<Tr> <Td> mmHg </Td> <Td> 107 </Td> <Td> 75 - 100 </Td> <Td> 30 - 40 </Td> </Tr> <P> The free oxygen partial pressure in the body of a living vertebrate organism is highest in the respiratory system, and decreases along any arterial system, peripheral tissues, and venous system, respectively . Partial pressure is the pressure that oxygen would have if it alone occupied the volume . </P> <P> Free oxygen gas was almost nonexistent in Earth's atmosphere before photosynthetic archaea and bacteria evolved, probably about 3.5 billion years ago . Free oxygen first appeared in significant quantities during the Paleoproterozoic eon (between 3.0 and 2.3 billion years ago). For the first billion years, any free oxygen produced by these organisms combined with dissolved iron in the oceans to form banded iron formations . When such oxygen sinks became saturated, free oxygen began to outgas from the oceans 3--2.7 billion years ago, reaching 10% of its present level around 1.7 billion years ago . </P> <P> The presence of large amounts of dissolved and free oxygen in the oceans and atmosphere may have driven most of the extant anaerobic organisms to extinction during the Great Oxygenation Event (oxygen catastrophe) about 2.4 billion years ago . Cellular respiration using O 2 enables aerobic organisms to produce much more ATP than anaerobic organisms . Cellular respiration of O 2 occurs in all eukaryotes, including all complex multicellular organisms such as plants and animals . </P>

Who was the first person to discover that air is essentially a mixture of two gases