Cells

[Skye]

There's no point arguing that the boundaries of a particular concept are arbitrary (or blurred) because this applies to all concepts. We might as well give up thinking.

[Guest pythagoros]

"Micelles[/b'] form from amphipathc lipids that position the hydrophobic tails in the center of the lipid aggregations with the polar head groups facing outward" Micelles form spontaneously when amphipathic lipids are placed in water. It has been suggested that the first cell was just a piece of DNA or RNA, capable of reproducing, contained within a micelle. Because they lack fossil records they really have no idea what life was life before the earliest found prokaryotes.

 

I KNEW the musk oxen theory of life was correct ! just kidding

But the image of a circle of musk oxen protecting its young

was the first image that popped into my head when I read your description of a lipid wall encircling the "defenseless" DNA ;-)

[Arg]

We might as well give up thinking.

I agree. I'd also like to take this opportunity to say that it seems like many already have!

[admiral_ju00]

Here's is a nice paper that might shed some more light:

 

The coevolution theory of the origin of the genetic code

[Rakdos]

microspheres

 

 

 

Last year I had to take a bio class. Early in the book, it talks about Cell Theory. In second semester it talked about cells evolving from bubbles of lipids(it had a name for them, but i don't remember it). They aren't compatible. Which one is in the standard model for biology?

[ydoaPs]

if life arose in this manner, then would there not be billions of different types of life(not species, types)? like cells that don't use DNA or RNA, but something else all togather.

[LucidDreamer]

Good point. It's all speculation anyway. The argument that I can make for the pro-biotic soup argument is that early life evolved in an extreme environment like hydrothermal vents. This environment had a limited amount of energy and there was an enormous amount of competition. Any small chemical advantage allowed that organism to greatly out compete the other organisms. This very specific and limited environment did not encourage diversity.

 

The basic chemical mechanisms today like left-handed amino acids, DNA with four bases, RNA, ATP, etc. probably evolved before life evolved the ability to take advantage of the enormous amount of energy given off by the sun with its many different kinds of environments that encourages diversity. The DNA/RNA/protein system was already firmly embedded and it would have been very difficult to evolve a new system after a certain point because you have to change the whole system, which is all tied together, all at once. Evolution works on small mutations.

[Skye]

if life arose in this manner, then would there not be billions of different types of life(not species, types)? like cells that don't use DNA or RNA, but something else all togather.

Maybe our ancestors ate them.

[Sayonara]

if life arose in this manner, then would there not be billions of different types of life(not species, types)? like cells that don't use DNA or RNA, but something else all togather.

Competition would quickly put an end to the least efficient systems.

[admiral_ju00]

Competition would quickly put an end to the least efficient systems.

 

if life arose in this manner' date=' then would there not be billions of different types of life(not species, types)? like cells that don't use DNA or RNA, but something else all togather.[/quote']

 

In addition to what Sayo said, Mutation, Natural selection and Genetic Drift does not agree with you there.

 

{edit}

 

Forgot to add this: Organisms that are highly specialized, do not last very long outside of the condition, environment - niche - that they are so custom tailored to. Meaning there HAS to be some sort of a mechanism that will enable them to differentiate and adapt to new environments. Otherwise they will go extinct once removed from their niche or a new element is introduced to it, etc.

[LucidDreamer]

Of course, there is always the theory of pansporia.

[ydoaPs]

there would still be more than one type, unless something selected with EXTREME prejudice.

[Sayonara]

there would still be more than one type, unless something selected with EXTREME prejudice.

Like, say, competition?

 

Consider that if a mere phenotypic expression is being selected against, such as coloration, it will most likely have a much less drastic effect on a species' survival than would selection against the very mechanism by which they pass on their information.

 

It is possible (in fact, I'd say likely) that other systems of information preservation have existed on Earth in the past, but it would only take a slight selective nudge from competing systems to obliterate them entirely.

[LucidDreamer]

You have to admit that it is a bit strange that all life on Earth has as much chemistry in common as it does. It would mean that all life on Earth originates from one ancestor that already had the modern mechanisms of replication, translation, transcription, phospholipid membranes, about 20 or so amino acids, energy in the form of ATP, etc. That’s a lot of chemistry without much deviation surviving. It seems like other mechanisms would have survived through the exploitation of a niche. I guess it just shows how fragile early life's existence was.

[Sayonara]

It is more evidence of the fact that exploiting a niche is not enough to guarantee species survival.

