More than any other element, oxygen shapes the current biosphere. It is produced by photosynthesis and it is used to respire most of the organic matter at the Earth surface. However, this is not always been the case. During the first 2 billion years of Earth history, oxygen concentrations were likely over 100,000 times less than present levels. Could an aerobic biosphere have existed with so little oxygen? This question is addressed through growth experiments conducted on E. coli utilizing special oxygen sensors with ultra-low oxygen-detection limits. We find that E. coli can grow and respire oxygen at oxygen concentrations < 2nM which is 100,000 times less than found today in air-saturated water. After about 2.4 billion years ago, oxygen concentrations rose to levels that are uncertain. There is a prevailing view, however, that these levels were insufficient to allow respiration by animals thus preventing their evolution. The minimum levels of oxygen required for animals (as a group), however, is poorly known. We attempt to define the levels required for animal respiration through a variety of respiration and behavior experiments on two different marine sponges. We show that sponges feed and respire at oxygen concentrations ranging between 1 and 4% of present levels. This, therefore, might be viewed as a possible minimum oxygen requirement by early animals. We provide further evidence from the geologic record to show that atmospheric oxygen may have attained these levels well before animals, implying that other factors controlled the timing of animal evolution.