Protocells: Milestones and Recent Advances

Abstract

to assemble astonishing pieces of a complicated puzzle, and realized that further advancement requires input from multiple branches of science, not just biology as the primary life science. Detailed hypotheses have been established about the different scenarios of the emergence of life, including the "RNA world," [1] the "lipid world," [2] "replicator first," [3] "metabolism first," [4][5][6] and others. Although the origin of life is still surrounded by many open questions, our understanding of chemical, physicochemical, and biochemical processes possibly involved in the ancient events preceding Darwinian evolution has seen much progress. We have come a long way from Leduc's physicochemical, inorganic matter-centered view on the beginning of the evolution, yet the matter of the transition from nonliving to living matter still remains largely unsolved, and one of the great scientific problems of our time.The phylogenetic tree of different living domains reflects that life has evolved from simple to more complex structures, i.e., from single-to multicellular organisms. The oldest fossil evidence dating back 3.5 Gy (billion years) comes from stromatolites, [7][8][9][10] microorganismal residues in sedimentary rocks. [11] There appears to be a gap of knowledge regarding the period of evolution between the first primitive hypothetical cells and the fossilized ancient bacteria, which can be considered as an already advanced form of life. [12] It is highly likely that intermediate primitive cell precursors preceded the single-cell organisms. The hypothetical prebiotic structures that were the stepping stone to first self-sustaining living cells are commonly termed "protocells." The possibility of a strong link between the formation of protocells and the origin of life can today be reasonably assumed.One cannot easily proceed in the context of the evolution of cell-based organisms without briefly illuminating the concept of life as we know it on our planet. Over time, different requirements have been proposed for an entity to be considered alive. According to Tibor Ganti's chemoton model, [13] a protocell contains three autocatalytic subsystems: a membrane subsystem that keeps the components together and intact, a metabolic subsystem that captures energy and material resources, and an information subsystem that processes and transfers heritable information to progeny. To be considered alive, these subsystems must be unified and function co-operatively for the survival and evolution of the supersystem. Pohorille and Deamer suggested a modified set of 7 criteria related to the chemoton. [14] At about the same time, Oro defined the requirements by 10 characteristic features. [15] Despite their differences, these descriptions align well with NASA's broader definition of life: "a self-sustaining chemical system capable of Darwinian evolution."The origin of life is still one of humankind's great mysteries. At the transition between nonliving and living matter, protocells, initially featureless aggregates of abiotic matter, ga...

Distinguishing cellular from abiotic spheroidal microstructures in the ca. 3.4 Ga Strelley Pool Formation

“…However, these organic‐walled spheres (acritarchs) differ from those reported here by their taphonomy as they occur as a large population, they are preserved as hollow folded wrinkled carbonaceous vesicles compressed parallel to bedding in shales and they keep their integrity following acid extraction (Javaux et al, 2010). Auto‐assembly of organic molecules may form large vesicles in laboratory experiments (Chen & Walde, 2010; Deamer, 2021; Gözen et al, 2022; McMahon & Cosmidis, 2021) but their preservation in the rock record and taphonomy is unknown and needs to be tested in different preservation windows and lithologies (shale and chert) (Javaux et al, 2010). Another interpretation for large vesicles is giant sulfur bacteria (Czaja, Beukes & Osterhout, 2016), suggested by an association with numerous sulfides (Sugitani et al, 2015b; Figure 1) that could have precipitated in an H 2 S‐rich environment favorable to sulfide oxidation carried out by such bacteria.…”

Growth, replication and division enable evolution of coacervate protocells

“…For a detailed account of the properties and advantages of different protocell types, we refer the reader to several recent reviews. 7,8,24,[27][28][29] Interestingly, it is becoming increasingly clear that coacervates can interact strongly with membranes, and that this interaction may have a functional role in modern cells. [30][31][32][33] As a consequence, it may be plausible that hybrid protocells with a coacervate core and a lipid-based membrane formed at some point during the emergence of cells.…”

“…1 In recent years, many chemical systems have been reported in which one or more of these hallmarks have been reconstituted, as steps on the way to creating a living, viable protocell (a primitive cell). 5 It is not known what the initial chemical systems or protocells precisely looked like, but they must have originated from the abiotic molecules that were likely present on early Earth, [6][7][8] and they must have been able to undergo a primitive form of open-ended evolution in order to increase in complexity and gain new functions. 4 This implies that a chemical ancestor of modern life should have been capable of autonomous proliferation and thus mimic the cell cycle of growthreplication-division described above.…”

From vesicles toward protocells and minimal cells

“…The hypothetical precursor systems which are thought to have formed in prebiotic times before the first cells emerged are often called “protocells”. 10–15 Such hypothetical protocells consisted of molecules present in prebiotic times. Another approach towards understanding the emergence of the first cells is to consider – and try to build – “minimal cells”.…”