Communication : The key to defining "life", "death" and the force driving evolution. "Organic chemistry-based-" versus "artificial" life.

Plausible definitions of `'life'' and `'death'' can be simply derived by combining the principles of compartmental organisation of the living state, with the principles of communication and those of Ilya Prigogine's `'dissipative systems''. Prokaryotes have only one compartmental level, but all other living systems have several, up to about fifteen in the tentative classification system we propose. From the principles of communication and information it can be understood why `'life'' is more than just part-and-parcel of chemistry and physics : information in itself has no units of force or energy. Since communication is the cornerstone of life, a living entity dies when it irreversibly looses its ability to communicate at its highest level of compartmental organization. It is not important that lower levels of compartmental organisation, if present, retain their ability to communicate. Since `'death'' is the irreversible end of `'life'', it follows that a compartment starts to live when it acquires the ability to communicate at its highest level of compartmentalisation. Therefore, a `'life activity'' (L) of compartment S at moment t is the total sum of all acts of communication (C) performed by this compartment (with its different levels of organization, from 1 to j) at moment t. This can be mathematically expressed as: [GRAPHICS] Biological life as contrasted to artificial life, cannot be sustained without transmembrane gradients because of their crucial role in communication. Therefore, `'Life'' could not exist before some primordial aggregate compartmentalized and acquired the ability to sustain a gradient over its limiting membrane and thus established a communication channel. Communication at the level of the plasma membrane requires a moderately a `'leaky'' membrane to make transmembrane ion fluxes possible : thus `'life'' started with an imperfect (leaky) membrane in combination with a chemical gradient (which is by definition a thermodynamically far-from-equilibrium state) established through the membrane. Sustaining a chemical gradient requires energy, part of which is used to create order out of disorder. These are elements of dissipative systems. Gradient formation, which is a crucial event in life, which is often neglected in many fields of biology, is the primary force to self-selection and evolution. Thus, life on one hand and self selection and evolution on the other are inseparable as the two sides of a coin. Communication is not only the very essence of `'life'' but at the same time, it is a major driving force of (its own) evolution. This approach leads to a holistic type of biology in which communication plays a central role, and for which the name `'dissipative biology'' or `'non equilibrium biology'' is proposed. Our approach also allows to make the distinction between `'organic chemistry-based life'' and `'artificial (man-made) life''.