Some scientists analyzed microfossils in a rock about 3.5 billion years ago in Australia. They used a secondary ion mass spectrometer to separate carbon-12 from carbon-13 in each microfossil, and measured both. Proportionately, they found that the fossils contained a primitive group of organisms, so they speculated that life appeared on Earth as early as 3.5 billion years ago. To reach the above conclusions, the prerequisites that need to be added are:. ['As early as 4.3 billion years ago, liquid water existed on Earth.', 'The carbon-13 and carbon-12 ratios of microfossils have characteristics of biological and metabolic functions.', "Methane was an important component of the early Earth's atmosphere before the advent of oxygen and was a simple organic matter.", 'Stones suspected of being a biological remains more than three billion years ago have been found in both Greenland and Canada.']