Understanding through Discussion


Welcome! You are not logged in. [ Login ]
EvC Forum active members: 85 (8915 total)
Current session began: 
Page Loaded: 07-18-2019 8:08 PM
28 online now:
AZPaul3, edge, jar, Jon, Louis Morelli (5 members, 23 visitors)
Chatting now:  Chat room empty
Newest Member: 4petdinos
Post Volume:
Total: 856,974 Year: 12,010/19,786 Month: 1,791/2,641 Week: 300/708 Day: 75/52 Hour: 1/0


Thread  Details

Email This Thread
Newer Topic | Older Topic
  
Author Topic:   Abiogenesis - Or Better Living Through Chemistry
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


(1)
Message 1 of 85 (5654)
02-27-2002 4:57 AM


Apologies in advance for the length of this post. Enjoy!

Even though abiogenesis – the origin of life from non-life – is not related to the validity or falsehood of evolutionary theory, it is an interesting subject in its own right. Although evolutionary theory does not rest on the truth of abiogenesis, creationists in particular seem to demand that a non-supernatural origin of life be “proven” before evolution can be accepted. It is in that sense that I will undertake to provide a brief synopsis of the various hypotheses, and discuss in general terms both the positive aspects and the potential problems with each. Consider this the “Reader’s Digest Condensed Version” of abiogenesis.

The discussion of the origin of life is one of the most complex and contentious issues in science today. Because the issue is so complex, there are many, including even some scientists (almost all from outside the biological sciences), who claim that it was, in fact, impossible for life to have arisen through solely natural processes. They believe there was required some Divine Spark, or Supreme Designer, standing outside all known universal laws to initiate the process. Finally, they believe that life contains some level of organization below which recourse to merely physical laws cannot explain how it came into being – a First Event from which all else flows.

Although our knowledge in some areas may be weak or we may be missing some details today, natural abiogenesis studies begin with the premise that there is nothing unknowable in nature. Life, in the final analysis, makes perfect sense using known physical laws. The actions of a Supreme Being are not required to explain it. Reason, analogies drawn from modern organisms, and the results of scientific research from disciplines as diverse as astronomy, astrophysics, microbiology, parasitology, chemistry, biology, genetics and geology, as well as dozens of others, provide sufficient explanation. There was no First Event.

There are currently three main scientific hypotheses for how life arose on Earth. All three have their adherents, and all three are actively being researched by some of the finest scientific minds on the planet. All three have both empirical and inferential support (for elements of the theory), but all three contain certain assumptions that must be true for the theory to be valid. All three contain elements that have been shown either in the lab or in nature to lend credence to their hypothesis.

There is one unifying thread that ties all three hypotheses together which must be understood at the outset: all three hypotheses rest on a foundation of organic chemistry. Don’t get confused. “Organic” here does not mean “living” or “coming from life”. Organic chemistry is nothing more than the chemistry of carbon. It happens to be enormously richer than the chemistry of other elements - and thus able to support life - because of the unique associative properties of the carbon atom. In all likelihood the first building blocks of life arose as do all natural chemical compounds - spontaneously, according to the rules of thermodynamics. In one way of looking at it: we ARE carbon.

1. The Biotic Soup Hypothesis

This is arguably the most well-known (and least understood!) hypothesis. In essence, the hypothesis argues that chemicals available on the Hadean Earth from either terrestrial or extraterrestrial sources combined via standard chemical reactions to form biologically significant macromolecules. The biotic soup hypothesis uses an ammonia-methane atmosphere as a starting point, and naturally occurring electrical storms and radioactivity as the energy source.

Spectroscopic analysis by astronomers has revealed that space is permeated by an extremely tenuous cloud of microscopic particles, called interstellar dust, containing a variety of combinations of carbon, hydrogen, oxygen, nitrogen and, sometimes, sulfur or silicon. These are mostly highly reactive free atoms and small molecules that would hardly remain intact under conditions on earth, but in space could interact to form more stable, typical organic compounds, many of them similar to substances found in living organisms. That such processes indeed take place is demonstrated by the presence of amino acids and other biologically significant compounds on celestial bodies – for example, the meteorite that fell in 1969 in Murchison, Australia, Comet Halley (which was analyzed by the Giotto spacecraft during its 1985 passage), and Saturn's satellite Titan, the seas of which are believed to be made of hydrocarbons (based on the Voyager fly-bys) and which contains an atmosphere with significant organic compounds.

