when inferring life history as aspects of dental development in Pliocene and early Pleistocene fossils are distinct from modern humans and African apes, and recent work has challenged the predictive power of primate-wide associations between hominoid first molar emergence and certain life history variables.     

Bipedalism and quadrupedalism in Megatheriurn: an attempt at biomechanical reconstruction

Biomechanical reconstruction is increasingly being applied to the study of the mode of life of fossil animals. Different footprints from the fossil mammal Megatherium sp., the giant ground sloth, seem to indicate that it was able to use either bipedal or quadrupedal locomotion. By means of the estimation of the body mass of the type of the species Megatherium americanum , and using the published tracks, different mechanical parameters, such as speed, Froude number, indicators of athletic ability and bending and resistance moments of the vertebral column were calculated in both bipedal and quadrupedal conditions. Results on leg parameters are not conclusive as to the kind of locomotion to which Megatherium sp. was better adapted, but the calculations on the moments of resistance of the vertebral column and on the bending moment at breaking of the femur seem to indicate that Megatherium sp. presented adaptations to bipedalism. MEGATHERIUM, mammals, legs, vertebral column, locomotion, biomechanics, reconstruction .     

The Origins of the RNA World

The general notion of an “RNA World” is that, in the early development of life on the Earth, genetic continuity was assured by the replication of RNA and genetically encoded proteins were not involved as catalysts. There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life. However, arguments regarding whether life on Earth began with RNA are more tenuous. It might be imagined that all of the components of RNA were available in some prebiotic pool, and that these components assembled into replicating, evolving polynucleotides without the prior existence of any evolved macromolecules. A thorough consideration of this “RNA-first” view of the origin of life must reconcile concerns regarding the intractable mixtures that are obtained in experiments designed to simulate the chemistry of the primitive Earth. Perhaps these concerns will eventually be resolved, and recent experimental findings provide some reason for optimism. However, the problem of the origin of the RNA World is far from being solved, and it is fruitful to consider the alternative possibility that RNA was preceded by some other replicating, evolving molecule, just as DNA and proteins were preceded by RNA.     

Oldest Evidence of Toolmaking Hominins in a Grassland-Dominated Ecosystem

Background

Major biological and cultural innovations in late Pliocene hominin evolution are frequently linked to the spread or fluctuating presence of C₄ grass in African ecosystems. Whereas the deep sea record of global climatic change provides indirect evidence for an increase in C₄ vegetation with a shift towards a cooler, drier and more variable global climatic regime beginning approximately 3 million years ago (Ma), evidence for grassland-dominated ecosystems in continental Africa and hominin activities within such ecosystems have been lacking.

Methodology/Principal Findings

We report stable isotopic analyses of pedogenic carbonates and ungulate enamel, as well as faunal data from ∼2.0 Ma archeological occurrences at Kanjera South, Kenya. These document repeated hominin activities within a grassland-dominated ecosystem.

Conclusions/Significance

These data demonstrate what hitherto had been speculated based on indirect evidence: that grassland-dominated ecosystems did in fact exist during the Plio-Pleistocene, and that early Homo was active in open settings. Comparison with other Oldowan occurrences indicates that by 2.0 Ma hominins, almost certainly of the genus Homo, used a broad spectrum of habitats in East Africa, from open grassland to riparian forest. This strongly contrasts with the habitat usage of Australopithecus, and may signal an important shift in hominin landscape usage.     

The Projection of a Test Genome onto a Reference Population and Applications to Humans and Archaic Hominins

We introduce a method for comparing a test genome with numerous genomes from a reference population. Sites in the test genome are given a weight, w, that depends on the allele frequency, x, in the reference population. The projection of the test genome onto the reference population is the average weight for each x, w¯(x). The weight is assigned in such a way that, if the test genome is a random sample from the reference population, then w¯(x)=1. Using analytic theory, numerical analysis, and simulations, we show how the projection depends on the time of population splitting, the history of admixture, and changes in past population size. The projection is sensitive to small amounts of past admixture, the direction of admixture, and admixture from a population not sampled (a ghost population). We compute the projections of several human and two archaic genomes onto three reference populations from the 1000 Genomes project—Europeans, Han Chinese, and Yoruba—and discuss the consistency of our analysis with previously published results for European and Yoruba demographic history. Including higher amounts of admixture between Europeans and Yoruba soon after their separation and low amounts of admixture more recently can resolve discrepancies between the projections and demographic inferences from some previous studies.     

The Origins and Evolution of Culture

This article outlines a deductive theory that creates a new way to think about the origins and evolution of culture. It is Darwinian in the sense that it posits that novel concepts and behavior, like novel genes, appear randomly and are subject to selection on the basis of specific criteria that are established by the properties of living things. The theory permits us to hypothesize properties of the genome that generate culture and to infer the conditions under which selection would favor the origins of culture. Theoretical deductions lead to the conclusion that the organisms that create culture actively participate in the creation of descendants who exhibit increasing cultural abilities and who generate increases in productivity and more reliable flows of resources. Culture is not something that has evolved solely and relatively recently in the hominid line of evolution. Fossil evidence suggests that culture may have existed at least 50 million years ago, and may have originated more than 200 million years ago.     

The Evolutionary Origins of Hierarchy

Hierarchical organization—the recursive composition of sub-modules—is ubiquitous in biological networks, including neural, metabolic, ecological, and genetic regulatory networks, and in human-made systems, such as large organizations and the Internet. To date, most research on hierarchy in networks has been limited to quantifying this property. However, an open, important question in evolutionary biology is why hierarchical organization evolves in the first place. It has recently been shown that modularity evolves because of the presence of a cost for network connections. Here we investigate whether such connection costs also tend to cause a hierarchical organization of such modules. In computational simulations, we find that networks without a connection cost do not evolve to be hierarchical, even when the task has a hierarchical structure. However, with a connection cost, networks evolve to be both modular and hierarchical, and these networks exhibit higher overall performance and evolvability (i.e. faster adaptation to new environments). Additional analyses confirm that hierarchy independently improves adaptability after controlling for modularity. Overall, our results suggest that the same force–the cost of connections–promotes the evolution of both hierarchy and modularity, and that these properties are important drivers of network performance and adaptability. In addition to shedding light on the emergence of hierarchy across the many domains in which it appears, these findings will also accelerate future research into evolving more complex, intelligent computational brains in the fields of artificial intelligence and robotics.