Raman Imaging Spectroscopy of a Putative Microfossil from the ∼3.46 Ga Apex Chert: Insights from Quartz Grain Orientation.

The utility of nondestructive laser Raman for testing the biogenicity of microfossil-like structures in ancient rocks is promising, yet results from deposits like the ∼3.46 Ga Apex chert remain contentious. The essence of the debate is that associated microstructures, which are not purported to be microfossils, also contain reduced carbon that displays Raman D- and G-band peaks similar to those seen in the purported microfossils. This has led to the hypothesis that all features including reported microfossils are due to compression of nonfossil carbon during crystal growth around quartz spherulites or more angular crystals. In this scenario, the precursor to this macromolecular carbon may or may not have been of biogenic origin, while the arcuate and linear features described would be pseudofossils. To test this hypothesis, we have undertaken 2-D micro-Raman imaging of the Eoleptonema apex holotype and associated features using instrumentation with a high spatial and spectral resolution. In addition to this, we utilized the ratio of two Raman active quartz mode intensities (I129/I461) to assess quartz grain orientation and grain-splitting artifacts. These data lead us to conclude that the holotype of Eoleptonema apex is a sheet-shaped pseudofossil that appears to be a carbon infilled intragranular crack; therefore other holotypes should be carefully reexamined for syngenicity.

A billion years of environmental stability and the emergence of eukaryotes: new data from northern Australia.

Carbon isotopes through 6km of fully cored drill holes in 1.7 to 1.5 Ga carbonates of the Mount Isa and McArthur basins, Australia (which host the earliest known eukaryote biomarkers) provide the most comprehensive and best-dated delta 13C stratigraphy yet obtained from such ancient rocks. Both basins reveal remarkably stable temporal delta 13C trends (mean of -0.6% +/- 2% PDB [Peedee belemnite]) and confirm the impression of delta 13C stasis between 2.0 and 1.0 Ga, which, together with other evidence, suggest a prolonged period of stability in crustal dynamics, redox state of surface environments, and planetary climate. This delta 13C stasis is consistent with great stability in the carbon cycle controlled, we suggest, by P limitation of primary productivity. Recent evidence shows that P depletion is a major factor in obligate associations between photosymbionts and host cells. We argue that a billion years of stability in the carbon and nutrient cycles may have been the driving force that propelled prokaryotes toward photosymbiosis and the emergence of the autotrophic eukaryote cell.

A comparison of lateral peristriate and striate neocortical ablations in the rat.

The effects of striate, lateral peristriate and sham ablations upon the post-operative reacquisition of visual discriminations were studied in two experiments. The results of Experiment 1 indicated that lesions restricted to the lateral peristriate neocortex rendered the rat incapable of resolving an oblique stripe discrimination. Seven of the 9 rats with striate ablations were unable to form the discrimination. The histological findings indicated that the deficit following lateral peristriate injury was not due to interruption of the geniculostriate system. However, the behavioral impairment in destriate rats corresponded to the degree of unintentional injury in lateral peristriate neocortex, but not medial peristriate tissue. In Experiment 2, the effects of these lesions were studied upon the post-operative reacquisition of either a non-reversal or reversal of a preoperatively acquired brightness discrimination. Animals with striate damage demonstrated considerable savings on the non-reversed brightness discrimination, and they acquired the reversal at a rate equal to that seen in animals with sham ablations. Rats with peristriate ablations showed no savings on the non-reversed brightness habit, and they required more extensive training than both the striate and sham groups on the reversal. These findings suggest that the frequently reported loss of a preoperative brightness habit following large posterior neocortex ablations is due to damage in the lateral peristriate neocortex rather than damage of the geniculostriate system. These results, taken with other literature, suggest an important role of the lateral peristriate neocortex in visuospatial sensory function.