Melatonin in Plants and Animals

Molecular oxygen (O2) began to rise in the Earth’s atmosphere around 2.5 billion years ago due to its persistent release from photosynthetic bacteria that had evolved an estimated billion years earlier [1, 2]. The rise of atmospheric O2 was a highly selective pressure for the evolution of organisms to use O2 as the basis of their metabolism [3]. During aerobic metabolism, reactive oxygen species (ROS) are invariably generated when O2 accepts leaked electrons from the electron transport chain (ETC) [4]. It is estimated that up to 4% of the O2 consumed by organisms during aerobic metabolism is eventually reduced to ROS [5]. These large amounts of ROS are toxic to cells and organisms, inducing the development of complex and effective mechanisms to neutralize them; this initially occurred in early life forms such as bacteria and subsequent unicellular organisms [6].