  In the modern search for life elsewhere in the Universe, we are broadly
looking for the following: the planets similar to Earth - physical indicators
of habitability, and the manifestation of life - the biological signatures. A
biosignature is a measured parameter that has a high probability of being
caused by the living organisms, either atmospheric gas species or some surface
features. Therefore, the focus of a search is on a product or phenomena
produced by the living systems, mostly by microorganisms as these are the most
abundant on our planet like, say, methane. However, we may need to distinguish
the terms `biosignature' and `bioindicator'. A biosignature is what living
organisms produce - a bioproduct, while a bioindicator may be anything
necessary for life as we know it, such as water or a rocky planet. Oxygen in
this case is a double biomarker; first, it is a byproduct of oxygenic
photosynthesis and, second, it is a signature of a complex life, because
complex highly organized life requires high levels of oxygen. It is possible
that there are other such bioindicators. For example, in the atmospheric
compositions of terrestrial planets in our Solar System (including Titan),
argon is one of the major constituents, moreover it was recently acknowledged
to be a `biologically' active gas, exhibiting organprotective and
neuroprotective properties, especially under hypoxic conditions. Here we
propose that argon in the atmosphere of a rocky planet is a bioindicator of a
highly organized life, provided that the planet is already deemed potentially
habitable: with water, atmosphere, and of a certain age allowing for the
complex life to evolve. We also delineate its possible detection methods.
