Which Came First?

[foodchain]

The cell or the code? Such as the cell or RNA/DNA.

[insane_alien]

RNA came first. its the simplest. DNA and very simple cells would have came next

[ecoli]

RNA came before cells? I'm not so sure about that. Weren't protobionts nothing more than droplets of organic chemicals surrounded by a membrane? They didn't have RNA yet, did they?

[SkepticLance]

Answer : we don't know.

 

Without doubt, some kind of replicating molecule came first. The best current candidate is RNA, but we cannot be sure. The trouble is, molecules don't leave fossils!

[ecoli]

Without doubt, some kind of replicating molecule came first.

 

Why?

[dttom]

To discuss which, the code or the cell, came first, it is about biological substance, that is cell or replicator, otherwise, one could certainly say that any inorganic element or compound came first, far before any thing regarded as 'living' came first. The question is how do we define life, or so, how do we define a cell. Does a membrane-bound chemical is a cell? Then we could just wrap a chemical with dialysis tubing then we have created a cell! So let say it a cell as an entity at least contain some replicator, so that such an entity could propagate throughout the population, utimately forming what we now see as the ecosystem. Hence I believe it is replicator, or the code, came first, otherwise no 'cell' is formed, and no evolution could be observed, which we usually use the term in biology but not simply chemistry.

[SkepticLance]

ecoli asked why the replicating molecule came first.

 

One answer is that a cell is a much more complex structure and would thus take longer to evolve.

 

Another answer is that the properties of the replicating molecule are needed to create and replicate cells. No cell can reproduce and evolve without the replicating and encoding abilities of the molecule.

 

A third answer is that this is the logical extenstion of the word begun in the 1960's which proved that organic molecules can be created under natural and ancient conditions that occurred 4 billion years ago. If the first step towards life is the creation of simple organic molecules such as amino acids, and purines, then the second step is to form these into polymers. The third step is to form, from simple polymers, a complex molecule that can replicate. To form a cell would have to follow after.

[foodchain]

ecoli asked why the replicating molecule came first.

 

One answer is that a cell is a much more complex structure and would thus take longer to evolve.

 

Another answer is that the properties of the replicating molecule are needed to create and replicate cells. No cell can reproduce and evolve without the replicating and encoding abilities of the molecule.

 

A third answer is that this is the logical extenstion of the word begun in the 1960's which proved that organic molecules can be created under natural and ancient conditions that occurred 4 billion years ago. If the first step towards life is the creation of simple organic molecules such as amino acids, and purines, then the second step is to form these into polymers. The third step is to form, from simple polymers, a complex molecule that can replicate. To form a cell would have to follow after.

 

Yes, but then you must logically assume that the "code" then made the cell? I just don’t see naked polymers being able to make it in the wild so to speak to that point in my opinion. I also have a very hard time putting life in a common sense to this, it has to be one of those simply physical things so to speak; I see the "protocell" as something required really. For instance, chemically speaking, what would motivate "code" which is really what, to produce a cell? On the other hand though, "code" in a cell would have a buffer really or something of a medium in which to operate via. The cell is a highly conserved biological entity, present from single to multi-celled biological systems overall, we can see all kinds of variation, but even in the virus, you have some kind of body or cell really. A cell chemically speaking, could basically via chemistry in my unsubstantiated with factual data type of opinion I could see basically in environmental change self assemble itself some stability. Such as a "protocell" in various chemistries, or environments adapting a chemistry for a stable equilibrium, which could tie into chemotaxis at some point. If memory serves many amino acids exist in nature that are not used by biological systems.

[CharonY]

Ahem, just a bit of nitpicking, but the "code" is the coupling of anticodons presented by tRNAs coupled to an amino acid. This came up much later.

 

Now the notion of self-relplicating molecules is quite clearly the easiest to comprehend, and is the generally accepted hypothesis. As mentioned above it is also quite clear that these molecules are lilely to precede whole cells, at least what we understand as cells nowadays. How cells were assembled, however, is to my knowledge still a mystery.

 

And whether DNA or RNA were those self-replicating molecules (they are the likeliest candidates due to their low complexity and occurence in every single organism) it is still a bit debated. Many assume that it is RNA, however others believe that RNA is too unstable alone.

[foodchain]

RNA came before cells? I'm not so sure about that. Weren't protobionts nothing more than droplets of organic chemicals surrounded by a membrane? They didn't have RNA yet, did they?

 

Here are some links to follow this up.

