  Since time immemorial life has been viewed as being fragile, yet over the
past few decades it has been found that many extreme environments are inhabited
by organisms known as extremophiles.Knowledge of their emergence, adaptability,
and limitations seems to provide a guideline for the search of
extra-terrestrial life, since some extremophiles presumably can survive in
extreme environments such as Mars, Europa, and Enceladus. Due to
physicochemical constraints, the first life necessarily came into existence at
the lower limit of lifes conceivable complexity.Thus, the first life could not
have been an extremophile, furthermore, since biological evolution occurs over
time, then the dual knowledge regarding what specific extremophiles are capable
of, and to the analogue environment on extreme worlds, will not be sufficient
as a search criterion.This is because, even though an extremophile can live in
an extreme environment here-and-now, its ancestor however could not live in
that very same environment in the past, which means that no contemporary
extremophiles exist in that environment.Furthermore, a theoretical framework
should be able to predict whether extremophiles can be considered a special or
general case in the galaxy.Thus, a question is raised: does Earths continuous
inhabitability represent an extreme or average value for planets? Thus,
dependent on whether it is difficult or easy for worlds to maintain this
inhabitability, the search for extra-terrestrial life with a focus on
extremophiles will either represent a search after dying worlds, or a search
after special life on living worlds, where one focus too narrowly on extreme
values.
