The origin of life

Microfossil finds have been firmly established at about 3.5 Ga (giga annee = 10(9) years), but no rocks older than about 4.0 Ga have been demonstrated , leaving the history of the first 0.6 Ga missing. This gap has been filled by models of the solar system. The origin of the ocean, atmosphere, and mu ch crustal material apparently lies in a heavy rain of comets, subsequent t o the catastrophic Moon-forming event. The earliest microfossils are those of the Apex chert in Australia, about 3.5 Ga old. 'Prebiotic' simulations o f possible biochemistry have made some progress in recent years, but many o bstacles remain, and there is no agreement as to the course of development. The 'ribose nucleic acid (RNA) World', aboriginal 'clay genes', and cataly sis on iron-sulfide precipitates are not ruled out. The search for the 'las t common ancestor' has reached a point between the Bacteria and the Archaea . It is possible that this organism may have been a thermophile, similar to many modern hot spring organisms. But it is likely to have been an autotro ph, and a late development after the true origin of life. Even more specula tive are suggestions about the origins of metabolic sequences, in particula r the origin of the genetic code. Since all modern organisms share this cod e (and many other things), there had to be a long history of development du ring the blank period of Earth history. (C) 1999 Elsevier Science B.V. All rights reserved.