Questions on the Urey-Miller experiments (OrgMol. formation)

[davidbenari]

My knowledge of Earth science is basically null so please bear with me.My first question relates to the soundness of assuming a primitive atmosphere rich with H2O CH4 NH3 and H2 gases (Also not too much presence of O2, as the Urey-Miller atmosphere has to be a "reducing atmosphere"). The wikipedia article mentions this has been criticised but I was left wondering what exactly can you criticize about that? What was the composition of the primitive atmosphere? Where did O2 come from then? What's so special about a "reducing atmosphere"?

Also, versions of these experiments use 100,000 Volt sparks to initiate chemical reactions. I was searching for the typical potential difference in a thunderbolt and couldn't find anything. What is it? Does this question make sense?

On the other hand, thunderbolts assume clouds, I think. What evidence is there for the formation of clouds in the primitive atmosphere?

And my final question is: There exist 500+/- different amino acids. I don't know if they are all found in nature. The ones that are common to life are only 20. Does that mean that all over Earth one can only find 20 different amino acids? Or can one find those actual 500? Why would life prefer 20 only out of all those?

Thanks!

 

[D H]

Where did O2 come from then? What's so special about a "reducing atmosphere"?

Oxygen in the form of oxygen molecules is not very stable. It reacts with lots and lots of stuff. An oxygen atmosphere without a steady resupply of oxygen wouldn't last very long. The primordial (pre-life) atmosphere is almost universally assumed to have been reducing as opposed to oxidizing precisely because oxygen isn't stable. The Miller-Urey experiment was an initial attempt at showing how organic chemicals can arise without life. The first forms of life were almost certainly very primitive and very simple organisms, and they most likely formed from and took advantage of organic compounds that in turn formed from non-biological mechanisms.

With regard to where the oxygen in our atmosphere originated, it came from life, cyanobacteria in particular. There is very solid evidence of this. Most, but far from all, of the iron in the Earth sank to the Earth's core during the formation of the Earth. Iron is a highly reducing chemical. You've seen what happens to iron left outdoors: It quickly turns to rust. No free oxygen, no rust.

Free iron is rather soluble. The Earth's first oceans contained a lot of dissolved iron. When cyanobacteria first formed, all of the waste oxygen produced by photosynthesis went into turning that dissolved free iron into rust. As rust is much less soluble than is free iron, that rust formed by this oxidation event came out of solution and sank to the bottom of the seas. There is evidence of this worldwide in the banded iron formations. Almost all of our easily attainable iron is mined from the rust created by that very primitive photosynthesizing life.

 

[davidbenari]

Were cyanobacteria sea creatures in the beginning? Also, what you said about iron bands, does this mean that the ocean's surface became "less deep" gradually? What would motivate such organised bands?

I'm also confused as to why people say the Urey-Miller atmosphere was similar to that of a volcano eruption. I am confused by this because I think volcano eruptions require the presence of oxygen. Am I wrong in that?

Do you know of any documentary on youtube or something about primitive Earth which you would recommend?

Do you happen to know something of the other questions I posed in my original post?

Thanks a lot, your answer was very interesting.

 

[Evo]

Do you know of any documentary on youtube or something about primitive Earth which you would recommend?

There is this documentary, but the video doesn't work for me. You may be able to view it.

http://www.bbc.co.uk/science/earth/earth_timeline/first_lifeYou might like this documentary, although it barely touches on stromatolites (formed by the cyanobacteria DH mentioned).

First life with David Attenborough– Arrival BBC (2010)

BBC’s First Life is a marvelous encapsulation of life’s evolution in two episodes. Episode one “Arrival” covers major events until the Cambrian Explosion. If you are here because you want to know about the origins of life you may be slightly frustrated. The episode will briefly mention underwater hydrothermal vents as possible locations that life may have started and will mention about stromatolites which appeared in geological history around 3.5 billion years ago and that’s it. It will not talk about how the first living cell with a replication machinery evolved and you will not hear about RNA World Hypothesis as well. The major focus will be on Ediacaran when http://www2.mcdaniel.edu/Biology/PGclass/webpagepictures2/5kingdoms.htm were already established. A documentary on pre-Cambrian era was badly needed and this documentary series fills the gap successfully.

http://naturedocumentaries.org/3063/first-life-david-attenborough-arrival-bbc-2010/

 

[davidbenari]

Nice, Evo.

I'll check them out.