Joined: Jan. 2003
Over in this Internet Infidels thread, I had posted some interesting recent research into the evolution of life before the Last Universal Common Ancestor (LUA, LCA, LUCA) of all known cellular Earth life. To summarize, both papers I'd looked at point to the earliest Earth life as being closer to prebiotic chemistry than more recent life. And the second paper points to evidence of much less complexity in the earliest life.
The enormous volume of genetic data collected over the last few decades, including the sequencing of over 100 genomes, has made possible the reconstruction of several of the genes and proteins contained by the Last Universal Common Ancestor.
The amino-acid content of these proteins is interesting; according to the work of Brooks DJ, Fresco JR, Lesk AM, Singh M, the LUCA's proteins were enhanced in amino acids known to be produced in prebiotic-synthesis experiments, and depleted in amino acids known to be rare or absent in such experiments. This adds support to the hypothesis that the original source of amino acids was prebiotic synthesis; the earliest organisms simply eat some Primordial Soup.
By comparison, Brian K. Davis's work focuses on 10 proteins, and uses a different criterion for assigning amino-acid origin time; how many metabolic steps are necessary to produce some amino acid from a Krebs-Cycle predecessor. Aspartate and glutamate, for example, score very low, while lysine and arginine score very high. The low scorers are also those relatively abundant in prebiotic syntheses, which suggest that biosynthesis of them was developed as a substitute for Primordial-Soup eating (the Horowitz hypothesis).
The "code age" of a protein he determined by finding the average score of its amino acids; he used this to work out the proteins' order of appearance.
The oldest of these proteins was ferredoxin, a biosynthesis enzyme that contains iron-sulfur clusters and that transfers electrons (hydrogen-atom equivalents). This protein he reconstructs as having a negatively-charged tail; this can stick to positively-charged objects like mineral surfaces with their metal ions -- which is consistent with the view of Gunter Wachtershauser that life originated from iron-sulfur-associated chemical reactions on mineral surfaces, and that the Krebs Cycle dates from this time. Note that the Krebs Cycle's members are all acids -- negatively-charged ions -- meaning that they can stick to mineral surfaces.
This suggests that the earliest life had not had well-defined cells, that it had been a sort of Haeckelian Urschleim living in the mud of hydrothermal vents.
Not much younger than ferredoxin is a protein involved in cell division and an ATPase component that resides in cell membranes; as a consequence, nearly all the rest of Brian Davis's scenario takes place in distinct cells, including the acquisition of "difficult" amino acids like the benzene-ring and alkaline ones.
Also after the origin of cells but before the LUCA is the origin of DNA; enzymes for synthesizing DNA nucleotides from RNA ones, copying DNA to RNA, and copying RNA to DNA date from this period. So DNA is younger than both RNA and proteins.
However, DNA-to-DNA copying systems are much more difficult to place in this period, since those of the (eu)bacterial and the archaeo-eukaryotic lineages are very different, suggesting separate elaboration -- or even separate origin. The LUCA could have had a DNA-RNA genome, with DNA being copied to RNA and back.
Brian Davis's paper did not address the RNA-world question, but his work suggests that an RNA world, if it had existed, had been pre-cellular.
An interesting result is that the earliest Earth life is closer to various sorts of prebiotic chemistry than later Earth life. This poses an interesting conundrum for the hypothesis that some designer had "seeded" the Earth with some organism that became the ancestor of all its later life. Why this choice of "seed"? Why not a "seed" with a chemistry more like that of present-day organisms?
Also interesting is the absence from the earliest life of DNA, distinct cells, and several amino acids; this indicates the absence of the enzyme systems necessary for constructing and handling them. Thus, the origin of life has to account for much less complexity than one would expect from present-day cell architecture.
Brooks DJ, Fresco JR, Lesk AM, Singh M.
Evolution of amino acid frequencies in proteins over deep time: inferred order of introduction of amino acids into the genetic code.
Mol Biol Evol 2002 Oct;19(10):1645-55
At this PubMed entry.
Molecular evolution before the origin of species.
Prog Biophys Mol Biol 2002 May-Jul;79(1-3):77-133
At this PubMed entry.