Brian: According to Pflug(**) there are banded iron formations in the Isua rocks
of Greenland, 3.8 billion years old.
So we shave of a few hundred million years... <g> How much time would abiogenesis require ? Fossil evidence of life suggest that 3-3.5 Billion years ago life existed.
**Hans D. Pflug, "Early Geological Record and the Origin of Life,"
<Naturwissenschaften> 71:63-68, 1984.