It is not certain that the first living beings consisted of cells. This is actually very unlikely.

The very first living being that was able to reproduce itself could not have had a closed cell membrane, because that cannot function without pores and pumps provided by peptides/proteins, which control the exchange of substances through the membrane.

The very first self-reproducing chemical system is likely to have been an open structure formed by organic substances attached to the surface of a mineral containing the catalysts for the metabolic reactions, e.g. an iron sulfide impurified with nickel and cobalt.

There is a hypothesis, which seems very plausible, that before the appearance of closed cells, the living beings consisted of discoidal fatty membranes with peptides attached to them, which obstructed pores in minerals located in hydrothermal vents, where the environment provided a flux of ions equivalent with the flux of ions that must be provided by ionic pumps in any closed living cell.

It is likely that only after the development of active trans-membrane ionic pumps, it became possible for living beings to take the form of free closed cells, unattached to minerals.

It is not clear which happened first, the transition to free closed cells or the apparition of self-replicating RNA.

The first membranes must have also been formed by molecules with hydrophobic and hydrophilic parts like the present lipid bilayers. However the molecules must have been different and simpler. Perhaps the first membranes were made just of free fatty acids, but with short chains. They certainly must have been much more permeable than today, to allow the passing of some molecules for which there were not yet adequate transporters. In any case, the phospholipids that are dominant today in membranes are likely to have appeared only some time later than the first life forms.

There is not much positive evidence about the early forms of life, but there is a lot of evidence about many things that are impossible, so we can say for sure that they could not have existed in the first forms of life.

We can say with absolute certainly that no other function of RNA could have existed before it acquired the ability of self-replication. More precisely, if any such function would have happened accidentally in any living being it would have been immediately lost forever, because it could not have been transmitted to descendants.

We can also say with absolute certainty that the self-replication of RNA could never appear otherwise than in an environment where a source of energy ensured a continuous production of the 4 monomers required for making RNA.

That environment must have been an already existing living being, not only because there is no other known environment that could produce phosphorylated nucleotides, but also because without such an already living host there would not have been a path of evolution for the self-replicating RNA, where it acquired extra abilities of catalyzing other chemical reactions, culminating in the ability of synthesizing proteins, such that eventually the mechanism of making proteins via RNA has substituted in most cases the mechanisms for making non-ribosomal peptides.

There are many other things that we know that they could not have existed in the first living beings. Most of the living beings that we know, including many anaerobic bacteria from places without light about which it is frequent to see in the popular but incompetent press claims that they live independently of the Sun, depend either directly or indirectly on the phototrophic living beings, which capture solar energy.

There are only two kinds of bacteria or archaea that are really independent of the solar energy, which do either acetogenesis or methanogenesis. We can rule out with certainty both phototrophy and methanogenesis as sources of energy for the first living beings, which leaves only acetogenesis, a process for which there is ample evidence that it already existed in the ancestor of all present living beings.

Besides the source of energy, we also have a pretty good knowledge about the chemical composition of the first living beings.

While a human needs around 20 chemical elements, much less are needed for the simplest self-reproducible life forms.

5 non-metals are certainly needed in any life form for the organic substances: H, C, N, O and S. These happen to also be the most abundant non-metals in the Universe (not counting noble gases). The minimal set of metabolic reactions for a self-reproducing chemical system powered by acetogenesis requires at least 3 catalytic metals, iron, nickel and cobalt. The consumption of carbon dioxide instead of the less abundant carbon monoxide and of dinitrogen instead of the less abundant ammonia requires an additional catalytic metal, either molybdenum or tungsten. This extra catalyst may be a later addition to the living beings, enabling them to also live in less reducing environments, where carbon monoxide and ammonia became scarce. Besides organic substances and catalysts, there is a necessity for the first living beings to have abundant potassium ions in their environment, in order to neutralize the excess of organic acids from living matter. While in the present oceans sodium is more abundant, forcing all living beings to have means to expel sodium from their cells, it is likely that in the early oceans potassium was more abundant because it is easier dissolved from the volcanic rocks than any other component, so it was the first to become abundant in sea water, until the concentration of other ions has caught up with it after the passing of time has brought them in solution too.

This brings the total to 9 chemical elements that are certain to have been used in the first self-replicating living beings. With the later increase in complexity, other chemical elements must have been added, the most important being the addition of phoshphorus (as phosphoric acid) and of magnesium, which have lead to the use of ATP as a dehydrating agent, presumably replacing the earlier use of thioesters, which had as a byproduct the appearance of RNA.

There are a lot of unknowns, but there are also a lot of restrictions to the space of possible solutions, so there are chances that eventually it would become possible to demonstrate a self-replicating system of chemical reactions that could be similar with the first forms of life, even if it is unlikely that the solution is unique, so we might never know the precise details how it happened.