From molecular to cellular form: modeling the first major transition during the arising of life

It has long been suggested that Darwinian evolution may have started at the molecular level and subsequently proceeded to a level with membrane boundary, i.e., of protocells. The transformation has been referred to as "the first major transition leading to life". However, so far, we actually have little knowledge about the relevant evolutionary mechanisms - and even about the plausibility - of such a transition. Here, based upon the scenario of the RNA world, we performed a computer simulation study to address this issue.

First, it was shown that at the molecular level, after the spread of one ribozyme (RNA replicase), another ribozyme (nucleotide synthetase) may emerge naturally in the system, and the two ribozymes would cooperate to spread in the naked scene. Then, when empty vesicles absorb the two ribozymes via "cytophagy", the resulting protocells may spread in the system and substitute the naked ribozymes. As for the driven power of such a transition, it was demonstrated that the membrane boundary's roles to ensure the cooperation between the two ribozymes and to prevent invasion of parasites are important. Beyond that, remarkably, it was found that another two factors may also have been significant: a possibly higher mobility of the raw materials in the environment (free water) and the protocells' potential capability to move around actively. Finally, the permeability of the membrane to raw materials was shown to be a major problem regarding the disadvantage for the cellular form.

The transition from the molecular level to the cellular level may have occurred naturally in early history of evolution. The evolutionary mechanisms for this process were complex. Besides the membrane boundary's roles to guarantee the molecular cooperation and to resist parasites, the greater chance for the protocells to access raw materials - either due to the diffusion of raw materials outside or the protocells' active movement, should also be highlighted, which may have at least to an extent compensated the disadvantage regarding the membrane's blocking effect against raw materials. The present study represents an effort of systematical exploration on this significant transition during the arising of life.