Figure 1.

(A) Possible approaches to the historical reconstruction. Two complementary approaches exist: top-down and bottom-up. In the former, the idea is to simplify the cellular architecture and cellular biochemistry by removing redundant or dispensable functions. These are functions that can be either replaced by providing chemicals or taken over by simpler chemicals easily synthesized by, e.g., “non-coded” protein catalysts, or performed, perhaps less efficiently, by other catalysts in the cells. The process should be repeated until a very simple putative “protocell” stage (vide infra) is attained. This is likely a point in time at which biology did not yet exist, but instead pure chemistry defined the protocellular reaction network. The latter approach is based on the use of molecule sets that can self-assemble into chemical aggregates and systems that will then be able to perform an increasingly more complex chemistry. These systems are precursors of protocells that preceded the emergence of ancestral cells. (B) Putative representation of a protocell (adapted from [3]). Independently of the type of chemicals involved, e.g., pure RNA catalysts/”genetic” information or peptide/RNA, a protocell should contain three components: a compartment, a catalytic and energy harvesting machinery, and an information system. These components should work in an interconnected fashion to achieve the prolonged activity necessary for the protocell evolution. The interconnectedness in the systems is visible if one considers the various arrows between molecules/components: The catalytic machinery is defined and controlled by the information component (I) and the compartment (via encapsulation), whose molecular species are in turn produced by the catalytic machinery (II: information replication, III: amphiphile production, IV: energy harvesting and chemical replication, and V: catalyst amplification, which can lead to VI: replication process of the whole protocell). The compartment will also define the access of the protocell to environmental resources and, in part, the energy harvesting capabilities. It will also be instrumental in the replication (VI). It might also permit an interface-driven multiphase chemistry (see text below). Molecular precursors (i.e., resources to build protocell chemicals) are highlighted by black dotted structures or frames. Original chemicals of the protocell are highlighted by thick grey dotted frames. Products of the catalytic machinery are placed over a grey background. The involvement of catalysts is depicted by dashed arrows, that of information components with a plain arrow, and that of the compartment (expect the encapsulation) by dotted arrows. Note that the energy-related aspect would be involved in all chemical syntheses but, for the sake of clarity, is only shown once.