Biology Reference
In-Depth Information
Ernst
Mayr (1998)
argued that the three-domain arrangement was unjusti-
fied because the number of groups known for Archaebacteria and Eubacteria
was far exceeded by the number of eukaryotic species, and the
phenotypic
diversity is orders of magnitude less for the Archaebacteria and Eubacteria
than the Eukaryota. However, separation of life into three domains has become
less controversial (
Cavalier-Smith 1998, Mayr 1998, Brochier-Armanet et al.
2011, Reynaud and Devos 2011
). More than 100 archaeal genomes had been
sequenced by 2011, and their analysis “provides an increasingly complex pic-
ture of archaeal phylogeny and evolution
…
” and supports the three-domain
arrangement (
Brochier-Armanet et al. 2011, Fournier et al. 2011
). However, the
evolutionary relationship between the three domains of life remains difficult to
resolve (
Reynaud and Devos 2011
). Several hypotheses have been proposed as to
the origin of the Eukaryota.
12.7.3 Origin of Eukaryota
Molecular, geological, and paleontological evidence suggests that the Eukaryota
originated a very long time ago (
Margulis 1970, 1996; Katz 1998, 1999; Guy and
Ettema 2011; Parfrey et al. 2011
;
Figure 12.6
). Estimates of eukaryotic origins,
however, vary (
Parfrey et al. 2011
), with the fossil record suggesting eukaryotes
were present
≈
1800 million years ago and possible biomarkers as old as 2700
million years old have been found.
Parfrey et al. (2011)
provide molecular-clock
estimates that suggest the origin of eukaryotes occurred 1866-1679 million
years ago, which they claim is congruent with the fossil record.
Eukaryotic cells developed from an ancestor that contained an endosymbiont
that originated from an
α
-proteobacterium and that microorganism evolved to
become a mitochondrion. This suggests that bacteria evolved before the L
ast
U
niversal
C
ommon
A
ncestor
(
LUCA
) of all eukaryotes and that the host cell that
engulfed the bacterium could have been an archaebacterium (
Guy and Ettema
2011, Kelly et al. 2011
). The rRNA genealogy and analyses of ancient gene dupli-
cations of protein-coding genes support the sister status of Archaebacteria and
Eukaryota (
Katz 1998
). However, this view had to be modified after complete
genome sequences were obtained from a variety of organisms. It is believed
that eukaryotic genomes are
chimeric
, derived from both archaebacterial and
eubacterial lineages (
Golding and Gupta 1995, Rivera et al. 1998, Lang et al.
1999, Roger 1999, Kelly et al. 2011, Reynaud and Devos 2011
).
The three domains share an RNA polymerase, ribosomes, membrane-protein
systems, and a common genetic code, indicating that they are derived from a
common ancestor (
Reynaud and Devos 2011
). However, each domain has unique
features. Archaebacteria and Eubacteria both have a circular chromosome
