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View Full Version : Hints on Earliest Earth Life: The Primordial Slime?



lpetrich
2002-Nov-18, 02:12 AM
There's been some very interesting research on that subject -- research made possible by the abundance of genomes that have now been sequenced. At the most recent count (http://ergo.integratedgenomics.com/GOLD/), it is 113. Most of these are of prokaryotes (nucleus-less organisms, one-celled or colonial), but that is actually OK for this purpose, since they have the greatest divergences and the most metabolic variety. One finds this out using familiar tools of evolutionary biology, and one can work out what their Last Universal Common Ancestor (LUCA or LUA or LCA) (http://caspar.bgsu.edu/~courses/evolution/xLectures/Lect02_04.html) had been like.

It was a rather sophisticated organism, with a complete RNA-to-protein translation system, full-scale biosynthesis, including carbon fixation, etc. It thus depended on various inorganic-chemical reactions for energy, being "chemolithotrophic", something like various present-day bacteria. And although it had had DNA, its DNA handling was not well-developed. But its proteins had a curious feature (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12270892&dopt=Abstract): its amino-acid usage was closer to the amino-acid abundances expected from prebiotic-chemistry "Primordial Soup" experiments.

With the help of evolutionary-biology tools, we can look back even further (http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12225777&dopt=Abstract), using evidence of gene duplications, like genes which look like concatenated duplicates of some earlier gene. And also features of the RNA-to-protein genetic code, and how many metabolic steps are necessary to build various amino acids.

As a result, there are some tantalizing hints of life before cells, as deduced from features of a protein called ferredoxin. It does the equivalent of transferring hydrogen from one molecule to another, an important metabolic process. Its inferred ancestral amino acids are mostly the fairly-simple, easy-to-build ones that could easily have come from some Primordial Soup, and it has an acidic, negatively-charged "tail" that can easily stick to mineral surfaces, with their positively-charged metal ions. Ferredoxin's "working parts" are also mineral -- an iron-sulfur complex.

This is consistent with the Wachterhauser picture of life having originated from some iron-sulfur-organic chemistry taking place in hot-spring mud and the like.

And the persistence of ferredoxin from that time suggests that the earliest life on Earth had not been divided up into cells, but had instead been one big mass that lived in the interstices of soil grains in hot-spring mud and similar habitats, and that had metabolized on those grains' surfaces.

This is much like Ernst Haeckel's proposed Urschleim, a pre-cellular stage of life. In his day, it had even been "identified" on ocean floors, as Bathybius haeckelii. However, Bathybius could be produced by adding a certain preservative to ocean-floor mud, making it a false alarm.

There are still some unanswered questions, like the origin of RNA (DNA is a RNA derivative, however). However, primordial-soup-like amino-acid abundances and evidence of iron-sulfur mineral-surface metabolism are all consistent with various hypotheses about prebiotic environments.

Which adds support for the proposition that the origin of life could happen elsewhere in the Universe under conditions that are much like some early-Earth environments.

(edited to make the title catchier)

<font size=-1>[ This Message was edited by: lpetrich on 2002-11-17 22:48 ]</font>

jkmccrann
2005-Nov-08, 11:03 AM
Very interesting background. Explaining and coming up with a workable theory on how life originated on Earth from the so-called `primordial soup' will go a long way towards dispelling the flimsy science of the IDers.

It would also obviously give us a better idea of how likely it is that life has arisen elsewhere, as I'm sure it has.

Benign Terrorist
2005-Nov-08, 07:09 PM
I am not so sure that prokaryotic life had greater diversity and genetic adaptability than eukaryotic life. Seems like it was the eukaryotes that got that bandwagon really rolling.

As for life outside of a cell, someone ran an experiment in the 60's in which he got the single stranded RNA from an AIDS virus to replicate in a solution that had only replicase and additional nucleotides. The nucleotides were reasonable because they were abundant in the early oceans. However, the replicase was a stretch.

What he found was that shorter RNA strands reproduced faster than long ones. This had the effect of favoring strands that lost unneeded sections to their makeup. They reproduced faster, so they grabbed the available resources faster - the additional nucleotides. In about 74 generations, his sample quickly shortened from around 4,500 nucleotides down to 220, where the process stopped shortening. The experiment was reproducible; other researchers got exactly the same results.

TheBlackCat
2005-Nov-09, 12:32 AM
All eukaryotes have very similar basic biochemistires. Bacteria biochemistries vary a lot. I think that is what they mean.

Replicase may be a stretch, but RNA is a catalyst, it is perfectly reasonable for there to be an auto-catalytic form of RNA that could catalyze its own synthesis. Humans may even have synthesized one at some point, I am not sure.

Ilya
2005-Nov-09, 02:35 AM
As for life outside of a cell, someone ran an experiment in the 60's in which he got the single stranded RNA from an AIDS virus to replicate in a solution that had only replicase and additional nucleotides. The nucleotides were reasonable because they were abundant in the early oceans. However, the replicase was a stretch.

Where did he get an AIDS virus in 1960's? It was not even recognized until late 70's, if not 1980.

Blob
2005-Nov-12, 06:17 AM
Hum,

&#60;slightly off topic&#62;

it has recently been discovered that certain clay minerals at the bottom of the ocean may have acted as incubators for the first organic molecules on Earth.

Clay structures in and around mid Atlantic ocean floor black smokers (volcanic vents) providing a haven for molecules brought up from the Earth's interior

Lumps of clay (smectite) that build up on the inside walls of the vents capture molecules , then protected them from the 300 to 400 C temperatures in those vents.
Smectite owes its protective properties to the layers of silicate it is formed from, allowing it to expand easily and let water, ions and molecules to flow inside.
The clay deposits eventually break away and spill out onto the ocean floor, where they release the complex molecules, such as methanol, into the cooler surrounding waters.

http://geology.asu.edu/people/faculty/lwilliams/

&#60;/slightly off topic&#62;