"Earth-Science Reviews, 47(1-2): 71-93
The origin of life
J.H. McClendon
105 Bush Street, Ashland, OR, USA
Microfossil finds have been firmly established at about 3.5 Ga
(gigaanne=10^9 years), but no rocks older than about 4.0 Ga have been
demonstrated, leaving the history of the first 0.6 missing. This gap has
been filled by models of the solar system. The origin of the ocean,
atmosphere, and much crustal material apparently lies in a heavy rain
ofcomets, subsequent to the catastrophic Moon-forming event The earliest
microfossils are those of the Apex chert in Australia, about 3.5 Ga old.
'Prebiotc' simulations of possible biochemistry have made some progress in
recent years, but many obstacles remain, and there is no agreement as to
the course of development. The 'ribose nucleic acid (RNA) World',
aboriginal 'clay genes', and catalysis on iron-sulfide precipitates are not
ruled out. The search for the 'last 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 autotroph, and a late development after the
true origin of life. Even more speculative are suggestions about the origin
of metabolic sequences, in particular the origin of the genetic code. Since
all modern organisms share this code (and many other thlngs), there had to
be a long history of development during the blank period of Earth history."