UC Riverside

Since life took its earliest footholds, organisms have been in a perpetual arms race. Evolutionary trial and error has produced increasingly complex organisms. All with a single purpose, to utilize available resources as efficiently as possible and reproduce. This progression has led to things like photosynthesis and respiration, phagocytosis, multicellularity, differentiated cell types, neural networks, and apparently consciousness. This dissertation presents one account of the history of the push-and-pull relationship between life and the environment as both evolved together, centered around two case studies that contributed to the knowledgebase on which the larger argument is founded.Because the net outcome of biological activity is the concentration of reduced material, the net outcome is also environmental oxidation, so long as some of that material still exists. Oxygenic photosynthesis was not necessarily the path of least resistance to ecological success, but once the machinery was assembled, the substrates were nearly limitless in many environments. This intuitively leads to the utilization of O2 in respiration—is by far the best candidate for the job, assuming it is ambiently present. That statement suggests implicitly that evolution will continually exploit its environment to gain a competitive advantage. This effectively frames one side of a long-standing debate concerning the role that environmental O2 concentrations played the evolution of the eukaryotic clade. The chapters herein elaborate on this argument. It begins with a case study of the 1.4-billion-year-old Xiamaling Formation, presenting iron speciation, sulfur isotope, trace metal, and molybdenum isotope data; concluding that while the Xiamaling Formation was likely deposited in a restricted setting, it still holds geochemical indications that global oceans could have been more well oxygenated at the time than many previously thought. The middle chapters explore these ideas further. The dissertation closes with another case study, Meiklejohn Peak. This study presents paired carbon isotope, sulfur isotope, and iodine concentration data, along with detailed sedimentology. The formation explored in this chapter was deposited during one of the largest pulses of diversification in the history of animal life, and multiple independent geochemical proxies suggest rising environmental O2 concentrations accompanied this event.