The modern chemical composition of the Earth is mostly Fe, Mg, Si, and O, with the other elements contributing 5% of the total. Life originated as a result of chemical reactions occurring (largely) in the atmosphere followed by reactions in the primeval oceans and lakes. The atmosphere at the end of the Hadean Period (~4-4.2 gya) is primarily composed of variable amounts of CO2, N2, SO2, H2S, S, HCl, B2O3, and smaller quantities of H2, CH4, SO3, NH3 and HF (but no O2), due partly to outgassing from volcanoes, and partly to the reaction of condensing water vapor (formed as the Earth cooled) with minerals such as nitrides (hence NH3), carbides (hence CH4, CO, etc.) and sulfides (hence H2S). There was no free oxygen (any free O2 would have reacted with P, Si and metals such as molten iron to give minerals e.g. iron oxides, silicates, phosphates, etc.). This atmosphere readily lends itself to the formation of small organic molecules, which in turn readily combine to form more complex macromolecules.

In the lab, tantalizing experiments attempting to re-produce the atmospheric conditions of the early Earth have produced astonishing results. As early as the 1950’s Harold Urey used simple electrical stimulation of a hydrogen-methane-ammonia atmosphere and – in just a few days – saw over 15% of the methane/carbon converted to amino acids: one of the key building blocks of proteins and hence life. Although his postulated atmosphere was probably inaccurate, the same amino acids in nearly the same proportions have been discovered in the Murchison asteroid. Since Urey, besides amino acids and other organic acids, experiments have yielded complex sugars as well as purine and pyrimidine bases, and even adenine: some of the components of the nucleic acids DNA and RNA, the genetic repositories of the codes of all life. Stanley Miller is STILL working on the problem at UC San Diego.

There are a few problems with this hypothesis. In the first place, it is impossible at this remove to determine the exact chemical composition of the early atmosphere – hence whereas the chemical reactions are quite straightforward, the relative yield is open to interpretation. In addition, many of the small organic molecule precursors such as HCN and HCHO are volatile and would break down readily in the atmosphere. It is postulated that these precursors were absorbed into the primeval ocean where they would be shielded from the damaging UV (<300 nm), creating the “organic soup” of the Urey-Miller experiments.

2. Cairns-Smith Crystal Matrix Hypothesis

One of the problems with the biotic soup hypothesis, even assuming the chemical reactions were as stated, is how these macromolecules – randomly distributed as they were – were able spontaneously to form the key biological macromolecules such as peptides and nucleic acids. The probability of the formation of these crucial biological molecules from a random mixture of organic chemicals is vanishingly small. In addition, these molecules had to be self-replicating. The chemist Alexander Cairns-Smith proposed that inorganic materials, rather than organic, represented the first replicators.

The fundamental problem he was trying to address is the requirement that the first "life" (using the term very loosely) had to have been self-replicating. Cairns-Smith speculated that the earliest replicators were not organic at all, but rather were self-replicating crystals that were later superseded by the rise of the far-more-efficient organic replicators. In this view, the first replicators were crystals of the type that exist in clay or mud along riverbanks; they transmitted their "genetic" information through the natural tendency of these types of molecules to fit together into a geometric pattern.

The fundamental characteristic of crystals as replicators must be hereditary variation, or inheritance. Fortunately, crystals in nature display this pattern: they may be perfectly aligned until a specific point is reached, in which a flaw has accumulated (these are quite common in natural crystals). This flaw has a tendency to percolate down the subsequent layers of crystal, setting up a rudimentary system of heredity. Furthermore, atoms of the crystal's substance may be more attracted to certain geometric patterns than they are to others. This sets up a kind of "differential reproduction" which then leads logically to a form of natural selection.

