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Time for a change
Prokaryote: gene-sequence comparisons show the tree of life consists of bacteria, eukarya and archaea.
The use of the term ‘prokaryote’ fails to recognize that an idea aboutlife’s origins has been proved wrong.
Norman R. Pace
The explosive accumulation of gene
sequences over the past few decades has
brought a new perspective on life and its
history. Some of the resultsindicate that
we need to reassess our understanding of
the course of evolution at the most fundamental
The current textbook paradigm for biological
diversity and evolution is based on
what Iwill call the prokaryote/eukaryote
model. This posits that there are two kinds
of cells: prokaryotic, those without nuclei
(specifically, without nuclear membranes)
and eukaryotic, those with aclassical
membrane-bounded nucleus. The model
further posits that the former gave rise to
the latter. The historical antecedents of this
model are complex and rooted in the nineteenth
century; forexample, German biologist
Ernst Haeckel positioned ‘monera’
(masses of protoplasm without a nucleus,
later termed ‘prokaryotes’) at the base of
his four-kingdoms phylogenetic tree.
Therecognition that the main eukaryotic
organelles, mitochondria and chloroplasts,
were derived from bacteria by
symbiosis between the bacteria and an
ancestral eukaryotic cell prompted speculation
on asimilar origin for the eukaryotic
nucleus. And the discovery of archaea
— microbes that in many molecular ways
resemble eukaryotes more than bacteria
— resulted in proposals for archaeal origins
fornuclear and cytoplasmic components
of eukaryotic cells. Such proposals
have sustained the concept that prokaryotes
evolved into eukaryotes — an evolutionary
model invoked by the terms
themselves.Molecular-sequence comparisons, first of
ribosomal RNA genes in the late 1970s and
of many other genes since, replaced analyses
based on morphological subjectivities
(such as the presence or...
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