14 SETI in Rocky Exoplanets …
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temperature seems to be essential for the functioning of brains. Since homeotherms
emerged from Darwinian evolution of multicellular poikilotherms, we may say that
the thermal limits of multicellular poikilotherms are relevant for all multicellular life,
including homeotherms.
The approximate thermal limits of multicellular poikilotherms (plants, invertebrates and ectothermic vertebrates) with active metabolism are 0 ≤ T (
◦ C) ≤ 50 [8].
Quite interestingly, the same limits are also relevant for the biological production of
atmospheric O 2 because the metabolism of the main O 2 producers (cyanobacteria and
plants) drops outside this temperature interval [7]. It is hard to overemphasize the role
of oxygen in this context, since the aerobic metabolism is much more efficient that
anaereobic metabolism and the presence of significative amounts of atmospheric O 2
might be a necessary condition for the emergence of multicellular life in any planet
[10].
The thermal limits 0 ≤ T (
◦ C) ≤ 50 are more stringent than the liquid-water temperature range commonly adopted in studies of habitability. These stringent limits
can be used to narrow the search of optimal targets for the SETI program.
14.2.1 How Universal Are the Thermal Limits of Terrestrial
Life?
A comprehensive mechanistic understanding of the effects of temperature on biological processes is still lacking. In spite of this, the physical nature of the main
mechanisms of thermal response at work in terrestrial life suggest that the same
mechanisms would also be at work in other forms of chemical life.
At the molecular level, there are strong indications that life processes based on
genetic and catalytic molecules require the existence of a network of hydrogen-bond
interactions [11]. Among cosmically abundant elements and molecules, CNO elements and water have unique capabilities to form molecular groups that can interact
via hydrogen bonds. Therefore, the basic aspects of terrestrial biochemistry (CHON
elements and water) are likely to be universal for any type of life life based on genetic
and catalytic molecules.
For water-based life, the water freezing point T = 0
◦ C, which is almost independent of the ambient pressure, is likely to be a universal lower bound, because
frozen water would hamper the mobility of biomolecules and, in addition, would
make impossible for molecular motors [12] to harvest the kinetic energy of Brownian motion [7]. A universal upper bound is set by the temperature at which thermal
energy denatures the molecular structures most sensitive to heat. Since intramolecular hydrogen bonds are essential for shaping molecular structures, the low binding
energy of hydrogen bonds can be used, in principle, to set universal upper limits to
the temperature of life processes [11].
At the supramolecular level, the thermal tolerance must narrow as complexity increases, because the number of molecular structures potentially limiting the
129
temperature seems to be essential for the functioning of brains. Since homeotherms
emerged from Darwinian evolution of multicellular poikilotherms, we may say that
the thermal limits of multicellular poikilotherms are relevant for all multicellular life,
including homeotherms.
The approximate thermal limits of multicellular poikilotherms (plants, invertebrates and ectothermic vertebrates) with active metabolism are 0 ≤ T (
◦ C) ≤ 50 [8].
Quite interestingly, the same limits are also relevant for the biological production of
atmospheric O 2 because the metabolism of the main O 2 producers (cyanobacteria and
plants) drops outside this temperature interval [7]. It is hard to overemphasize the role
of oxygen in this context, since the aerobic metabolism is much more efficient that
anaereobic metabolism and the presence of significative amounts of atmospheric O 2
might be a necessary condition for the emergence of multicellular life in any planet
[10].
The thermal limits 0 ≤ T (
◦ C) ≤ 50 are more stringent than the liquid-water temperature range commonly adopted in studies of habitability. These stringent limits
can be used to narrow the search of optimal targets for the SETI program.
14.2.1 How Universal Are the Thermal Limits of Terrestrial
Life?
A comprehensive mechanistic understanding of the effects of temperature on biological processes is still lacking. In spite of this, the physical nature of the main
mechanisms of thermal response at work in terrestrial life suggest that the same
mechanisms would also be at work in other forms of chemical life.
At the molecular level, there are strong indications that life processes based on
genetic and catalytic molecules require the existence of a network of hydrogen-bond
interactions [11]. Among cosmically abundant elements and molecules, CNO elements and water have unique capabilities to form molecular groups that can interact
via hydrogen bonds. Therefore, the basic aspects of terrestrial biochemistry (CHON
elements and water) are likely to be universal for any type of life life based on genetic
and catalytic molecules.
For water-based life, the water freezing point T = 0
◦ C, which is almost independent of the ambient pressure, is likely to be a universal lower bound, because
frozen water would hamper the mobility of biomolecules and, in addition, would
make impossible for molecular motors [12] to harvest the kinetic energy of Brownian motion [7]. A universal upper bound is set by the temperature at which thermal
energy denatures the molecular structures most sensitive to heat. Since intramolecular hydrogen bonds are essential for shaping molecular structures, the low binding
energy of hydrogen bonds can be used, in principle, to set universal upper limits to
the temperature of life processes [11].
At the supramolecular level, the thermal tolerance must narrow as complexity increases, because the number of molecular structures potentially limiting the