Geology: Requirement for Hydrothermal Settings
In addition to abundant chemical elements such as carbon, oxygen, nitrogen, and
hydrogen, biological systems contain a number of microelements, often at levels
far exceeding those in the surrounding environment (see [171] for a comprehensive
survey). In particular, transition metals are often involved in enzyme catalytic
centres [90,172]. The concentration of such metals in modern cells is many orders
of magnitude larger than that in sea water (see Table 1); the ion accumulation
is accomplished by sophisticated transport systems and demands ion-tight
membranes to prevent the escape of trapped metal ions out of the cell [173,174].
However, the ion-tight membranes, as argued elsewhere [175,176], seem to be
a relatively late evolutionary acquisition. Here we encounter a paradox. On the
one hand, the emergence of metal-containing RNA and protein domains – as a
result of their eventual stabilization by available transition metal ions – implies
an abundance of these ions. On the other hand, the equilibrium concentration of
such ions in sea water is very low (see Table 1). This paradox is routinely resolved
by invoking hydrothermal settings as potential cradles of life [113-123,177]. In