The metabolic strengths, weaknesses, opportunities and threats of the metabolic ability to split water brought about a proliferation of biological systems, produced a toxic oxygenic environment, and were responsible for the development of antioxidant defence mechanisms. Evolution is driven by heritable adaptations which improve environmental 'fit'. Hence aerobic respiration, using oxygen as a nutrient, came to predominate in biological systems, and antioxidant defence mechanisms which prevent and neutralise toxic oxygen intermediates have become widespread, varied, coordinated and effective. Antioxidant defences are not infallible however. In humans, reactive oxygen species-induced damage is associated with the ageing process, and with chronic diseases including cancer and coronary heart disease. Interestingly, some important antioxidants, including ascorbic acid and the tocopherols, cannot be synthesised by humans and must be taken in the diet. Another antioxidant, uric acid, is found in much higher concentrations in humans than in other mammals, and levels are also affected by diet. In humans, therefore, antioxidant defence against toxic oxygen intermediates is species specific and heavily influenced by nutrition. In this article, the atmospheric and metabolic changes which produced both the threat and opportunity offered by an oxygenic environment are outlined. An overview of oxygen toxicity, and adaptations to oxidative stress in terms of evolution of antioxidant defences, is presented. Finally, suggested benefits underlying our curious inability to manufacture ascorbic acid, and the possible role of uric acid in human antioxidant defence, are briefly discussed with particular reference to nutrition and toxicology.