Ice-core analysis.
Lead and silver ore mining powered ancient economies, but previously reported measurements of lead pollution in Arctic ice were based on sparse sampling and inconclusive dating. Joseph McConnell et al. (pp. 5726–5731) used precisely dated measurements of lead pollution in Greenland ice from 1100 BCE to 800 CE to uncover links between estimated lead emissions and historical events that affected the economy, including imperial expansion, wars, and major plagues. The authors measured lead concentrations in 423 m of ice core from the North Greenland Ice Core Project and combined the measurements with an annual-layer-counting approach to develop a chronology of Greenland ice. The authors found that lead pollution increased during periods of prosperity, such as the Phoenician expansion, and peaked under the Roman Empire. In contrast, lead pollution was low during periods of instability, such as the aftermath of the Antonine plague. Furthermore, the authors report that the bullion in Rome’s coinage, the denarius, reflected fluctuations in estimated lead emissions. According to the authors, the record of European lead pollution in Greenland ice provides insights into economic activities, including lead and silver mining, in ancient societies. — T.G.
Four continuous states of carbon nanotubes in m-cresol.
Carbon nanotubes have attractive electrical, thermal, and mechanical properties for a broad range of applications. Some types of nanotubes are mass-manufactured in powder form, but further processing is required to generate disaggregated and usable forms of the materials. Kevin Chiou et al. (pp. 5703–5708) report that relatively abundant and inexpensive industrial solvents called cresols can disperse various types of carbon nanotube powders at high concentrations. Spectroscopy experiments confirmed that cresols do not induce permanent chemical changes to the nanotube surface and can be removed by evaporation or washing. Increasing the concentration of nanotubes results in a continuous transition between four states, including dilute dispersions, thick pastes, free-standing gels, and playdough-like materials. As a proof of concept, the authors used the gel to print a cup-shaped structure that dried into a stiff solid object. By contrast, blobs of segregated nanotubes mixed in the common solvent N-methyl-2-pyrrolidone (NMP) clogged the nozzles used for 3D printing. While the nanotube/m-cresol dough could be kneaded or rolled without fracture, the nanotube/NMP mixtures disintegrated upon kneading. According to the authors, the nanotube/m-cresol states can be readily used in a broad array of material-processing techniques to form desirable structures and polymer composites. — J.W.
Myelin content in lateral inflated left hemisphere cortex in human (Left) and chimpanzee (Right). The white line represents the location of coronal slice at the corpus callosum genu; the pink and blue lines represent conservative and liberal PFC delineations, respectively.
Humans’ advanced cognitive abilities are thought to be the result of possessing the largest cerebral cortex among primates. However, whether the human prefrontal cortex (PFC), which is implicated in cognition, is larger than that of other primates remains unclear, largely due to methodological differences in conflicting reports. Chad Donahue et al. (pp. E5183–E5192) compared cortical tissue volumes from structural MRI images of 60 humans, 29 chimpanzees, and 19 macaques using liberal and conservative approaches to delineate the PFC. Human PFC was found to contain 1.9-fold and 1.2-fold more cortical gray matter than that of macaques and chimpanzees, respectively. Humans also had 2.4-fold and 1.7-fold more subcortical white matter beneath the PFC than macaques and chimpanzees, respectively. Whereas the PFC occupied approximately the same surface area as the primary visual cortex area V1 in macaques, the PFC surface area was sixfold greater than that of V1 in humans. Based on a conservative demarcation of the PFC, the authors conclude that in absolute terms, the human PFC is 4.5 times larger than that of chimpanzees, suggesting a relatively larger neural endowment in humans compared with other primates. An accurate tally of the relative size differences between distinct brain regions among primate species might yield evolutionary insights into brain function, according to the authors. — P.N.
Remains of the hominin species Homo naledi discovered in the Dinaledi Chamber, South Africa, date between 236,000 and 335,000 years ago, concurrent with early ancestors of modern humans. H. naledi had a small endocranial volume, suggesting a small brain, compared with other Homo species, despite sharing certain aspects of cranial anatomy with these species. To compare the brain morphology of H. naledi to that of other hominids, Ralph Holloway et al. (pp. 5738–5743) created digital impressions of the interior of the skull, or endocasts, from four H. naledi cranial fragments. The inner cranial surface can preserve impressions of brain surface features. An endocast of a nearly complete H. naledi left hemisphere exhibited no fronto-orbital sulcus, a feature found in the brains of apes and Australopithecus but not in human brains. Instead, similar to modern humans and other Homo species, the H. naledi endocast exhibited a frontal operculum formed by expansion of the frontal lobes relative to apes. H. naledi endocasts also exhibited occipital lobe asymmetry and lunate sulcus morphology similar to that seen in modern humans. The results suggest that H. naledi shared certain aspects of brain organization with modern humans and that these features were ancestral to the Homo genus, according to the authors. — B.D.
