Testing the Cambrian explosion hypothesis by using a molecular dating technique

Molecular studies have the potential to shed light on the origin of the animal phyla by providing independent estimates of the divergence times, but have been criticized for failing to account adequately for variation in rate of evolution. A method of dating divergence times from molecular data addresses the criticisms of earlier studies and provides more realistic, but wider, confidence intervals. The data are not compatible with the Cambrian explosion hypothesis as an explanation for the origin of metazoan phyla, and provide additional support for an extended period of Precambrian metazoan diversification.

Oceanic minerals: Their origin, nature of their environment, and significance

The chemical and isotopic compositions of oceanic biogenic and authigenic minerals contain invaluable information on the evolution of seawater, hence on the history of interaction between tectonics, climate, ocean circulation, and the evolution of life. Important advances and greater understanding of (a) key minor and trace element cycles with various residence times, (b) isotopic sources and sinks and fractionation behaviors, and (c) potential diagenetic problems, as well as developments in high-precision instrumentation, recently have been achieved. These advances provided new compelling evidence that neither gradualism nor uniformitarianism can explain many of the new important discoveries obtained from the chemistry and isotopic compositions of oceanic minerals. Presently, the best-developed geochemical proxies in biogenic carbonates are 18O/16O and Sr/Ca ratios (possibly Mg/Ca) for temperature; 87Sr/86Sr for input sources, Cd/Ca and Ba/Ca ratios for phosphate and alkalinity concentrations, respectively, thus also for ocean circulation; 13C/12C for ocean productivity; B isotopes for seawater pH;, U, Th isotopes, and 14C for dating; and Sr and Mn concentrations for diagenesis. The oceanic authigenic minerals most widely used for chemical paleoceanography are barite, evaporite sulfates, and hydrogenous ferromanganese nodules. Marine barite is an effective alternative monitor of seawater 87Sr/86Sr, especially where carbonates are diagenetically altered or absent. It also provides a high-resolution record of seawater sulfate S isotopes, (evaporite sulfates only carry an episodic record), with new insights on factors affecting the S and C cycles and atmospheric oxygen. High-resolution studies of Sr, Nd, and Pb isotopes of well-dated ferromanganese nodules contain invaluable records on climate driven changes in oceanic circulation.

Co-Permeability of 3H-Labeled Water and 14C-Labeled Organic Acids across Isolated Plant Cuticles1 : Investigating Cuticular Paths of Diffusion and Predicting Cuticular Transpiration

Penetration of 3H-labeled water (3H2O) and the 14C-labeled organic acids benzoic acid ([14C]BA), salicylic acid ([14C]SA), and 2,4-dichlorophenoxyacetic acid ([14C]2,4-D) were measured simultaneously in isolated cuticular membranes of Prunus laurocerasus L., Ginkgo biloba L., and Juglans regia L. For each of the three pairs of compounds (3H2O/[14C]BA, 3H2O/[14C]SA, and 3H2O/[14C]2,4-D) rates of cuticular water penetration were highly correlated with the rates of penetration of the organic acids. Therefore, water and organic acids penetrated the cuticles by the same routes. With the combination 3H2O/[14C]BA, co-permeability was measured with isolated cuticles of nine other plant species. Permeances of 3H2O of all 12 investigated species were highly correlated with the permeances of [14C]BA (r2 = 0.95). Thus, cuticular transpiration can be predicted from BA permeance. The application of this experimental method, together with the established prediction equation, offers the opportunity to answer several important questions about cuticular transport physiology in future investigations.

Genetic absolute dating based on microsatellites and the origin of modern humans.

We introduce a new genetic distance for microsatellite loci, incorporating features of the stepwise mutation model, and test its performance on microsatellite polymorphisms in humans, chimpanzees, and gorillas. We find that it performs well in determining the relations among the primates, but less well than other distance measures (not based on the stepwise mutation model) in determining the relations among closely related human populations. However, the deepest split in the human phylogeny seems to be accurately reconstructed by the new distance and separates African and non-African populations. The new distance is independent of population size and therefore allows direct estimation of divergence times if the mutation rate is known. Based on 30 microsatellite polymorphisms and a recently reported average mutation rate of 5.6 x 10(-4) at 15 dinucleotide microsatellites, we estimate that the deepest split in the human phylogeny occurred about 156,000 years ago. Unlike most previous estimates, ours requires no external calibration of the rate of molecular evolution. We can use such calibrations, however, to test our estimate.