The hypothetical crystals described above may very well begin a basic process of cumulative selection. Certain crystals may have the property of altering streams or other water sources for their own "benefit", such as by increasing the likelihood of more of the same material being deposited in the same location. Crystals may also encourage the formation of "spores" by breaking easily into subsequent "generations" Those crystals that broke into generations most easily would be selected for; these generations would invariably contain mutations on occasion and would intensify the competition between rival variants.

In time, the crystals could evolve a sort of "phenotype" by altering other materials in their environment. These materials could be used to further the crystal's replication by inhibiting rival crystals from forming or promoting the parent crystal's reproduction. Cairns-Smith's hypothesis is that the materials used by the crystals for self-replication later turned out to be even more efficient replicators in their own right – the earliest peptide-RNA – which ultimately replaced their inorganic substrates. This process of replacement might repeat for several cycles, or the first products used by the crystals may have been the ancestors of modern replicators - i.e., RNA and eventually DNA.

The principal difficulty with Cairns-Smith’s hypothesis is the fact that clay doesn't necessarily form a lattice/matrix that is perfectly designed for the arrangement of biologically significant molecules. Since there are a rather large number of potential arrangements, getting the precise arrangement necessary to act as a catalyst for a specific molecule is pretty problematic. Finally, the type of clays best suited for this type of “inorganic evolution” are usually found in riparian zones – the smaller biological molecules are fairly unstable when subjected to unshielded UV. It remains to be seen whether such processes could occur in such a way that these molecules could persist long enough to form stable compounds.

However, as with Miller, Cairns-Smith’s organic replicator overthrow of the inorganics only needed to occur once…

3.a. Submarine Hot Springs Hypothesis – Electrochemical Variant

In this hypothesis, life is believed to have begun at the sites of warm submarine springs where chemical energy was focused and the mixing of spring water with seawater could lead to the precipitation of chemicals. The precipitation of chemicals on mixing of solutions can form a barrier preventing further mixing and precipitation. This barrier can also provide a template for the assembly of chains of organic molecules, and act as a catalyst for electrochemical reactions. This hypothetical precipitate, again operating in a naturally occurring biotic soup, consisted mainly of small groups of iron and sulfur atoms. Iron-sulfur groups still play an essential electrochemical catalytic role in all living cells.

As a boundary, the precipitate concentrated organic molecules such as amino acids. These formed at depth below the spring where water and its dissolved chemicals reacted with rocks containing Fe and iron-rich minerals. The boundary also concentrated other chemicals that could participate in chemical reactions.

As a catalyst the groups of FeSiO4 and Fe3O4 could activate molecular hydrogen (and probably methane which consists of carbon and hydrogen) which also formed at depth in the spring. The hydrogen is essential for the synthesis of organic molecules. Electrons produced as a by-product (and representing the dissipation of energy) are transferred to a type of iron, known as ferric iron, dissolved in seawater. (The ferric iron is produced from dissolved ferrous iron (richer in electrons) at the ocean's surface by sunlight. The same processes cause the reddening of the surface of Mars as iron-bearing minerals have “rusted”.

As a template, the iron sulfide precipitate (consisting of small crystals of only a hundred atoms or so), could bond chemically to, and assemble a sequence of, the molecular components of RNA. Acid springs of high temperature, coupled to emergent magma plumes, emit ferrous iron and other transition metals to the ocean. Solar energy oxidizes some iron to the ferric state, generating a dispersed positive terminal. Cooler alkaline waters emanate from the deep ocean floor, bearing hydrogen, methane, ammonia, formaldehyde, cyanide and hydrosulfide - molecules reduced from water and carbon oxides by reaction with ferrous silicate, residual nickeliferous iron and ferrous sulfide. Where these waters seep into the ocean, mounds, comprising layers of ferrous sulfide and green rust flocculants and films, arise. These mounds are where the reduced molecules are filtered and adsorbed. Concentrated, they react to form glyceraldehyde, amino acids, and the components of nucleosides.

