From Carl Woese: Proc. Natl. Acad. Sci. USA Vol. 95, pp. 6854ö 6859,
June 1998
A concept of the universal ancestor turns on more than phylogenetic trees,
however. The Archaea and Bacteria share large number of metabolic genes
that are not found in eukaryotes (18, 20). If these two ÎÎ prokaryoticââ
groups span the primary phylogenetic divide and their genes are vertically
(genealogically) inherited, then the universal ancestor must have had all
of these genes, these many functions: This distribution of genes would make
the ancestor a prototroph with a complete tricarboxylic acid cycle,
polysaccharide me-tabolism, both sulfur oxidation and reduction, and
nitrogen fixation; it was motile by means of flagella; it had a regulated
cell cycle, and more. This is not the simple ancestor, limited in metabolic
capabilities, that biologists originally intuited. That ancestor can
explain neither this broad distribution of diverse metabolic functions nor
the early origin of autotrophy implied by this distribution. The ancestor
that this broad spread of metabolic genes demands is totipotent (21), a
genetically rich and complex entity, as rich and complex as any modern
cell÷ seemingly more so.
Yet the totipotent ancestor also fails: it cannot explain the manner of the
ancestorâs evolution, i.e., how it became so miraculously complex in so
short a time and just as rapidly gave rise to the ancestors of the three
primary lines of descent. All of this apparently happened in far less than
1 billion years, whereas evolution within each of the three primary lines
of descent has been going on for over 3 billion years now with outcomes
that donât even begin to compare with the spectac-ular ones associated with
the ancestor and its original offspring (4)÷ yet experience teaches that
complex, integrated struc-tures change more slowly than do simple ones.
Moreover, the totipotent ancestor associates physiologies that have not
been observed together in any modern lineage and asks that all ofthis come
about through vertical inheritance. Thus, we are left with no consistent
and satisfactory picture of the universal ancestor. It is time to question
underlying assumptions.
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