Exact(3)
The adaptive significance of multicellularity was analyzed in detail by others, cited in my refs [ 1- 4].
These models made an important contribution in detailing possible pathways to cellular life and towards the eukaryotic cell, but many alternative syntrophic relationships at the root of the eukaryotes were suggested, for examples see my refs. 3 6.
The idea of a primitive, pre-cellular common ancestor dates back to the first molecular trees of life, when Fox and Woese concluded that this organism might have been a progenote, i.e. an organism without a tight coupling between geno- and phenotype (my refs. 12 & 13).
Similar(57)
However, several shared derived characters of ATPases (my ref. 15) and elongation factors (my ref. 16) suggest that the root is located outside the clade comprised by the archaea and the eukaryotic nucleocytoplasm.
Apparently, the MRCA of all known cellular organisms was not devoid of membranes, but already had a complex targeting machinery for membrane proteins (my ref. 9), terminal oxidases (my ref. 10 and Simonetta Gribaldo, pers. communication) and ATP synthases driven by transmembrane electricochemical ion gradients (my ref. 11).
To a large extent lipids based on fatty acids, long chain alcohols, and even non-biogenic lipids, for example extracted from the Murchison meteorite (my ref. 21), appear mechanistically equivalent.
Pre-cells, or cells with small, possibly partial genomes (my ref. 23) should do just fine for the indicated stages, as long as there is a moderate level of transfer allowing for recombination and for molecular parasites to evolve.
One way to arrive at a more primitive MRCA of the three domains is to place the root of the tree of life on the eukaryotic branch as suggested by Forterre and collaborators (my ref. 14).
If the ester linked fatty acid based membrane lipids were a later bacterial invention, it would not be surprising to find this pathway also in eukaryotes, because all known eukaryotes apparently evolved from ancestors that once possessed mitochondria (my ref. 20).
My son refs too and one time I used him as my partner.
This point is illustrated by our temporal estimates from the mitochondrial 16S gene which we obtained applying a slower substitution rate, which ranged from 0.03 1% per lineage per My (see refs. in the Discussion).
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