The fluids are prevented from mixing thoroughly with the surrounding ocean by the spontaneous precipitation of a barrier of colloidal iron compounds. Nucleotides can then assemble in green rust. The thermal potential begins to be dissipated but the chemical potential is dammed. Though the hydrothermal solution is constrained, electrons escape from adsorbed hydrogen through the conducting layers of iron monosulfide, drawn to reduce the photolytic ferric iron.

There is invasion of the iron sulfide/hydroxide barrier by protons, pyrophosphate and carbonic acid, through iron sulfide-walled micro-channels. The newly formed nucleotides poison the iron sulfide but combine with peptides, producing pRNA. The side chains of particular amino acids register to fitting nucleotide triplet clefts. Keyed in, the amino acids are polymerized, through acid-base catalysis, to alpha chains by invading protons. The resulting short protopeptides sequester ready-made iron sulfide clusters to form ferredoxins, ubiquitous proteins with the longest evolutionary pedigree. These take over the role of catalyst and electron transfer agent from the iron sulfides, promote further chemosynthesis and so support the electrochemical reactor from which they sprang.

The principal problem with this hypothesis is the reliability of the invasion and precipitation scenario. To wit, how effective is the sulfide barrier and the green rust substrate at providing a template for biological macromolecules? In addition, to be more plausible, the hypothesis must assume a fairly high concentration of chemical precursors. Especially since, unlike the evaporation-concentration element of Miller’s biotic soup hypothesis, there is no specific mechanism for concentrating these molecules into sufficiently close proximity for the electrochemical bonding to take place.

3.b. Submarine Hotsprings Hypothesis – Flow Reactor Variant

Similar to the above, however instead of a postulated electrochemical mound as one of the poles, this theory using the high-temperature (300-400 C) energy found in the cracking front of submarine steam vents (“black smokers”) to provide the necessary energy.

In a hot spring, the flow of heat is constrained by the structure and constitutive properties of magma, rock and water; the gravity field, etc. The model proposes that in addition to the coherent flow of fluid, these constraints produce phase space trajectories which lead to the creation of high-energy molecular and macromolecular structures in which the particles are locked, or frozen, into coherence. The cooling particles fall and are trapped into the potential energy wells, i.e. attractors, provided by the constraining forces that bind matter together. In other words, life is an emergent property of the high-temperature chemistry and physical constraints present in the flow reactor of a submarine magma pipe.

The products of rapid heating and quenching at the cracking front follow a highly constrained trajectory, rapidly mixing with cool sea water flowing upward through a matrix of fractured rocks of enormous surface area, lined with a highly catalytic surface of clay minerals (note the use of Cairns-Smith’s clay matrix). They began their ascent as hot (~350° - 600°C), acid (pH~3.6), highly reducing fluids, and approach a low temperature end member which is cold (~2° - 20°C), slightly alkaline (pH 7.9), and oxidizing. A fraction of the thermal energy traveling from mantle to ocean is trapped into the high energy bonds of organic molecules, which remain as static equilibrium structures. The process is thermodynamically analogous to the emergence of matter from energy during the expansion and cooling of the early Universe.

As with the other hypotheses, this one also relies on the presence of organic molecules in the primeval ocean. However, unlike the biotic soup idea, these precursor molecules would be concentrated by the geophysical properties of the mantle at the site of the hotsprings.

Conclusion

Regardless of which of the above hypotheses ultimately leads to the creation of self-sustaining biomolecules, all show that life is merely an inherent property of chemical reactions. Any time conditions are appropriate, life (as we know it) should arise. And once we get self-replicating molecules, evolution (heritable variation, random mutation, and natural selection) + time are sufficient to explain the amazing diversity of modern life.

Science has yet to provide evidence for any of these hypotheses beyond reasonable doubt. But since all are “brand new” ideas, the only thing lacking is time… Stay tuned!


