Enumerate important effects of Oxygen an evolution in biology
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Oxygen in the Evolution of Complex Life and the Price We Pay
Victor J. Thannickal
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Aerobic organisms use molecular oxygen (dioxygen; O2) to generate chemical energy in the form of adenine triphosphate (ATP). Energy transforms cellular structure to function, a defining property of life. Due to its favorable thermodynamic properties, O2 appears to have been selected during biological evolution to serve as the terminal electron acceptor in the reduction of carbon-based fuels to generate ATP by oxidative phosphorylation. That O2 is essential to sustain human life is perhaps best illustrated during an acute cardiopulmonary arrest—commonly referred to as “code blue” on the medical wards. Indeed, the “A-B-Cs” of basic life support is to ensure gas-exchange via the lungs and O2 delivery to internal organs: “A” for airway, “B” for breathing, and “C” for circulation. A less well appreciated role of O2 is in the evolution of organismal size, multicellularity, and biological complexity. An understanding of the key role of O2 in the evolution of complex life and mammalian physiology may provide novel insights of O2, and its metabolites (reactive oxygen species), in the pathophysiology of diseases that affect the lung.
O2 AND METAZOAN EVOLUTION
Multiple lines of evidence from evolutionary biology (1, 2), geochemistry (3), and systems biology (4) build a compelling case for a central role of O2 in the evolution of complex multicellular life on earth (5, 6). The oldest life forms are estimated to have been present over 3.7 billion years (Gyr) ago (7). In the relative absence of O2 and under the strong reducing conditions that existed on the primordial earth's surface, early unicellular life forms relied on metabolic pathways that used electron acceptors such as CO2 and SO4. Between about 2.5 and 2.2 Gyr ago, however, the earth's atmospheric concentration of O2 rose significantly (3). This rise in atmospheric O2 is thought to have been due to biological and geological factors that led to an increase in O2 production relative to its consumption. Biological processes, most notably the emergence of cyanobacteria capable of oxygenic photosynthesis (8), were an important source of O2 production during this early period. Geological events such as the burial of organic carbon (9) and a shift from submarine to subaerial volcanism (10) accounted for decreased O2 consumption.