 

The species that uses X to encode its information may be better at exploiting a niche than the species that use DNA/RNA, but if X is not as effective at preserving information then it will give way to RNA/DNA no matter the population-level circumstances.

[ydoaPs]

Like' date=' say, competition?

 

Consider that if a mere phenotypic expression is being selected against, such as coloration, it will most likely have a much less drastic effect on a species' survival than would selection against the very mechanism by which they pass on their information.

 

It is possible (in fact, I'd say [i']likely[/i]) that other systems of information preservation have existed on Earth in the past, but it would only take a slight selective nudge from competing systems to obliterate them entirely.

 

that is like saying one thing selects for us and every other spiecies becomes extinct. it doesn't work like that.

[Sayonara]

No, that is not what it is saying.

 

Read it again.

[ydoaPs]

there would still be more than one. the other wouldn't neccesarily die out. the lifeforms with DNA would just be mor numerous

[Sayonara]

It doesn't work that way.

[ydoaPs]

if DNA worked better than EVERYTHING else, then there would still be remnants of the other types.

[Sayonara]

No there would not. There would be no living species of that kind for the reasons I stated, and sub-cellular evidence is not likely to be around after almost four billion years, now is it?

[ydoaPs]

u have yet to say why the others would no longer be around

[Sayonara]

I've already told you why, but for the sake of clarity I will expand on it:

 

When competition occurs, and there is a species with one or a few traits that make it less suited to the local ecology than its competitors, then realistically one of two things can happen to preserve the information of the less suited species:

 

(a) The population can migrate and attempt to find a less competitive or more resource-rich habitat,

(b) Selection can drive the population to adapt so that they are not competitively disadvantaged.

 

The first one only works if there are suitable habitats within range of the species under threat, and if the more competitive species are not already there (or cannot reach the new habitat).

 

The second one only works where information is being passed on. A species can't adapt otherwise.

 

In the case of the species I first mentioned, neither of these are going to be a particular problem, especially if geographic dispersal is possible and there is a high rate of reproduction.

 

However, where we are discussing a species (or a kingdom rather) which does not share the DNA/RNA system, and assuming we are taking the lack of such organisms now to be evidence that they are extinct, it is reasonable to assume that the system in use in this kingdom (let's call it X) is competitively weak next to the DNA/RNA system.

 

In such a scenario, (a) would not help, especially over long periods of time. The wider range of the other kingdoms and the sheer diversity of species would mean that virtually any habitat that the X-system species might use will already have been colonised by organisms of at least one trophic level. This means that X-species can never be effective as exploitative producers (and are slightly unlikely to be primary consumers, although that is less of a problem).

 

The second problem is even more threatening. Because these species can't thrive, and because there is little chance of random drift or competitive selection throwing up a whole new system of information encoding, selection begins to occur without their genetic input. They themselves are being selected against. Their information is not being passed on to offspring with enough frequency or range to allow for sufficent diversification. It's not just a simple matter of features being selected against; their entire system of information preservation is being prevented from working. They cannot adapt to that.

[LucidDreamer]

Instead of the energy rich environment that we live in early life was more restricted. Early life organisms were probably chemoautotrophs that used Hydrogen sulfide, which is formed at vents in the ocean floor. This is a very specific environment with limited resources. I think for the theory of abiogenesis to work early life must have developed all of its current fundamental mechanisms (DNA, RNA, ATP etc) before it spread to other more fertile environments. If early life could have taken advantage of the sun with huge amounts of available energy and had an entire ocean to spread out in than more types of mechanisms would exist today.

 

I think that in order for what Sayonara has said to work early life must have been confined to a limited environment. I do not believe that competition could eliminate the other mechanisms throughout the entire planet with the incredible resources of the sun. Lots of evolutionary branches would form by taking advantage of various niches. One form would not be able to stamp out all of the others before these branches had time to evolve to forms that could compete.

[Sayonara]

What I said does not require that multiple systems formed at the same time.

 

The point I am trying to get across is that a weaker information system will collapse entirely when in competition with a stronger information system, as opposed to weaker genotypes competing against stronger genotypes, where there is at least some chance for adaptation.

 

The circumstances are fairly irrelevant, as the success with which information is preserved and diversified will over-ride all other competitive concerns. If it didn't we would see non-DNA/RNA cellular life.

 

By the way, taking hold of a niche and exploiting it does not in any way guarantee that a species will stay in that niche.