ajm51987

Hey All,

Does anyone have any sources ideas, theories, ect on how the earliest life forms formed on Earth? I remember reading something about lightning or ultaviolet rays causing protiens or ameino acids to link togther and that making something happen???

Thanks
-ajm

Answerer

Oh, Bible and Quran got lots of such stuff, go ask those 'righteous' creationists and muslims about it. Hehe, just kidding anyway.

lpetrich

Here are <a href="http://www.geocities.com/CapeCanaveral/Lab/2948/orgel.html" target="_blank">Leslie Orgel's pages</a> on the subject; he has done research into this question for a long time.

The question can be answered in two ways: bottom-up and top-down.

The bottom-up approach is to simulate early-Earth conditions in the lab and see what emerges. This approach is to take water, ammonia, methane, and other such simple compounds and expose them to various energy sources, like ultraviolet light, electric sparks, and so forth. When the starting materials are sufficiently hydrogen-rich, a large variety of organic molecules result, including several biological amino acids (the building blocks of proteins).

However, no new living things have been produced in such experiments, and the chemical mix is very different from that in present-day organisms. However, it is rather close to that of the organic materials in some meteorites, like the Murchison meteorite -- suggesting that Urey-Miller experiments are on the right track.

They may be incomplete, however; there are some putative sites for the origin of life, like ocean-floor hydrothermal vents, that are rather difficult to imitate in the lab. Gunter Wachterhauser has done a lot of work on how chemical reactions in such vents could have led to the first living thing.

I now turn to the top-down approach, which is to extrapolate backward from existing organisms with the tools of evolutionary biology and try to work out what the ultimate ancestor must have been like. And we do find some satisfying simplicity, although not as much as we might like.

The <a href="http://caspar.bgsu.edu/~courses/evolution/xLectures/Lect02_04.html" target="_blank">Last Universal Common Ancestor</a> is reconstructed as having these features:

Prokaryotic cell architecture (eukaryotic architecture a later invention)
Chemosynthetic metabolism (lives off of various chemical reactions)
Full-scale biosynthesis (does not need to eat organic molecules)
Full-scale DNA-to-RNA-to-protein mechanism

However,

DNA replication is poorly developed, and the LUCA may have used DNA-to-RNA-to-DNA instead of direct DNA-to-DNA. Existing DNA-replication systems were invented twice, once in each major branch of prokaryotes.

Looking back further, we find that RNA's have a central enzymatic role in RNA-to-protein translation. Also, some important coenzymes have bits of RNA in them -- and that snippets of RNA can act as enzymes. This leads to the "RNA world" hypothesis, in which some ancestral organism had had RNA's that both carried information and acted as enzymes. Proteins would originally be coenzymes that became the "whole" enzymes; DNA was (and still is!) a modification of RNA.

This leaves open an important question: where did the original RNA come from? Though nucleic-acid bases can be produced in prebiotic-synthesis experiments without much trouble, the same cannot be said of the ribose.

This has led to speculation that RNA had a predecessor, like "Peptide Nucleic Acids", but it is not clear what a plausible predecessor would have been.

On a more positive note, some research into the LUCA and its ancestors has come up with some interesting evidence on its ancestors.

<a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=122708 92&dopt=Abstract" target="_blank">A recent study</a> finds that the LUCA's reconstructed proteins had more than present-day ones of the amino acids commonly formed in prebiotic-synthesis experiments. Which is consistent with a scenario of some ancestor having eaten some "Primordial Soup".

<a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=122257 77&dopt=Abstract" target="_blank">Another recent one</a> finds that one very ancient protein, ferredoxin, was originally adapted for sticking to mineral surfaces, as opposed to sticking to cell membranes, as many proteins are. This is consistent with the Wachterhauser scenario of life originating as some chemical reactions on mineral surfaces in hydrothermal vents and the like.