  In an effort to derive temperature based criteria of habitability for
multicellular life, we investigated the thermal limits of terrestrial
poikilotherms, i.e. organisms whose body temperature and the functioning of all
vital processes is directly affected by the ambient temperature. Multicellular
poikilotherms are the most common and evolutionarily ancient form of complex
life on earth. The thermal limits for their active metabolism and reproduction
are bracketed by the temperature interval 0C<T<50C. The same interval applies
to the photosynthetic production of oxygen, an essential ingredient of complex
life, and for the generation of atmospheric biosignatures. Analysis of the main
mechanisms responsible for the thermal thresholds of terrestrial life suggests
that the same mechanisms would apply to other forms of chemical life. We
propose a habitability index for complex life, h050, representing the mean
orbital fraction of planetary surface that satisfies the temperature limits
0C<T<50C. With the aid of a climate model tailored for the calculation of the
surface temperature of Earth-like planets, we calculated h050 as a function of
planet insolation S, and atmospheric columnar mass Natm, for a few earth-like
atmospheric compositions. By displaying h050 as a function of S and Natm, we
built up an atmospheric mass habitable zone (AMHZ) for complex life. At
variance with the classic habitable zone, the inner edge of the complex life HZ
is not affected by the uncertainties inherent to the calculation of the runaway
greenhouse limit. The complex life HZ is significantly narrower than the HZ of
dry planets. Our calculations illustrate how changes in ambient conditions
dependent on S and Natm, such as temperature excursions and surface dose of
secondary particles of cosmic rays, may influence the type of life potentially
present at different epochs of planetary evolution inside the AMHZ.