Replies to this message:
 Message 2 by joz, posted 02-27-2002 10:55 AM Quetzal has responded
 Message 3 by John Paul, posted 02-27-2002 5:25 PM Quetzal has responded
 Message 37 by Brad McFall, posted 04-01-2002 1:45 PM Quetzal has not yet responded
 Message 38 by Andor, posted 06-03-2002 10:15 AM Quetzal has responded
 Message 43 by Bart007, posted 09-07-2002 8:31 PM Quetzal has responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 26 of 85 (5862)
03-01-2002 3:00 AM
Reply to: Message 2 by joz
02-27-2002 10:55 AM


quote:
Originally posted by joz:
Great post Quetzal, thanx.....

Q)I was under the impression that one of the problems with The Biotic Soup Hypothesis of abogenesis was the low abundance of sugars produced and the lack of long chain fatty acids (in the Urey/Miller experiments). Has anything new cropped up that makes this less of a problem?


Wow, try and get some work done for one day, and look what happens! I'll try and answer everyone's posts (or quibbles) as I get the opportunity.

Anyway, joz, you're absolutely correct. One of the crucial problems with the experiments was their apparent inability to synthesize complex sugars, specifically ribose. Ribosomal RNA, of course, was the autocatalytic self-replicator that Cech discovered. It wasn't so much that they didn't get sugars or that the chemistry wasn't correct (after all, you get HCHO forming photochemically in the atmosphere today, then by Formose reaction you get isomers like formaldehyde (CH2O)6 [detectable in modern rainwater]. It's a fairly simple step to re-arrange things into C6H12O6). Getting from there to ribose is mostly a question of concentration and energy with the right catalyst.

Miller's biggest problem was (and remains) trying to get rRNA to form spontaneously. That and the fact that RNA couldn't be the first replicator simply because it is really unstable and formed in very tiny quantities. It wasn't until only a couple of years ago that it was found peptides (which are REALLY easy) were able to bond to the 5' site on the nucleic acid forming a stable hybrid polymer: pRNA.

You still have major problems with concentration and getting the nucleic acids to line up properly - something that hypothesis 2 and both 3's accomplish by using inorganic templates. Fe4S4][SFeS]2), which is structurally identical to the active center of ferredoxin (the Fe4S4).


This message is a reply to:
 Message 2 by joz, posted 02-27-2002 10:55 AM joz has responded

Replies to this message:
 Message 30 by joz, posted 03-01-2002 11:32 AM Quetzal has responded
 Message 46 by Bart007, posted 09-15-2002 3:47 PM Quetzal has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 27 of 85 (5863)
03-01-2002 3:33 AM
Reply to: Message 3 by John Paul
02-27-2002 5:25 PM


quote:
Originally posted by John Paul:
The only reason people cling to the reducing atmosphere hypothesis is because it offers the best hope. The rocks don't substantiate that claim though.

Not entirely accurate. Agreed, Miller's first atmosphere (about 20% hydrogen) is probably waaay off. Current estimates are between .1 - 1%. The quibble, of course, only argues about free H2, not more stable compounds like HCN, HCHO, etc. It also neglects to consider the dissociation of H2O by UV light (and even beta radiation), which would be a continuous source of hydrogen. There ARE problems with Miller's atmosphere - this just isn't one of them.

quote:
We also have no evidence that DNA can form anywhere outside of a living cell, and the cell itself represents IC (irreducible complexity):

It looks like only life can beget life.


Strawman argument. You should reread the post - it doesn't even discuss DNA, let alone the first cell. Our genetic storage equipment is a much later evolutionary development.

quote:
The thing is, these days we can take and mix amino acids at will and in differing environments. Guess what? Nothing resembling the start of life, just a bunch of stirred up amino acids in a flask.

A misleading statement. Just about every possible precursor HAS been formed in the lab. We've got chemical catalysts, we've got spontaneously occuring amino acids, we've got spontaneously occuring peptides and nucleic acids. We've got lots of evolutionary change observed in RNA, for instance, once we do get the replicator going. Given a little more time, scientists will undoubtedly get the final, tiny step completed.

quote:
Life isn't just about chemical reactions. How long is it going to take before you realize that?

