ABSTRACT
Hypothesis: Production and conservation of energy are primary driving forces behind evolution of fatty acid structure with archaeal isoprenoids and omega-3s representing extreme forms.
Membrane evolution presumably begins with the rst living cells containing lipidbased membranes and continues to the present. This evolution covers over four billion years including the largely anaerobic period, itself encompassing more than half this time. As the title of this chapter suggests, we assert that changes in the biosphere throughout evolution have acted to select lipid chains providing the most benecial membrane architecture to the cell or organism as it adapts to an ever-changing environment. Foremost among these changes was the “oxygen revolution.” Molecular oxygen, O2, has been called the molecule that made the world, and its impact on membrane lipid structures is no exception. As shown in Figure 2.1, the evolution of O2-dependent pathways of membrane lipid biosynthesis, creating an explosion of new lipid conformations, is intimately linked to oxygenation of the biosphere. Clearly, this great atmospheric shift is a critical driver of membrane lipid evolution, but not the only one. We suggest that most anaerobic pathways of membrane lipid synthesis, including saturated, monounsaturated, methyl-branched, cyclopropane fatty acids, as well as isoprenoid chains of archaeal membranes, and cholesterol-like molecules such as hopanoids and cholesterol precursors up to squalene, all likely evolved during the anaerobic period. For simplicity, only the increase in complexity of fatty acid structures is shown in the illustration. Oxygenation of the atmosphere created conditions for evolution of fatty acid desaturases, enzymes that use molecular O2 for insertion of double bonds creating many new conformations. The further processing of squalene to cholesterol essential for growth of all animal cells is also dependent on O2. While the balance of total uxes for membrane fatty acids in the extant biosphere occurring via anaerobic versus aerobic pathways has yet to be established, the main point is that the strictly anaerobic period followed by transitional
semioxygenated periods likely served as an evolutionary proving ground for lipid shapes still prevalent in many cells today.
