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When people try to explain evolution, they tell me that evolution is a cumulative result of mutations & natural section of the more superior individuals of a particular species. I think I'm fairly convinced with this explanation.

But when I think about it, all of them assume that there was an organism, however simple, that was capable of self replication & occasionally mutate. How did such an organism come into existence? Can anyone explain this?

An answer I found on Reddit didn’t really convince me.

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If you want a good start, I would suggest looking at the RNA World hypothesis. –  bobthejoe Jun 30 '12 at 6:43
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Can we please leave out the entropy canard? It’s totally irrelevant. Earth isn’t a closed system. Period. Furthermore, this would be completely off topic on a biology site. –  Konrad Rudolph Jun 30 '12 at 13:09
    
I remember reading this entire article a few years ago, it was pretty revealing. en.wikipedia.org/wiki/Abiogenesis –  LanceLafontaine Jun 30 '12 at 15:19
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Entropy is not irrelevent for this question (how could it be in organic chemistry??). Earth is not a closed system,but in these considerations you have to consider the scale of the experiment. –  Poshpaws Jun 30 '12 at 19:23
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If you actually run the calculations, the second law of thermodynamics does show you something related to evolution: the entire evolution of the current biosphere could not have happened in much less than a month (physics.gmu.edu/~roerter/EvolutionEntropy.htm). As long as you take longer than a month (say 4 billion years), there's no problem at all. –  Noah Snyder Jul 5 '12 at 16:54

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up vote 11 down vote accepted

Evolution or (as Darwin called it) "descent with modification" is a theory which explains the origin of the species NOT the origin of life. How the first life arose is completely irrelevant to the theory of evolution. What evolution does explain is how and why we have such variety of life on earth all descending from the same organism.

What you're asking about is not a theory of "evolution" but rather a theory of "abiogenesis." Although there are many interesting hypothesis for how abiogenesis happened (e.g. the RNA world, the "metabolism first" theory, etc.), the fact is we simply do not know yet how life first arose. What we do know is that life first arose between 3.5 and 3.9 billion years ago. That's a really long time ago compared to lots of other important events in natural history (even the Cambrian explosion where modern animal phyla evolved was only half a billion years ago), and so it shouldn't be surprising that it's a hard problem.

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Yes, you are right. I am asking about a theory of "abiogenesis". But I do think that its related to Darwin's theory because it assumes that an organism with the required qualities existed. Isn't Darwin's theory based on an unproven assumption? Can you clarify? –  Green Noob Jul 5 '12 at 6:32
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We know that organisms with the required properties existed 3.4 billion years ago, by looking at fossils of bacteria. There's no unproven assumption there. Evolution describes the most recent 3.5-4 billion years of natural history. To go beyond that you need another theory. That's fine, that's how science works. You wouldn't say that gravity relies on unproved assumptions because it doesn't explain electricity or radioactive decay; it's not supposed to, it's supposed to explain gravity. –  Noah Snyder Jul 5 '12 at 16:37
    
Agreed. I have nothing against Darwin's theory. All I wanted was a convincing theory of abiogenesis. –  Green Noob Jul 6 '12 at 4:42
    
@GreenNoob The theory of evolution (only creationists call it Darwin's theory, just FYI) deals with changes in populations. It says that given a starting population, changes will occur because X,Y, and Z in a way that you seem reasonably convinced by. That starting population is an objective fact; we can see it in the fossil record. Further, the theory of evolution says nothing at all about where that population came from. It could be abiogenesis, aliens, or a deity; it really doesn't matter. Abiogenesis uses many of the same ideas as evolution, but it's a completely separate theory. –  Jason Patterson Sep 29 at 17:42

We don't know how self-replicating molecules first arose (and probably never will know exactly) but the Earth is large and had 500 million years (i.e. the prebiotic Earth timescale) or so to experiment in organic chemistry. The land-sea interface (such as tidal pools) are a good candidate site since these are areas where high concentrations of organic goodies can be found.

In this context, one focus that researchers have been looking at is self-replicating molecules.

For example, one lab in Cambridge,UK has come up with tC19Z.

tC19Z is the name of a RNA enzyme that acts like a self-replicating molecule. It can copy chunks of RNA almost 50% as long as itself. It can also make copies of other RNA enzymes. This molecule is not "alive" itself, but clearly demonstrates how greater complexity can arise.

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We don't know how self-replicating molecules first arose (and probably never will know exactly) Ouch, that hurt. You mean the whole of Darwin's theory rests on an assumption that is unprovable? –  Green Noob Jul 1 '12 at 14:32
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No, I just mean that how are we really ever going to know what the exact sequence of events was? I suppose it depends whether you are a contingency or convergence type! –  Poshpaws Jul 1 '12 at 14:44
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Although we don't know exactly, we do have lots of examples of ways it might have happened. Not knowing exactly which one it was is not a problem, the important thing is we can demonstrate that it could easily happen that way. –  Richard Smith-Unna Jul 2 '12 at 0:41
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@GreenNoob: Darwins Theory of Evolution doesn't deal with this aspect of life at all, and it rests upon two very provable assumptions: Life exists; and life changes. –  Williham Totland Jul 4 '12 at 22:09
    
@WillihamTotland Can you elaborate? –  Green Noob Jul 5 '12 at 6:26

They teach us in Physics that the entropy of an isolated system is always increasing or at least constant. Then how can an organism be born under these conditions?

The sun sends energy to the Earth, allowing for a decrease in entropy on Earth at the expense of the sun's entropy.