It most assuredly IS about chemical reactions. When will YOU realize that?

quote:
I do find this all interesting and I hope the research continues, privately funded of course. Proving all these purely natural scenarios (for the origins of life) may be the only way to get people focused on how we really got here.

I agree. Pretty soon the creationists will all be out of work.


This message is a reply to:
 Message 3 by John Paul, posted 02-27-2002 5:25 PM John Paul has not yet responded

Replies to this message:
 Message 39 by Dr_Tazimus_maximus, posted 06-03-2002 11:14 AM Quetzal has not yet responded
 Message 77 by DNAunion, posted 11-29-2003 2:50 PM Quetzal has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 28 of 85 (5864)
03-01-2002 3:49 AM
Reply to: Message 3 by John Paul
02-27-2002 5:25 PM


quote:
Originally quoted by John Paul from Jonathan Wells "Icons of Evolution":
It was concluded in the 1960s that the earth's primitive atmosphere was derived from volcanic outgassing, and consisted of water vapor, carbon dioxide, nitrogen and trace amounts of hydrogen. With most of the hydrogen being lost to space, there would be nothing to reduce the carbon dioxide and nitrogen, so methane and ammonia could not have been major constituents of the early atmosphere.

I missed this bit. One of the problems with Wells's argument I've already addressed - his statement is misleading as he implies there is NO H2. Submarine vents (which he fails to mention at all), photochemical dissociation of water, and lightning all produce free hydrogen. What's more, volcanic outgassing also produces already stable compounds such as H2S, methane, etc. Wells also neglects to mention the possible extraterrestrial origin of a lot of the organic compounds. You must have missed that part of my post.


This message is a reply to:
 Message 3 by John Paul, posted 02-27-2002 5:25 PM John Paul has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 29 of 85 (5866)
03-01-2002 4:11 AM
Reply to: Message 11 by TrueCreation
02-27-2002 9:28 PM


TC: You're questioning whether I actually wrote this? Are you accusing me of "cutting and pasting" without attribution? Better guess again, junior. The information is derived from numerous sources - my post is a synopsis of current research, and is part of a substantially longer essay I've been working on. If you want to do some of your own homework, you could start with:

1. Biotic soup: Start with Nasa's astrochemistry links, then go to PubMed and perform a search for Miller, Origin of Life, abiogenesis, etc. Eventually you'll be able to gather all of the same information. You might also try Sagan's "Demon Haunted World". Finally, pick up any random bio textbook (I recommend Campbell's "Biology").

2. Cairns-Smith: You'll have to buy his book, "Seven Clues to the Origin of Life". Then buy and read Dawkins's "Blind Watchmaker", which has a chapter devoted to the idea.

3.a. Electrochemical Battery: This is primarily a short synopsis of Russell and Hall's massive "Geochemical Origins of Life". Feel free to wade through it if you can follow the chemistry.

3.b. Submarine Flow Reactor: Again, a very short synopsis of John Corliss's "The Dynamics of Life". Feel free to look it up, although I think Corliss is now working at the University of Budapest.

Before you accuse or imply someone failed to attribute properly, you bloody well better have some evidence to back it up.


This message is a reply to:
 Message 11 by TrueCreation, posted 02-27-2002 9:28 PM TrueCreation has not yet responded

Replies to this message:
 Message 31 by joz, posted 03-01-2002 12:36 PM Quetzal has responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 32 of 85 (5978)
03-02-2002 4:31 AM
Reply to: Message 30 by joz
03-01-2002 11:32 AM


[QUOTE]Originally posted by joz:
[B] Q/Doesn`t repeated drying and dillution (as would be experienced on a shoreline) of the biotic soup produce peptide bonded chains that exhibit limited self replication?[B][/QUOTE]

It do indeed. In fact, Miller et al were sort of relying on this scenario to concentrate the chemistry. Unfortunately, complex long-chain macromolecules break down quickly in the presence of intense UV. Remember that at the ~4gya mark there wasn't any free oxygen (evidenced by ferrous - i.e., non-oxydized - iron in the oldest basement rocks). No O2 = no O-. Without the ozone shield, shoreline pools would be deleterious to these molecules. The only possibility would have been to suppose these reactions took place in the depths of the ocean - meaning that we'd have to assume an even higher percentage of organics in the primordial oceans - a difficult assumption. This is not an insurmountable problem, however. The "snowball Earth" hypothesis permits the concentration of biological macromolecules in shallow water because the ice provides the shielding. The evidence for snowball Earth to date is rather less than compelling, IMHO. ('Course, I'm one of those guys that needs to have his nose rubbed in something before he'll believe it .)


