<P> The first atmosphere consisted of gases in the solar nebula, primarily hydrogen . There were probably simple hydrides such as those now found in the gas giants (Jupiter and Saturn), notably water vapor, methane and ammonia . </P> <P> Outgassing from volcanism, supplemented by gases produced during the late heavy bombardment of Earth by huge asteroids, produced the next atmosphere, consisting largely of nitrogen plus carbon dioxide and inert gases . A major part of carbon - dioxide emissions dissolved in water and reacted with metals such as calcium and magnesium during weathering of crustal rocks to form carbonates that were deposited as sediments . Water - related sediments have been found that date from as early as 3.8 billion years ago . </P> <P> About 3.4 billion years ago, nitrogen formed the major part of the then stable "second atmosphere". The influence of life has to be taken into account rather soon in the history of the atmosphere, because hints of early life - forms appear as early as 3.5 billion years ago . How Earth at that time maintained a climate warm enough for liquid water and life, if the early Sun put out 30% lower solar radiance than today, is a puzzle known as the "faint young Sun paradox". </P> <P> The geological record however shows a continuous relatively warm surface during the complete early temperature record of Earth--with the exception of one cold glacial phase about 2.4 billion years ago . In the late Archean Eon an oxygen - containing atmosphere began to develop, apparently produced by photosynthesizing cyanobacteria (see Great Oxygenation Event), which have been found as stromatolite fossils from 2.7 billion years ago . The early basic carbon isotopy (isotope ratio proportions) strongly suggests conditions similar to the current, and that the fundamental features of the carbon cycle became established as early as 4 billion years ago . </P>

Which is found in the highest concentrations in the atmosphere