Biology Reference
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prokaryotes is not between archaebacteria and eubacteria (as suggested by Woese and coworkers)
but between organisms that have either a monoderm cell structure (i.e. prokaryotic cells surrounded
by a single membrane which includes all archaebacteria and gram-positive bacteria) or a diderm
cell structure (i.e. prokaryotic cell structure surrounded by an outer membrane which includes all
the gram-negative bacteria). Gupta (1998) further suggested that the root of the prokaryotic tree
or TOL lies in between (or within) archaebacteria and gram-positive bacteria. In this connection,
mention may be made of a eubacterial species
T. maritima
which had been treated as a gram-negative
bacterium but later was shown to be a gram-positive one. So along with archaebacteria the gram-
positive bacteria are infact represent ancestral lineages based on Earth's geological history and the
hot and anaerobic conditions prevailed at that time were conducive for the growth of these two
groups of organisms. Further it has been concluded that the ancestral eukaryotic cell is not a direct
descendent of the archaebacterial lineage but is in fact a chimera that resulted from a unique fusion
event between two different groups of prokaryotes such as a thermoacidophilic archaebacterium
(monoderm) and a gram-negative eubacterium (diderm) followed by the integration of their
genomes. Thus all eukaryotes (both amitochondriate and aplastidic) might have acquired the gene
contributions from both these sources and retained them.
On the other hand, Cavalier-Smith (2002) presented a rooted TOL depicting the evolution
of different bacterial groups from a root consisting of gram-negative bacteria. So in this respect
the concepts evolved by Cavalier-Smith (2001) mostly agree with the conclusions of Woese
et al
.
(1990) in recognizing the archaebacteria as a more recent group whose ancestors have given rise
to archaebacteria on the one hand and the earliest eukaryotic organisms on the other. However,
the difference lies in recognizing the root of the TOL. Accordingly, the roots identifi ed by the three
groups of workers are (i) ancestors between eubacteria and archaebacteria (Woese
et al
., 1990), (ii)
ancestors between archaebacteria and gram-positive bacteria (Gupta, 1998) and (iii) ancestors between
archaebacteria and gram-negative bacteria (Cavalier-Smith, 2001).
According to Cavalier-Smith (2002) life on planet Earth most likely has evolved ~3850 million
years (My) ago. Negibacteria (in Latin 'Negi' means negative, i.e. gram-negative bacteria) with
a double peptidoglycan layer of cell wall, porins, acyl ester phospholipids, photosynthesis and
chlorosomes appeared at the base ~3400 My ago that formed the root for the TOL. Evolution diverged
into cyanobacteria and Eobacteria (in Latin 'Eo' means dawn or early). At about ~2500 My ago
cyanobacterial branch gave rise to Eubacterial group comprising Posibacteria ('Posi' for gram-positive
bacteria with endospores-Endobacteria) and Actinobacteria (comprising Arthrobacteria, Streptomycetes
and Arbobacteria). Thermophilic ancestors of Eubacteria called as Neomura gave rise (~850 My
ago) to Archaebacteria and Eukaryota (Fig. 22). The line from Eobacteria diverged separately into
Spirochaetes (~2500 My ago after cyanobacteria have evolved), Planctobacteria and Proteobacteria. It
is the Proteobacteria that probably formed the ancestors for mitochondrial development during the
time of appearance of primitive eukaryotes which had mesophily and phagotrophy as their properties.
In this bacterial megaclassifi cation he recognized one Kingdom (as per Bacteriological Code) instead
of a Domain and Enterobacteriales as the Type order. He considered the term as 'Eubacteria' as a
useful grade name but not treated in his classifi cation as a taxon. The Bacteria are divided into two
new subkingdoms. Subkingdom I consists of Negibacteria (with a cell envelope of two distinct genetic
membranes) and Unibacteria (consisting of new phyla Archaebacteria and Posibacteria). A number of
morphological, palaentological and molecular data have been integrated to present a unifi ed picture
of bacterial cell evolution despite occasional LGT events. Archaebacteria and eukaryotes comprise the
clade neomura. There are some characters unique (8 of them) to Archaebacteria of which mention may
be made of the absence of Hsp90 chaperone, the splitting of RNA polymerase into two proteins and
