The mere presence of the building blocks (at any celestial body as well as on the Earth) as known to be essential of today's life, even those of the earliest remnants of life we are aware of, is neither a necessary nor a sufficient condition for the origin of life.
In addition, Eschenmoser et al. (2000) have shown experimentally that nucleic acids can work by other chemistry than today: RNA can be formed with different nucleobases, even with hexoses instead of the pentose ribose, with pyrophosphates; it may translate into different amino acids. However, the effectivity (in a Darwinian sense: the "fitness", as defined by Eigen and Schuster (1971)) of the processes above is generally lower. Thus it is very probable that evolution on the Earth started on a purely chemical stage with different, though similar, building blocks.
Even more important, it is by far not sufficient, as especially Nicolis and Prigogine (1977) have pointed out. At least we will give here some important necessary conditions beyond pure chemistry. Nevertheless, it is unsure whether the set of all necessary conditions being mentioned together is a sufficient condition. It is important only that "conditiones sine qua non" are discussed here in the view how comets and other celestial objects could have provided those.
As further necessary conditions, we may state as follows.
1. Life is systemic. Thus just chemicals do not constitute life, but a physical system, like a proto-cell, is subject of evolution. Cometary grains are preformed systems that in liquid water do have the necessary in/out property due to the large dielectric constant of water (environment) and the low one inside. Thus polar substances are eager to form a dipole layer as a hull. These substances need not be fatty acids from the beginning; from the view of cosmic dust chemistry porphyrine-like structures were more probable.
2. Life is dissipative, or biologically speaking, metabolic. Thus, for self-organization free energy flow must be provided from the beginning onward. At an already-organized state this would not cause a problem, the energy provided either by chemical energy (nutrients) or by solar photons. However, if there were, by chance, all necessary chemicals already present, life could not start, because this would establish near thermody-namic equilibrium. Thus precursors that react exothermally to building blocks can give the initial energetic kick into this nonequilibrium stage.
3. Life is self-organization, thus chemical reactions must be in part highly nonlinear (third order in concentrations). The only way to establish this chemically is chirality with enantiomeric asymmetry. Enantiomeric excess needs not to be provided from the cosmic beginnings, but it can be evolved by a non-Darwinian process on the Earth (tout-ou-rien-process) due to Eigen and Winkler (1975). The "wrong" enantiomers can then be recycled by UV-racemisation (Krueger and Kissel, 2001).
4. Life can be established only by a special diffusivity of the chemicals involved. This can be seen by a careful inspection of the relevant reaction-diffusion equations (Nicolis and Prigogine, 1977; Krueger and Kissel, 1989, 2001). Catalysts must be nondiffusive; cometary grains as starters form a cage to provide this property (Greenberg and Krueger, 1996). By contrast, substrates and products must be diffusive, i.e., small molecules easily solvable in water.
5. The system size, or the diameter of the hull mentioned in (1), must be within certain limits, very much dependent, i.e., on the actual diffusion constants mentioned in (4); for proto-cells with simple chemical assumptions sizes about 3 |m come out to be optimal.
There are some more special necessary conditions. As these are beyond the scope here, we refer to the literature cited above.
Was this article helpful?