This message is a reply to:
 Message 30 by joz, posted 03-01-2002 11:32 AM joz has not yet responded

Replies to this message:
 Message 34 by mark24, posted 03-04-2002 9:21 PM Quetzal has responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 33 of 85 (5979)
03-02-2002 4:37 AM
Reply to: Message 31 by joz
03-01-2002 12:36 PM


quote:
Originally posted by joz:
Any plans to post it here when its complete?

Errr, , the original intent was to develop an essay that answered some of the common creationist quibbles - to be posted in a forum such as this, or added to a website. It sort of takes things from the formation of the solar system to the edge of the Vendian and the diversification of bacteria, archaea, and eukaryotes. However, it has mostly become more of a hobby than a serious endeavor. IOW, the odds of ever actually completing it are rapidly growing vanishingly small. Reality check: I use it more as a "mine" for key points when I want to post something like the above. In addition, everytime I turn around there's more information to add...


This message is a reply to:
 Message 31 by joz, posted 03-01-2002 12:36 PM joz has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 35 of 85 (6143)
03-05-2002 4:05 AM
Reply to: Message 34 by mark24
03-04-2002 9:21 PM


quote:
Originally posted by mark24:
Quetzal,

Out of interest, how many "snowball earth" events were/are alleged?

I'm aware of the one just before the Cambrian explosion, & am dimly aware it wasn't the first. My reading suggests it may have been life itself that prevented the next "post" cambrian snowball, due to a change in gas concentrations, particularly oxygen. Mark


Besides the 600-700 mya series of glaciations (the Vendian snowball), I seem to remember Kirschvink claimed evidence for another period of "freeze-fry" series of consecutive disasters occurring 2.3 gya. He's postulating that arrangement of continents was the cause. George Williams (another bloody Australian) proposed that Earth was more oblique up to the end of the Proterozoic, leading to a fairly regular cycle of freezing. I don't think anyone's ever "proved" things one way or the other.

Here's a good paper on the "snowball earth" idea: "The Snowball Earth".

I don't have an answer for the "why not since?" question. Hoffman's conclusion in the paper seems to indicate there is no intrinsic reason why it couldn't. I don't think I'm going to lie awake at night worrying about it - if the timescales required are correct, the snowball is roughly every 1-1.5 billion years. We've got a ways to go before the next one...


This message is a reply to:
 Message 34 by mark24, posted 03-04-2002 9:21 PM mark24 has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 40 of 85 (10957)
06-04-2002 9:26 AM
Reply to: Message 38 by Andor
06-03-2002 10:15 AM


quote:
Originally posted by Andor:
Good exposition Quetzal
A question: During the time of prebiotic evolution, Earth was under an intense bombardment of spatial debris. Some of the impacts were so big as to evaporate all the water and eradicate the incipient life. Perhaps the possibility of life surviving (or even surging) in subterranean niches, should be added to the list.

Thanks Andor. I do remember reading something about that theory - do you have a link or reference? I seem to remember not being that enamored of it, at least for OOL, because the authors sort of glossed over any mechanism for concentration of precursor molecules. There's no question that during the Hadean period cometary and debris impacts could very easily have wiped out any potential replicators. The impact that created the moon nearly cancelled the entire experiment. However, by about 4 gya, most of the major impacts were over (based on moon samples, IIRC). With the bombardment period over, the atmosphere stabilized, the dust settled, we have the first Dawn, and life can start rocking.


