Selection in Prebiotic Chemistry: Why This… and not that?
First Luisi presents us with some basic facts. If you don’t agree with any of these, let me know.
Life on Earth started 3.5 to 3.9 billion years ago. Probably earlier, but we definitely have some microfossils (fossils of microscopic organisms) from 3.5 billion years ago. Of course, these were probably fully functional cells, so life probably started much earlier.
To much earlier than that though and you’re getting to the point where the surface of the Earth is molten. Life probably didn’t form then, but most likely, as soon as it was chemically possible.
Research into the origins of life have been going on since he early 1800s. But it really got a shot in the arm with the Miller-Urey experiments.
Luisi goes into some detail on the production of organic compounds by non-organic methods. This is quite significant. For example, one might think that urea (uric acid which is a major component of human urine) might only be formed from… well things that pee.
However, urea can be formed from non-organic compounds and common processes on Earth. This is also true of all the basic building blocks of life (click on the origins of life or peer-reviewed research links on the right to view a lot of my posts on these subjects).
But nucleic acids, sugars, amino acids, lipids, the basic materials of all proteins, fats, DNA, and RNA can easily be synthesized using base materials (methane, ammonia, nitrogen, carbon dioxide, etc) in common circumstances (for a primitive Earth). In fact, it is so easy to form these organic compounds that scientists have even discovered them in comets and nebula.
Luisi notes here that the “iron-sulfur world” view of the origin of life has some support because FeS and FeS2 are excellent catalysts for the chemical reactions that create some of these organic compounds.
It would not surprise me at all, if we learned that all the hypotheses on the origin of life are correct. It really was an RNA-iron-sulfur-panspermia-membrane world. But that’s just my opinion.
Luisi shows a table of all the sources for organic compounds from non-organic sources. He breaks the data down into
- UV photolysis – Millery-Urey type synthesis
- Electrical discharge – also Miller-Urey (for example, lightening caused by volcanic explosions that produce megatons of CO2, nitrates, sulfurs, ect)
- Shocks from impacts – comets and asteroids entering the atmosphere causing high temperature reactions
- hydrothermal vents – iron-sulfur world synthesis
- interplanetary dust – cometary and asteroid debris (for example, the Perseid meteor shower)
- Comets – comet impacts (not including the fact that much of the Earth’s water may be from comet impacts)
The total for all of this is estimated at 1011 kilograms of organic material per year. That’s 100 million metric tons of organic compounds per year on the early Earth.
That kind of production is not happening today for several reasons. One is the presence of atmospheric and dissolved oxygen, which is highly disruptive to the chemical reactions that would form these compounds. A second reason is the presence of life, which rapidly absorbs any compounds produced this way as free nutrients and/or energy.
Here’s where I will get a little thick.
Why this… and not that?
Why are the materials that is universal in life on this world that way? Why aren’t there other options for life?
The simple answer is, it was under thermodynamic control. That same determinism that shows that if you have pure iron and pure oxygen in the same environment, you will end up with rust.
For example, why did the Miller-Urey experiment only produce α-amino acids instead of β-amino acids? Well, the α-aminos are more thermodynamically stable under the conditions in the experiment. That’s all. Very deterministic.
This applies to many of the things that creationists would claim must be evidence of a designer. In fact, it is much easier to make a case for some kind of designer making these decisions (ready for creationists to quotemine). However, this can actually be tested experimentally (and note that no creationists have done so).
Why do human DNA and RNA use 5 carbon sugars instead of the much more common 6 carbon sugars? The answer, again is thermodynamics (BTW, that’s how energy moves through these systems). The 6-carbon sugars are very stable. In fact, they are too stable.
Albert Eschenmoser built RNA using 6-carbon sugars and found that the RNAs couldn’t replicate. The 6-carbon sugar is so stable and so chemically strong that once the strand of RNA was copied, the two strands couldn’t separate. That’s a major problem for replication in biologic systems.
Now Eschenmoser says that it seems to be likely that many forms of RNA-like systems developed about the same time. The reason that we have the one we have now is because it was most efficient. In other words, survival of the fittest.
Luisi spends the rest of the chapter talking about homochirality, the ‘handedness’ of many organic molecules. Think of it this way. Certain molecules have ‘mirror-image’ molecules that are almost exactly the same. They have the same numbers of each type of atom with the same types of bonds between atoms, but they are mirror images of each other.
Because of the super-specific nature of biologic systems, you cannot use a D-amino acid where an L-amino acid goes, even though the are effectively the same molecule.
Now, in nature, one would think that equal numbers of L- and D- forms of a molecule would be produced and that the fact that only L-amino acids are present in nature and biologic organisms only use D-sugars is evidence of something odd going on. Perhaps a designer stepped in to make it that way.
Unfortunately for creationists, this is a non-issue for biologists. There are actually many ways in which one form of amino acid would be produced over the other and why the L-amino acid/D-sugar system exists. I’ll briefly list these and happily provide references to those that wish to explore this in detail.
Methods of generating chiral molecules
- parity violation (5 papers) – basically, even though the molecules are almost exactly alike, there is a small, but important thermodynamic difference between them that favors one form over the other.
- circularly polarized light –
- magnetochiral anisotropy
- chiral crystallization
Any of these methods have been shown to generate one form of a chiral molecule over another. The chiral crystallization concept has the advantage of causing the spread of a particular form. Basically, because some crystals also have a ‘handedness’, but only one of the handed surfaces can be exposed, then any compounds that form on the crystal must have a particular handedness as well. They in turn will promote the handedness in other molecules.
Now, what about D-sugars and L-amino acids. Well, the simple fact is, if we had sugars and amino acids of the same handedness (L- or D-), then our biologic systems just wouldn’t work.
If any of the above methods resulted in more L-amino acids, then only the complimentary D-sugars would be used by those systems. In other words, given L-amino acids as a starting point, D-sugars would have been selected for while L-sugars would have been ignored.
Luisi concludes that the basics of the origin of life are under thermodynamic control and these processes are deterministic. If you were to repeat the Miller-Urey experiment, then you would get the same results that they did. This holds true for much of the precursor chemical reactions.
It’s pretty much trivial to show that, given almost any variety of plausible conditions on the primitive Earth, organic compounds will form.
Again, I would like to point out that we will never know exactly how life on the early Earth formed. In fact, it easily may have been many combinations of these hypotheses and models. The point we are establishing with all this research is that there is no chemical or physical problems with any of it. This is what creationists would have us believe and it is simply not true.
Next, I believe we will begin to explore the complex systems of macromolecules.