TOWARD A HOLISTIC ASSESSMENT OF PROTEROZOIC ANTECEDENTS OF ANIMALS - EPIGENETIC REGULATION EVOLVED IN RESPONSE TO CHANGING REDOX CONDITIONS AND IS THEN CO-OPTED FOR THE EVOLUTION OF COMPLEX METAZOA

After many cell divisions producing the same type of cell appropriate to the initial environment, single celled eukaryotes respond to changing redox conditions via alteration of epigenetic inheritance to form a distinct new cell type. Cysts, protected metabolically quiescent cells produced through epigenetic response to redox perturbation, are one such cell type. Cysts occur in a wide variety of unicellular eukaryotes suggesting their ancient ancestry, and such cysts are recorded in the fossil record starting in the middle Proterozoic presumably in response to fluctuating redox settings in marine environments. Epigenetic mechanisms are also critical to multicellular development as they permit cells to stably go through the cell cycle for multiple cycles before signaling resets them. Such behaviors are critical to the growth of epithelia and organs. Intriguingly, redox cues, often employing chemical species typical of marine redox gradients, are critical to much epigenetic signaling that regulates epigenetic cell type information in the tissues of even higher organisms. Returning to the Proterozoic marine setting, with increasing size, organisms can extend across, modify, and metabolically benefit from redox boundaries such as those that are found at the marine sediment water interface. Such phenomena that modify benthic boundary conditions are typical of a greater organization that was perhaps initiated in the Cryogenian in association with larger scale perturbations in deep sea oxygenation. Finally organisms with epithelial partitions evolve with features such as guts. Guts can be seen as partitions within organisms that separate environments with different redox potentials providing a range of metabolic and competitive benefits. This can be seen as initiating additional and epithelial partitions such as respiratory and circulatory structures. These can both be seen as responses to changing Neoproterozoic marine redox conditions but also as additional partitions that employ redox signalling mechanisms to maintain transepithelial conditions and appropriate vascularisation. Thus, a set of successive temporal periods from the middle Proterozoic into the Phanerozoic can be identified that vary in their environmental redox conditions. These also support a parallel evolutionary succession involving increasingly complex epigenetic control of biological structures that employ mechanisms of cellular control in development that are themselves redox responsive reflecting their Proterozoic evolutionary origins.