Upon further decrease in the amount of CO2 in the atmosphere and, accordingly,
in atmospheric pressure, the delivery of Zn-rich fluids to the surface of continents
would have gradually ceased, so that fresh, photosynthetically active ZnS surfaces
could no longer form in sub-aerial settings. After that, ZnS-rich hydrothermal
edifices could persist only deeply at the sea floor, ultimately clustering around the
mid-ocean ridges. The organisms that remained confined to the sub-aerial settings
would have found alternative ways to reduce CO2. Moreover, in the absence of a
zinc supply, they were forced to confront unfamiliar minerals, in particular those
containing iron. At that time the dominating transition metal ion in sea water
would have been Fe2+ [90,220,221]. Iron, unlike zinc, is redox-active and can
generate harmful hydroxyl radicals [222-225]. It would seem that the expatriates
of the Zn world had to be full-fledged organisms with reliable replication machinery,
robust metabolism, and protective envelopes. The story of how they could
populate the Earth is beyond the problem of the origin of life proper and hence
remains out of the scope of this communication; this topic, however, is addressed
in the accompanying article [226]