Anthony Poole: Abstract
The Nature of the Last Universal Common Ancestor
Rooting the tree of life has long been considered the primary method
for establishing nature of the Last Universal Common Ancestor
(LUCA). However, even if the right tree is recovered, it is impossible
to extrapolate the nature of the LUCA simply from the inferred
relationships between modern organisms. Comparative genomics
approaches have shed greater light on this problem, but are hampered
by incongruence and incomplete distribution of many genes. An
alternative approach, independent of the phylogenetic rooting problem,
is to try and reconstruct ancient biochemistry. The RNA world period
in the evolution of life predates the LUCA, and I have used RNA relics
to examine this problem. The majority of relics from the RNA world are
found in the eukaryotes, having been lost or replaced by protein
equivalents in archaea and bacteria. This suggests that the LUCA was
not a thermophile, and that many features of prokaryote biochemistry
are in fact derived, with ancestral pathways having been retained in
the eukaryote lineage.
A second issue I will consider is Carl Woese?s hypothesis, that early
life was dominated by horizontal gene transfer. He envisages the three
lineages, archaea, bacteria and eukaryotes emerging independently. I
will argue that his descriptive model is problematic, on the basis of
previously published work on simple replicators.
Lars Arvestad
Last modified: Wed Sep 15 16:40:16 CEST 2004