 

http://images.google.com/imgres?imgurl=http://www.biology.iupui.edu/biocourses/N100/images/19lipiddrop.gif&imgrefurl=http://www.biology.iupui.edu/biocourses/N100/ch8life.html&h=237&w=333&sz=42&hl=en&start=5&um=1&tbnid=AwKIaO_012yw8M:&tbnh=85&tbnw=119&prev=/images%3Fq%3Dprotobionts%26svnum%3D10%26um%3D1%26hl%3Den

 

http://www.daviddarling.info/encyclopedia/P/protobiont.html

[SkepticLance]

One theory is that of the primeval soup. Early in the history of Earth, enough organic molecules washed down from the air to form solutions of organics in the early seas of our planet. As previously stated, the simple compounds formed polymers which eventually formed at least one self replicating molecule.

 

That molecule would be in the soup. Thus, it would be surrounded by a solution of organics. The organic molecules would be the 'food' used to support replication. A molecule replicating in a soup would not need the internal chemistry of a cell, since the soup provides that chemistry.

 

After some time, there would be an enormous quantity of self replicating molecules, now in competition for the remaining 'food'. Changes would occur in those molecules in the way that mutations change nucleic acids today. The best fitted changes would survive and the others die. Thus, evolution at a molecular level would be under way.

 

RNA molecules sometimes have the special ability that they can be catalysts for chemical change. This is a major reason why many biologists believe RNA came first. DNA based life forms need proteins to act as catalysts. If we assume that the first self replicating molecule was some form of RNA, then the evolution of these molecules would lead to a form that could catalyse chemistry that benefited that molecule. If that chemistry involved making fatty acids, then those fatty acids could form a layer around the molecule, like a cell membrane. This would be the first 'cell'.

 

Of course, this is all speculation. My scenario above might be quite wrong!

[foodchain]

One theory is that of the primeval soup. Early in the history of Earth, enough organic molecules washed down from the air to form solutions of organics in the early seas of our planet. As previously stated, the simple compounds formed polymers which eventually formed at least one self replicating molecule.

 

That molecule would be in the soup. Thus, it would be surrounded by a solution of organics. The organic molecules would be the 'food' used to support replication. A molecule replicating in a soup would not need the internal chemistry of a cell, since the soup provides that chemistry.

 

After some time, there would be an enormous quantity of self replicating molecules, now in competition for the remaining 'food'. Changes would occur in those molecules in the way that mutations change nucleic acids today. The best fitted changes would survive and the others die. Thus, evolution at a molecular level would be under way.

 

RNA molecules sometimes have the special ability that they can be catalysts for chemical change. This is a major reason why many biologists believe RNA came first. DNA based life forms need proteins to act as catalysts. If we assume that the first self replicating molecule was some form of RNA, then the evolution of these molecules would lead to a form that could catalyse chemistry that benefited that molecule. If that chemistry involved making fatty acids, then those fatty acids could form a layer around the molecule, like a cell membrane. This would be the first 'cell'.

 

Of course, this is all speculation. My scenario above might be quite wrong!

 

Maybe biofilm is some hold over of that?

 

I just have a hard time even with the primordial soup seeing a polymer or monomers simply being able to hold their own. RNA can act as an enzyme though which makes things easier for me to digest it as a reality. Then again RNA itself is a collection of chemicals, to me it just seems easier to envision some membrane or cell in which equilibrium would work in. As for the molecules themselves, they would be free to sift around in the wild, from colloidal solutions, or clay, to lava and sea vents and such, various different ph levels and so on. The cell would have to do the same, but its a medium that I could see in a primordial soup to have an easier time simply maintaining I guess, though I know close to nothing in context of the reality of what is known currently, so my opinion is not very valid either. Maybe they came at the same time?

[SkepticLance]

To foodchain.

 

In this topic, speculation becomes the main thrust of opinion. This is because real facts are minimal.

 

One fact we know is that, if you take a simulated atmosphere of Earth as of 4 billion years ago (water vapour, methane, CO2 etc) and put strong energy into it (electric sparks simulating lightning, Ultra violet, or high temperatures simulating local conditions around meteor entry) then we see the formation of a large number of organic molecules such as amino acids and the raw materials of nucleaic acids.

 

Deduction suggests that this process over millions of years, with rain washing the organics into ponds, lakes and the sea, will create the primeval soup. This is deduction, not fact.

 

Other researchers have shown that some of the organic raw materials, exposed to such substances as montmorillonite (a type of clay), and calcite crystals, will adhere to the crystal surface in such a way as to line up, and chemically bond, forming a polymer. This is fact, not deduction.

 

Deduction suggests that, over a long period, such polymers will increase in concentration in the primeval soup. Deduction also suggests that chemical reactions may occur between polymers and other substances.

 

Speculation suggests that a self replicating molecule may form. Speculation also suggests a form of molecular evolution may follow.

 

Another fact is that, in rocks in northern Canada, organic residues have been found dated to 3.8 billion years. What this means is open to interpretation.

