The compartmentalistic approach
II. The compartmentalistic approach
The main conceptual tool of the prebiotic RNA world is self-replication. The main conceptual tool of the compartmentalistic approach – if such a word does not exist, we need to create it . . . – is so that everything starts within a closed spherical boundary. The argument in favour of this view is very basic: all life on Earth is cellular, namely based on closed compartments. If we take all cells of this world – the argument sets forth – and squeeze their content into the vast ocean, we will have all the RNA and DNA in the world – and no life. Also inside the cells there are other kinds of mutually interacting compartments, and in fact the main functions of life can be seen as an interaction between the inner world and the external medium; such functions are guaranteed by the flux of material and information through the boundary of the compartments. If such an asset is so important for life, the compartmentalistic approach then suggests that in order to consider the origin of life we have to start from a primitive semi-permeable closed boundary. This scenario is reinforced by the fact that cell-like compartments – e. g., vesicles –
form spontaneously with molecules of prebiotic origin, as illustrated in Figure 2.2 .
30 Approaches to the definition of life
Surfactant
Ionic
Amphiphilic Hydrophilic
Figure 2.2 The spontaneous self-aggregation of membranogenic surfactants into a vesicle, with an interior water pool that can host water-soluble molecules. If this self-aggregation takes place also in the presence of hydrophobic molecules, and/or ionic molecules, these can organize themselves into the bilayer or on the surface of the vesicle. A realistic scenario of the emergence of life can be based on a gradual transition from random mixtures of simple organic molecules to spatially ordered assemblies, displaying primitive forms of cellular compartmentation, self- reproduction, and catalysis.
This kind of approach, although with different wording and emphasis, has been in the literature for some time. Actually the first books on the origin of life emphasize the importance of a cell-like compartment as the prime act of the evolution that eventually leads to the self-reproducing cells; Oparin ( 1953 ), Morowitz ( 1992 ) and Dyson ( 1985 ) all discuss this concept. Several other authors have worked on these ideas, most notably David Deamer (for example Deamer, 1985 and 1998 ; Pohorille and Deamer, 2002 ); my own group (Oberholzer et al., 1995, 1999 ; Luisi and Oberholzer, 2001 ; Luisi, 2002a ); Lancet and his group (Segre and Lancet, 2000 ; Segre et al., 2001 ); as well as the groups of Ourisson and Nakatani (Nomura et al ., 2002 ; Takakura et al., 2003 ); of Noireax and Libchaber, 2004 ; and of Yomo (Ishikawa et al., 2004 ). From all this work, as discussed later on in detail in this book, it appears that it is possible to insert enzymes, nucleic acids and other biochemicals inside these compartments, so as to have in principle the beginning of a possible metabolic pathway. We will also see in detail that such compartments (micelles and vesicles) are capable of an autocatalytic self-reproduction mechanism.
The basic working idea of compartmentalism is that these primitive shells have encapsulated some simple peptide catalysts together with other molecules, and that a primitive protocell metabolism may have started in this way. However, the question of how the primitive metabolism really started is still unanswered – and in particular how that particular metabolism could have started, that led the way to
III. The “prebiotic metabolism” approach 31
Figure 2.3 The relation between the compartmentalistic approach and the prebiotic RNA world; and the missing link (items 1 and 2).
RNA. Thus, the microcompartment approach seems capable of giving the sponta- neous foundations of the elementary basic protocells, but presently cannot proceed further; the prebiotic RNA approach appears to start from the roof (from already formed RNA molecules) and is unable to explain how this roof came about – as
illustrated in Figure 2.3 .
It seems that there is a missing link between the scenarios of the prebiotic RNA- world and the compartmentalistic approach: and the missing link is how to make RNA by a prebiotic sequence or network of internalized reactions.