As I understand it, a unique tree of life is what we are left with. I guess it is possible that our tree replaced precursors, but I am not sure if there is evidence of that. I think not. At any rate, the postulated last common ancestor seems surprisingly sophisticated.
Forgive me as in matters biological I don't have the memory so much as I do for history and so I have to lean on sources, here's what I read about the original life form:
The genetic code was based on DNA.
The DNA was composed of four nucleotides (deoxyadenosine, deoxycytidine, deoxythymidine and deoxyguanosine), to the exclusion of other possible deoxynucleotides.
The genetic code was composed of three-nucleotide codons, thus producing 64 different codons. Since only 20 amino acids were used, multiple codons code for the same amino acids.
The DNA was kept double-stranded by a template-dependent DNA polymerase.
The integrity of the DNA was maintained by a group of maintenance enzymes, including DNA topoisomerase, DNA ligase and other DNA repair enzymes. The DNA was also protected by DNA-binding proteins such as histones.
The genetic code was expressed via RNA intermediates, which were single-stranded.
RNA was produced by a DNA-dependent RNA polymerase using nucleotides similar to those of DNA with the exception of thymidine in DNA was replaced by uridine in RNA.
Note however that other studies suggest that LUCA may have lacked DNA and been defined wholly through RNA [10]
The genetic code was expressed into proteins.
All other properties of the organism were the result of protein functions.
Proteins were assembled from free amino acids by translation of an mRNA by ribosomes, tRNA and a group of related proteins.
Ribosomes were composed of two subunits, one big and one small.
Each ribosomal subunit was composed of a core of ribosomal RNA surrounded by ribosomal proteins.
The RNA molecules (rRNA and tRNA) played an important role in the catalytic activity of the ribosomes.
Only 20 amino acids were used, to the exclusion of countless non-standard amino acids.
Only the L-isomers of the amino acids were used.
Glucose could be used as a source of energy and carbon; only the D-isomer was used.
ATP was used as an energy intermediate.
There were several hundred protein enzymes which catalyze chemical reactions that extract energy from fats, sugars, and amino acids, and that synthesize fats, sugars, amino acids, and nucleic acid bases using arbitrary chemical pathways.
The cell contained a water-based cytoplasm that was surrounded and effectively enclosed by a lipid bilayer membrane.
Inside the cell, the concentration of sodium was lower, and potassium was higher, than outside. This gradient was maintained by specific ion pumps.
The cell multiplied by duplicating all its contents followed by cellular division.
http://en.wikipedia.org/wiki/Last_universal_ancestor
That's a fairly amazing starting point, isn't it.
It will be interesting if we learn that Mars has its own independent tree and if it resembles life on earth.
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