 

Another fact is that, in rocks in Western Australia, structures have been seen that look remarkably like existing stromatolites (cyanobacteria), and this has been dated to 3.6 billion years ago.

 

If you throw together the facts, and the deductions, and season with a little speculation, you develop a picture which has a chance of actually being correct.

[Realitycheck]

I'm not exactly the abiogenesis guru myself, but let's see here. The RNA and DNA are supposed to provide all of the instructions to do everything in the cell, right? Let's also say that in this first cell, since RNA and DNA are very similar in structure chemically and that the first single-celled organism is relatively simple compared to humans, that we can envision a chemical that performs the functions of both RNA and DNA. Is that even feasible?

 

Now let us assume that we have a lot of this type of activity going on around the world, where pertinent; sea vents, volcanos, etc. That would give us plenty of trial combinations to work with. Now let's say that miraculously, over the course of a billion years, we get one trial combination of our hybrid RNA/DNA to form that works, the one combination that does it all for this little "cell" once it winds up in the fatty acid protocell/protobiont.

 

Of course, the one really big flaw to this plan is that while fatty acids might make up a cell wall, it bears very little resemblance to cytoplasm, but let's just say that this custom-made nucleic acid is built to compensate for the absolutely simplest cell structure. Maybe the fatty acid droplet baked in the sun for a while, drying out the cell wall, while the nucleic acid inside reacted with the fatty acids and other compounds inside, changing the chemical makeup of the inside, making the nucleic acid replicate.

Then maybe it got rained on and wound up in a little pool of water, where it started getting hydrated, until somehow, the replication and other reactions spawned by the excess of water surrounding it somehow caused it to split. Of course, it never could have happened without that one perfect trial combination of nucleic acid, meant just for that one specific combination of conditions that it would encounter.

[Realitycheck]

Oh yeah, now I am seeing lots of possibilities now that I have at last seen a diagram of a prokaryote. We don't need no stinking golgi apparatii, mitochondria, etc., not even a cell nucleus. This almost seems too easy to be true.

 

http://en.wikipedia.org/wiki/Image:Prokaryote_cell_diagram.svg

[Realitycheck]

This looks like a good article with lots of possibilities.

 

 

http://en.wikipedia.org/wiki/Protocell

[Realitycheck]

We can call it the Nucleic Acid Wizard. Does everything that it needs to form the first cell, and then, what's left over functions as the cell's nucleic acid. These are actually pretty hard to come by.

[lucaspa]

The cell or the code? Such as the cell or RNA/DNA.

 

Despite some of the definitive claims in the thread, this is matter of hot debate in abiogenesis circles. The most recent Scientific American has an article by Shapiro arguing against RNA first and arguing for cell first (Metabolism First).

 

I personally favor cell first by making cells from thermal proteins:

http://www.theharbinger.org/articles/rel_sci/fox.html

[lucaspa]

I'm not exactly the abiogenesis guru myself, but let's see here. The RNA and DNA are supposed to provide all of the instructions to do everything in the cell, right? Let's also say that in this first cell, since RNA and DNA are very similar in structure chemically and that the first single-celled organism is relatively simple compared to humans, that we can envision a chemical that performs the functions of both RNA and DNA. Is that even feasible?

 

Yes. RNA. RNA in most cells not used as the primary hereditary material. But in many viruses and some unicellular organisms it is.

 

Now let's say that miraculously, over the course of a billion years, we get one trial combination of our hybrid RNA/DNA to form that works, the one combination that does it all for this little "cell" once it winds up in the fatty acid protocell/protobiont.

 

Define "works". Most RNA World guys are looking for an RNA that catalyzes its own replication. That's what they mean by "works".

 

There are many, many RNAs that act as enzymes for one reaction or another. The simplest is only 3 bases long and catalyzes the hydrolysis of U from an RNA chain.

 

You don't need lipids to get a cell membrane. Even in modern cells, 60% of the cell membrane is protein. As you saw in the protocell article, proteins will make the entire cell membrane just fine.

[lucaspa]

We can call it the Nucleic Acid Wizard. Does everything that it needs to form the first cell, and then, what's left over functions as the cell's nucleic acid. These are actually pretty hard to come by.

 

No, protocells are frightfully easy to come by. There are so many varied conditions in which thermal proteins and protocells can form: a wide variety of atmospheres, amino acids, temperatures, etc. The 2 most relevant for abiogenesis research are:

 

1. Tidal pools. Have a solution of amino acids in a tidal pool that is evaporated by the sun. The result are proteins. Have the tide come back in and the result is protocells.

 

2. Hydrothermal vents. Amino acids are made at hydrothermal vents under today's conditions, and those same conditions will make the amino acids into proteins and then into protocells. So abiogenesis is occuring today at hydrothermal vents!