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>
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>