In
1977, Woese and Wolfe, using 16S ribosomal RNA sequences (Olsen
and Woese, 1993) overturned one of the major dogmas of biology.
Until then, all life on Earth belonged to one of two primary
lineages, the eukaryotes
(animals, plants, fungi and certain unicellular organisms such
as paramecia) and the prokaryotes
(all remaining microscopic organisms). The archaea
-- microorganisms that live in extreme environments without
oxygen in conditions thought to be reminiscent of Earth's early
environment -- changed that long-accepted view. Woese's molecular
studies of RNA sequences led to the realization that the archaea
were distinct from the two accepted classifications. His analytic
approach has since become the standard for identifying and classifying
microorganisms. Now three primary divisions of life are recognized:
eukaryotes, archaea and bacteria. According to Woese: "The
central task of biology in the new century will be to lay out
and elaborate this overarching framework of relationships among
living organisms," Woese said. "This endeavor will
help us to understand how the essential unit of all life, the
cell, came into being. It will help us to understand the evolutionary
interactions among microbial species that gave rise to, sustain,
and have the potential to drastically alter the nature of our
biosphere."
Comparison
of primary classification schemes:
(Color
key: prokaryotic - eukaryotic
- mixed)
Old
2-Kingdom System
|
Old
5-Kingdom System
|
The
New 3-Domain System
While Kingdom is still used, the concept
is of diminished relevance
|
Kingdoms
|
Kingdoms
|
Domains
|
Kingdoms
|
Plantae
|
Monera
(the prokaryotes)
|
Archaea
|
Estimates
range from 18 to 30 or more
|
Eubacteria
|
Unclear
|
Plantae
|
Eukarya
|
Fungi
|
Fungi
|
Plantae
(algae)
|
Protoctist
(e.g. algae, diatoms and different plankton)
|
Number
of others unclear
|
|
Animalia
|
Animalia
|
5
Kingdom System: The so-called 5 Kingdom
System was suggested in 1969 by Robert Whittaker. The 5 kingdoms
were Monera, Protista, Plants,
Animals and Fungi, with a primary differentiation between prokaryotes
and eukaryotes. Prokaryotes were given their own Kingdom Monera,
with the essential requirement being the distinct prokaryotic
cellular organization lacking a nucleus. In this system, prokaryotes
are bacteria including the Cyanophyta, or blue-green algae.
Note that despite being prokaryotic, the Cyanophyta have photosynthetic
pigments not present in the rest of the group, and they release
free oxygen as a by-product of their photosynthetic metabolic
system. Organisms in the other four kingdoms all have eukaryotic
cells with DNA-containing nucleus. Kingdom Proctotista (often
shortened to Protista) is the quite heterogeneous and rather
ambiguous It is most easily understood as a catch-all for all
eukaryotes that do not fit the definitions of plants, animals
or fungi. The plant, animal and fungi kingdoms are multicellular
eukaryotes that are mainly differentiated by their nutritional
modes.
-
Blank,
c., Could Cyanobacteria have Provided the Source of Oxidants
for Banded Iron Formation? 2002 Geological Society of America
Annual Meeting.
-
Cloud, P., Am. J. Sci. 272, 537 (1972).
- Karol,
K., et. al., The Closest Living Relatives of Land Plants, Science,
Vol 294, Issue 5550, 2351-2353 , 14 December 2001
-
Knoll,
A., Life on a Young Planet : The First Three Billion Years
of Evolution on Earth
-
Olcott,
A., Corsetti F, and Stanley A, A New Look at Stromatolite
Form Diversity, 2002 Geological Annual Meeting, Denver,
October 2002.
-
Evolution
of the Early Atmosphere, Hydrosphere, and Biosphere I: Constraints
from Ore Deposits, Session in the 2002 Geological Society
of America Annual Meeting.
- Gupta,
R. S. 1998. Protein phylogenies and signature sequences: A reappraisal
of evolutionary relationships among archaebacteria, eubacteria,
and eukaryotes. Microbiology and Molecular Biology Reviews 62:1435-1491.
- Gupta,
R. S. 1998. What are archaebacteria: Life's third domain or
monoderm prokaryotes related to Gram-positive bacteria? A new
proposal for the classification of prokaryotic organisms. Molecular
Microbiology 29:695-707.
- Olsen
G, Woese C. Ribosomal RNA: a key to phylogeny. FASEB J. 1993
Jan;7(1):113-23.
- Philippe,
H. and P. Forterre. 1999. The rooting of the universal tree
of life is not reliable. Journal of Molecular Evolution
49:509-523.
- Whittaker,
R.H. (1969). New concepts of kingdoms of organisms. Science
163, 150-160.
- Woese,
C. 1998. The universal ancestor. PNAS 95:6854-6859.
- Woese,
C. R., O. Kandler, and M. L. Wheelis. 1990. Towards a natural
system of organisms: proposal for the domains Archaea, Bacteria,
and Eucarya. PNAS 87:4576-457
- Wray,
G., Dating the Origin of Animals, Science, Vol 274, Issue
5295, 1993-1997 , 20 December 1996
|