<P> One of the reasons for interest in the early atmosphere and ocean is that they form the conditions under which life first arose . There are many models, but little consensus, on how life emerged from non-living chemicals; chemical systems created in the laboratory fall well short of the minimum complexity for a living organism . </P> <P> The first step in the emergence of life may have been chemical reactions that produced many of the simpler organic compounds, including nucleobases and amino acids, that are the building blocks of life . An experiment in 1953 by Stanley Miller and Harold Urey showed that such molecules could form in an atmosphere of water, methane, ammonia and hydrogen with the aid of sparks to mimic the effect of lightning . Although atmospheric composition was probably different from that used by Miller and Urey, later experiments with more realistic compositions also managed to synthesize organic molecules . Computer simulations show that extraterrestrial organic molecules could have formed in the protoplanetary disk before the formation of the Earth . </P> <P> Additional complexity could have been reached from at least three possible starting points: self - replication, an organism's ability to produce offspring that are similar to itself; metabolism, its ability to feed and repair itself; and external cell membranes, which allow food to enter and waste products to leave, but exclude unwanted substances . </P> <P> Even the simplest members of the three modern domains of life use DNA to record their "recipes" and a complex array of RNA and protein molecules to "read" these instructions and use them for growth, maintenance, and self - replication . </P>

How soon after the beginning of solar system formation 4.6 ga ago was earth’s accretion completed