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(sorry if the title seems like flame-bait - but it's a real question). I'm trying to understand what could have come before the first cell (What are the "minimum requirements" for a single cell?)

From what I've seen, the presented chemical evolution (aka abiogenesis) process centers around self-replicating RNA that gets trapped inside a lipid-based protocell and then somehow turns into a functioning cell. But it seems like a giant leap to go from that situation to the most basic cell we know about.

Instead of starting from basic molecule and trying to get more complex, what if we take a simple cell (which is actually really complicated) and try to get more simple - try to take out some of the pieces. Work our way backwards. The problem is, I don't know what pieces you can take out. It seems there are several molecular systems that need to be functioning for cell to be viable. Once you start to imagine a cell without them, it seems the cell will not be able to survive or replicate.

My question is, what would the step just before a "minimum cell" look like? Are there pieces we can take away and it would still be able to survive and replicate?

** Edit **

Based on some of the discussions in the comments, I wanted to clarify what I'm asking. This is a bit long - but hopefully it will help.

Evolution is a process in which replicating systems transfer beneficial permutations to 'offspring'. Usually the term is applied to "biological evolution) (meaning something living - from a cell to more complex organism) but I've also heard people refer to "chemical evolution" which is the process in which atoms combine to make molecules and amino acids and the amino acids combine to make RNA which can replicate. Or fatty molecules combine to make a lipid membrane, etc.

In this process to get to something more complex, you have to have gone through a simpler stage. To arrive at RNA, you had to have a system that would first produce nucleic acids. If the first life was in the oceans, you needed something that could survive in and out of water before you now have creatures on dry land. Each stage has to have a predecessor.

Generally question of the origin of life start at the raw materials and work their way towards the more complicated. Most of the discussion I've seen basically end with RNA in lipid cells + billions of years = a prokaryote. The difference between RNA in a protocell and a working basic cell is vast and I haven't seen an explanation on how to bridge that gap.

I'm approaching the question from the opposite direction. If we take the most basic, simple prokaryote cell - what came before it? Even such a simple cell is in actually very very complicated (semi-permeable membrane, nucleoid, ribosomes, cytoplasm, etc..) There are so many interconnected pieces that it seems are only valuable when working as a whole. Eg, could I have had a cell without ribosomes and then some how it mutated to have ribosomes? It seem not, because a cell without ribosomes would never have survived/replicated. The same with other components.. if I have some precursors without that piece, it would never have survived long enough to mutate to now have that component.

So my question is what could the predecessor to a basic cell have been, considering its complexity and the seeming interdependence on the pieces for the cells survival?

I hope that helps explain the question better - thanks for any insight you can provide!

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  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$ – Chris Jul 20 '16 at 19:43
  • $\begingroup$ ribosomes are made of RNA, Ribozymes are strands of RNA that act like enzymes ribosomes among them. So you have a smoother transition from RNA that catalyzes itself to RNA that catalyzes other strands of RNA, to RNA that translates other RNA into proteins. The human classification systems get in the way of your imagination, at the chemical level there is very little difference between many "essential" molecules many are in fact the same just arranged in slightly different ways. $\endgroup$ – John Nov 24 '18 at 16:41
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“Irreducible complexity” is, as you seem to realise, a weasel term used to imply that a thing (like a cell) cannot have arisen from something simpler (or at least “different”). In that sense the answer to the question in the title is a clear “no”, unless we accept supernatural explanations for the origin of life.

You go on to state that the heart of your question is “what could the predecessor to a basic cell have been, given its complexity...”. I can strongly recommend the classic text of A.I. Oparin, “Origin of Life”, published in an English translation in 1938(!).

This remarkable book contains a general exposition of the notion that living cells were preceded by “a gradual evolution of carbon and nitrogen compounds”, to quote the blurb of my personal copy.

The book is all the more remarkable in that its insights preceded the modern understanding of the nature of RNA and DNA - the type of “inheritance” with which the book is primarily concerned is not one of nucleic acid replication, but the relative abundance of specific chemical reactions - specifically, the so-called coacervates (spelt "coazervates" in the referenced book) and other manifestations of colloidal chemistry.

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There is no straightforward answer as to whether a "single cell" is irreducibly complex. Which cell are we talking about? Human mitochondria do not survive without their genome, but those of Cryptosporidium do persist without it. The Minimal Genome Project, led by biotechnologist Craig Venter, concluded with the creation of Mycoplasma laboratorium, the result of removing non-essential genes from Mycoplasma genitalium. This minimum cell had 382 genes.

Are 382 genes the minimum number of genes required for life then? No. That number refers to the total number of genes necessary for a particular organism, which happened to be Mycoplasma. Iterative trials of removing genes will reveal different numbers of essential genes for different organisms. Primates will require a high number of essential genes, whilst archaea could survive with fewer genes.

Regarding the transition from RNA-only to RNA-protein world, peptides function as cofactors for some ribozymes. Amino acids and peptides are known to have existed in the prebiotic environment and have been found in space (glycine has been found in comets, along with other 70 amino acids).

Regarding the formation of nucleotides, from Scientific American.

A New Route In the presence of phosphate, the raw materials for nucleobases and ribose first form 2-aminooxazole, a molecule that contains part of a sugar and part of a C or U nucleobase. Further reactions yield a full ribose-base block and then a full nucleotide. The reactions also produce “wrong” combinations of the original molecules, but after exposure to ultraviolet rays, only the “right” versions—the nucleotides—survive.

This answer is not complete, I will add more information and provide citations in the future.

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Eg, could I have had a cell without ribosomes and then some how it mutated to have ribosomes? It seem not, because a cell without ribosomes would never have survived/replicated.

The premise in the chain of reasoning that "we could hypothetically remove parts until we reach the bare minimum, but we can't so the cell came to be in its entirety is flawed". As populations evolve the organisms can gain and lose components. As an analogy consider the question of how humans can develop mouths. Our diet consists of chewing food which gives us energy to build structures like teeth and jaws, but we need teeth and jaws to chew food. A similar paradox. But of course we develop in the womb with an umbilical cord that we used to draw nutrients from our mothers that bypasses the mouth. This structure is not present in us after birth so we can't tell from looking at an adult human.

My question is, what would the step just before a "minimum cell" look like? Are there pieces we can take away and it would still be able to survive and replicate?

A very interesting question I think, but I also don't think it can be answered by looking at a modern cell and removing parts. The ancestor to cells may have lived through a very different system for example utilizing RNA to both catalyze essential reactions and store replication information as opposed to using proteins and DNA to do the same thing.

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