This message is a reply to:
 Message 38 by Andor, posted 06-03-2002 10:15 AM Andor has responded

Replies to this message:
 Message 41 by Andor, posted 06-04-2002 11:05 AM Quetzal has not yet responded

  
Quetzal
Member (Idle past 4067 days)
Posts: 3228
Joined: 01-09-2002


Message 44 of 85 (16889)
09-08-2002 4:31 AM
Reply to: Message 43 by Bart007
09-07-2002 8:31 PM


Bart - these are both excellent questions that deserve a response longer than I have time for (I'm jumping on a plane in about an hour, and have to finish packing). However, I didn't want to you to think I was ignoring your post. If you'd like, I'll try and respond more substantively when I return. In the interim, hopefully someone else will pick up the slack.

quote:
Originally posted by Bart007:

Q: Even though abiogenesis – the origin of life from non-life – is not related to the validity or falsehood of evolutionary theory, it is an interesting subject in its own right. Although evolutionary theory does not rest on the truth of abiogenesis, creationists in particular seem to demand that a non-supernatural origin of life be “proven” before evolution can be accepted.

B: I suppose your right, there is theistic evolution. But surely, if, hypothetically, it becomes clear that abiogenesis is not feasible by natural means, then there is no reason to accept materialistic evolution. Once a Creator is established as a neccessary condition for life, would not Creation make a lot more sense than evolution?


Hypothetically? "Creation" and a "creator" isn't necessarily the default hypothesis if chemical abiogenesis is "proven impossible". I suppose it could be one of the hypotheses. OTOH, in this case (where abiogenesis is impossible for some reason), positive evidence must be presented to support the creator version. IOW, you can't validate a theory by falsifying another. You can only validate a theory by providing positive evidence for it. Falsifying abiogenesis does nothing except falsify abiogenesis. It doesn't validate creationism. (sorry for the short answer)

quote:
Q: The modern chemical composition of the Earth is mostly Fe, Mg, Si, and O, with the other elements contributing 5% of the total. Life originated as a result of chemical reactions occurring (largely) in the atmosphere followed by reactions in the primeval oceans and lakes. The atmosphere at the end of the Hadean Period (~4-4.2 gya) is primarily composed of variable amounts of CO2, N2, SO2, H2S, S, HCl, B2O3, and smaller quantities of H2, CH4, SO3, NH3 and HF (but no O2), due partly to outgassing from volcanoes, and partly to the reaction of condensing water vapor (formed as the Earth cooled) with minerals such as nitrides (hence NH3), carbides (hence CH4, CO, etc.) and sulfides (hence H2S). There was no free oxygen (any free O2 ...

B: What if there was free O2, would that have any adverse affect on the formation of life's molecules, or are you simply pointing out that there simply was not any O2 in the prebiotic atmosphere?


Neat question. I'm not really sure I have the answer to that - my guess would be that significant free atmospheric O2 would change the chemistry postulated as necessary for what are thought to be the first replicators. Until the much-later-appearing cyanobacteria evolved the capability to synthesize superoxide dismutase around 2.2-1.9 gya, significant free 02 would have been pretty poisonous - as in fact it was (look up the "oxygen holocaust" in the mid-late Proterozoic - a lot of prokaryote families went extinct, and even the cyanobacterial families which had not developed this capability were also strongly effected).

OTOH, chemical analysis of the lowest basement rocks show the presence of non-oxydized iron (greenstone or banded iron formations) - indicating that whatever 02 was available either through outgassing or dissociation of water and other compounds was pretty quickly snatched up by other elements. It wasn't until the cyanobacterial waste built up in the atmosphere that we see evidence of free 02 (about 2-3%). There are a couple of very good geologists on this board that might be able to provide details.

Sorry about the short and reference-less reply. I really do have to run.


This message is a reply to:
 Message 43 by Bart007, posted 09-07-2002 8:31 PM Bart007 has not yet responded

  
Newer Topic | Older Topic
Jump to:


Copyright 2001-2018 by EvC Forum, All Rights Reserved

™ Version 4.0 Beta
Innovative software from Qwixotic © 2019