Reactive Oxygen Species: Radical Factors in the Evolution of Animal Life: A molecular timescale from Earth's earliest history to the rise of complex life.

Introduction of O to Earth's early biosphere stimulated remarkable evolutionary adaptations, and a wide range of electron acceptors allowed diverse, energy-yielding metabolic pathways. Enzymatic reduction of O yielded a several-fold increase in energy production, enabling evolution of multi-cellular animal life. However, utilization of O also presented major challenges as O and many of its derived reactive oxygen species (ROS) are highly toxic, possibly impeding multicellular evolution after the Great Oxidation Event. Remarkably, ROS, and especially hydrogen peroxide, seem to play a major part in early diversification and further development of cellular respiration and other oxygenic pathways, thus becoming an intricate part of evolution of complex life. Hence, although harnessing of chemical and thermo-dynamic properties of O for aerobic metabolism is generally considered to be an evolutionary milestone, the ability to use ROS for cell signaling and regulation may have been the first true breakthrough in development of complex life.