But when I think about it, all of them assume that there was an organism, however simple, that was capable of self replication & occasionally mutate. How did such an organism come into existence? Can anyone explain this?

That organism you're talking about is just a molecule that copies itself. Exactly how it has come about is not clear to me but it's not hard to imagine the possibility. A vast planet with molecules flying all over being bathed in ultraviolet light and if any molecule anywhere acquires the characteristic of copying itself, it will start growing exponentially and quickly spread all over the world.

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If any molecule anywhere acquires the characteristic of copying itself, it will start growing exponentially and quickly spread all over the world Don't you think that's a big 'if' for a scientific theory? –  Green Noob Jul 1 '12 at 14:28
    
Upvote for the "Sun sends energy to the Earth" part. Really cleared things up :) –  Green Noob Jul 1 '12 at 14:30
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No I think its a small if because its the only plausible explanation for the emergence of life I have ever encountered. –  Hermann Ingjaldsson Jul 1 '12 at 19:59
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@GreenNoob It's not at all a big if - there are lots of molecules which demonstrate that property, some are biological like DNA or RNA, and some, like crystals, are mineral. –  Richard Smith-Unna Jul 2 '12 at 0:39
    
@RichardSmith Can you give examples other than DNA or RNA? I don't think that such complex molecules originally existed in the primitive earth's atmosphere. –  Green Noob Jul 5 '12 at 6:37

This is an extremely interesting and extremely fundamental question, indeed, and thus far, biologists have failed at coming up with a satisfying answer.

We know that all the parts are there, we just don't know how they were arranged, or which ones go where.

The question is, in essence, composed of three sub-questions:

  1. How did the fundamental building blocks of life come about?
  2. How did the first self-replicating molecules come about?
  3. How did cell membranes come about?

The answer generally takes the form of "On primordial Earth, a small selection of the billions of organic compounds generated when UV-light hits a mess of carbon dioxide, nitrogen and water where captured in a tide pool where concentration and foam led to random chance producing self-replicating molecules in proto-cells."

This answer, while almost certainly true, is also incredibly dissatisfying, because all it tells us is what deductive logic has already taught us, almost intuitively.

Incidentally, the fact that all of this happens with a million to one odds isn't a problem: The Earth is big, and the time frame for this happening is along the lines of hundreds of millions of years: Anything that might happen once per year by a million to one shot would likely happen hundreds of times in that timeframe.

In any case, when it comes to evolution, or Darwin's Theory of Evolution, or any other theory of evolution, this is all irrelevant.

Evolution is something that happens in any sufficiently complex (open) system, assuming it has the capacity to change at all.

It is most easily observed in living organisms, because they are at the right scale, and incredibly diverse, but it happens on all scales of the universe.

In fact, the easiest way to explain how life first originated, is just to keep counting backwards when you reach the Last Common Ancestor (of All Life on Earth), and propose models for how this proto-bacterium could be even simpler, until you're left with CO₂, N₂ and H₂O, and other simple molecules.

At that end of the spectrum it is well-understood that e.g. H₂O "evolves" from H₂ and O₂, because H₂O has a quality that makes it more "fit" than either of its components, chemical stability.

Furthermore, H2 "evolves" from free hydrogen by a similar mechanism, and free hydrogen "evolves" from protons and electrons, because it has the property of being electrically neutral, which is also a desirable property.

Of course, at the level of protons and electrons, things get a little muddy, and evolution kind of breaks down as a method for explaining how things come about.

Edit: For reference: Current Models of Abiogenesis on Wikipedia.

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While many point to RNA, or a variant of it, as being the first molecule of "life" very few people know where it came from. Some suggest that it came from outer space because it's uncertain how the material for sugar-phosphate backbones could have developed on earth and that the perhaps these materials found their way here via meteorites. There are several hypotheses as to how an early earth environment may have promoted the properties of these molecules, but it's difficult to ascertain what exactly happened.

Somewhere within that abiogenesis wikipedia article is the mention of the role of deep-sea vents. The deep-sea vents and the currents that surround them basically facilitated a PCR reaction. Some of the early emerging DNA, maybe with some RNA and free nucleotides floating around, could have leveraged such an environment for replication and by sheer number (and some chance) entered into symbiotic relationship with other molecules to form the first cellular structures.

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If you are interested in this question, I highly recommend you look at the work of Jack Szostak - Nobel Prize winner at Harvard who is currently doing some of the best work in this area. His work is grounded in good experiments that point to how abiogenesis could have happened.

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I think that it started like this:

First stage:

Chemicals like Na, Cl, O₂, O₃, CO₂, CO, HCN, and H₂SO₄ react to form small molecules.

Second Stage:

Next those small molecules react to form macromolecules.

Third Stage:

DNA, being the most stable was first to replicate. A membrane eventually enveloped this and formed the nucleus

Fourth Stage:

Via Endosymbiosis it envelops mitochondria in an outer membrane

Fifth Stage:

Cell starts forming membrane and protein for all of its organelles. It is now a eukaryotic cell.

This is the DNA world Hypothesis.

Because DNA can be single, double, triple, or quadruple stranded in organisms(That would be ssDNA, dsDNA, tsDNA, and qsDNA respectively) it can do much more than just code for proteins or rRNA or tRNA.

In the single stranded state it can act like an enzyme forming deoxyribozymes.

In the triple and quadruple stranded states it can act inhibitory without needing another protein or methyl group.

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