|"Even if such relatively simple molecules, such amino acids (the building blocks of proteins), could populate the hypothetical primitive oceans, the formation of biologically active proteins could never occur. Under these conditions, the sequence of the amino acids would be purely by chance. There we 20 different amino acids in proteins. The average protein has 400 amino acids, but even 100 of these amino acids can be arranged in 20100, or 10130 different ways. The probability of just one such molecule arising by chance is thus equal to the number one followed by 130 zeros. This is essentially impossible, but to get life started would require billions of tons each of several hundred different proteins and equal quantities of even more complex DNA and RNA molecules." --- Reverend Duane Gish|
The following are various organic / biochemical reactions
that may have occurred on primitive earth. The reactions
are taken directly from the text Biochemistry by Geoffrey
Zubay, the second edition, 1988. To be honest, I though
this text was more comprehensive that it appears to be. In
order to address abiogenesis, one first must decide what
would be required for a primitive "living"
system. Based on the studies of Thomas Cech, Norman Pace,
Sidney Altman, and Alan Weiner, I would suggest that a
membrane encapsulated system containing RNA or an RNA like
molecule would be sufficient. This is based upon
experiments which have demonstrated that RNA can perform
1) act as a polymerase and direct template specific synthesis of RNA
2) act as a site specific nuclease to cleave RNA
3) act as polymerase and direct template independent synthesis of RNA
The result of these reactions is a molecule that under different ionic conditions can replicate, and release the products of replication via cleavage. To my way of thinking, in order to optimize the concentrations, and allow for somewhat adequete conditions for a self replicating system, it should be self contained, thus a membrane would be important if not required for our first "living" organism. It is quite possible that the earliest life forms performed these required reactions by nucleating in pockets of salt water saturated clays. Eventually however, a membrane is required. You should not from the above discussion assume that proteins are not required for this most primitive of scenarios.
Beyond this, there is circumstantial evidence that would support RNA's role in primitive life. First of all, it is completely ubiquitous and absolutely required for life of all known systems. No known biological systems can survive without RNA. DNA viruses have to go through an RNA intermediate. Not all RNA viruses require a DNA intermediate. This is an important distinction. Secondly, increasing evidence has demonstrated that it is the RNA in ribosomes that is critical for protein synthesis, not the proteins. It appears that the proteins are more of a scaffolding, while the RNA performs the catalytic function. Thus we have evidence of yet another role for RNA - that for polypeptide synthesis. Furthermore, RNA has been implicated in maintenance of telomeres, which is important to prevent loss of genetic information in each round of replication. Other groups have also implicated RNA as a catalyst involved in carbohydrate metabolism. From these examples it is clear that no other molecule is nearly as wide reaching in its biological implications as RNA.
Now, what is required to form an RNA molecule, and is it reasonable to expect that these molecules may have formed spontaneosly on primitive earth?
To answer the first part, you need bases, a sugar and phosphates. To answer the second part, the answer is yes, and no. Although the arguments are certainly not definitive, they are currently the best ones that I am aware of, although it is entirely possible that I have missed important research in this area in the last few years. The next message(s) will detail these reactions and my comments on them. Much to my regret, the text that I have does not supply the reactions for lipid synthesis or sugar synthesis. The lipid reactions I have completely forgotten and will have to ignore. The sugar reactions, I remember a bit more of, and will try to recount what I can.
First, I will discuss the biochemistry required for synthesis of the purine bases adenine and guanine. Under conditions postulated to have occurred on primitive earth, all of these reactions have been shown to occur, and the resulting end products are major products of the precursors.
H2N CN This is diaminoaleonitrile, a \ / relatively simple product, easily HCN ---> C synthesized from hydrogen cyanide || C / \ H2N CN | | Now add a little ionizing radiation | and another molecule of HCN and we V get: NC N \ / \ \ A mess. Organic molecules do C not lend themselves well to this || C media. Seriously though, you get C / 5-aminoimidazole-4-carbonitrile / \ N which is a direct precursor of H2N adenine. Just add HCN | | HCN V NH2 | N N // \/ \\ | || C \\ / \ / N N By adding H2O to 5-aminoinudazole-4 carbonitrile you get a precursor of guanine | | H2O V O Is it my imagination or are || N my drawings getting better? / \ / \\ anyways, now just add a little H2N || C cyanogen and voila! / \ / H2N N H | | (CN)2 | V O || N HN/ \ / \\ Here is guanine. So the purines | || C seem easy enough to make. Lets /\\ / \ / try some pyrimidines now. NH2 N N HFortunately at least one pathway for pyrimidine synthesis is a bit less complicated than for the purines. For the sake of brevity I will post it here, if you are genuinely curious, you can find all of this in the text cited in the first message.
NH2 O HC | || ||| NCO- //\ H2O / \ C ------> N C -----> HN C C | || | || N //\ / //\ / O N O N H H Cytosine UracilSo now we have four bases. The next step is the sugar. To me, this is the biggest problem of the whole thing. Not because sugars would not form spontaneously under these circumstances, but because of the exponential nature of stereoisomers that can form with each additional carbon atom. The number of separate 5 carbon sugars is high enough to make the selection of ribose seem prohibitive. Some researchers think that glycerol or another similar sugar may have evolved first, simulating the structure that would later be achieved through ribose. Such a structure might look like:
O Base \ | C H H / | C - C H OH OH Where as ribose looks like: * HOCH O Base \ / \ | C C /\ H H / | H C - C H OH OH * ** * denotes carbons involved in forming nucleotide polymers ** denotes hydroxyl groups required for RNA catalytic activity.As can be seen in the above diagrams glycerol supplies the critical catalytic hydroxyl, but lacks the carbons required for polymerization. To me, this is critical, and needs to be resolved, but until such a time it is the most current thinking. As for the phosphates, suffice it to say that they are added fairly easily. I will look for the lipid reactions, and if I can find them, I will post them along with the phosphate reactions. I hope everyone has found this interesting and informative. --- Jeff Otto
Read about the Urey-Miller experiments at the University
of Chicago in the 1950's and then follow it up with study
of the more recent work of Dr. Sidney Fox at the
University of Miami.
Urey-Miller created amino acids by discharging electricity through an atmospheric soup of chemicals. Much as lightning passing through a primordial Earth's atmosphere would have done. Sidney Fox at the University of Miami took those amino acids (created in the same way) and then, by heating them (to less than 150 degrees F) in conjunction with other aspartic and glutamic acids (also created through simulation experiments) and was able to polymerize them into proteinoid microspheres. Under a microscope, the microspheres look like primitive cells. In fact, artificially fossilized microspheres are indistinguishable from the earliest known microfossils that date back to about 3.5 BYA.
Although hesitant to claim that these were alive Dr. Fox stated that they were undeniably "protoalive". This is not an evasive answer. As Tim M. Berra says in "Evolution and the Myth of Creationism" (pg.75):
"For centuries, science knew nothing intermediate between non-living and living things, but today the distinction is not at all clear. Since life evolved from non-living matter, at some point we must arbitrarily draw a line and say that everything beyond that point is alive. Viruses, for example, appear to be alive when they infect a host, but seem to be non-living when outside a host."Since a single cell would appear to be the smallest unit that can be said to be alive, proteinoid microspheres may quite justifiably be called protocells, or, life.
These are just the early stages of these types of experiments. There is every likelihood that within the next couple hundred years man will be able to create self-replicating life of varying forms from purely chemical and natural elements under laboratory conditions.--- Simon Ewins
See also the Abiogenesis FAQ.