From: Damien Broderick (d.broderick@english.unimelb.edu.au)
Date: Thu Jun 20 2002 - 23:23:23 MDT
Copyright 1995-2002 UniScience News Net, Inc. All rights reserved.
Theory Challenges Darwin Doctrine Of
Common Descent
The driving force in evolving cellular life on Earth has
been horizontal gene transfer, in which the acquisition
of alien cellular components, including genes and
proteins, works to promote the evolution of recipient
cellular entities.
This is the theory of Carl Woese, a microbiologist at the
University of Illinois at Urbana-Champaign.
Woese presents his theory of cellular evolution, which
challenges long-held traditions and beliefs of biologists,
in today's issue of the Proceedings of the National
Academy of Sciences.
Life did not begin with one primordial cell, Woese's
theory holds. Instead, there were initially at least three
simple types of loosely constructed cellular
organizations.
They swam in a pool of genes, evolving in a communal
way that aided one another in bootstrapping into the
three distinct types of cells by sharing their evolutionary
inventions.
Cellular evolution, Woese argues, began in a communal
environment in which the loosely organized cells took
shape through extensive horizontal gene transfer.
Such a transfer previously had been recognized as
having a minor role in evolution, but the arrival of
microbial genomics, Woese says, is shedding a more
accurate light. Horizontal gene transfer, he argues, has
the capacity to rework entire genomes. With simple
primitive entities, this process can "completely erase an
organismal genealogical trace."
His theory challenges the longstanding Darwinian
assumption known as the Doctrine of Common Descent
-- that all life on Earth has descended from one original
primordial form.
"We cannot expect to explain cellular evolution if we
stay locked in the classical Darwinian mode of
thinking," Woese says. "The time has come for biology
to go beyond the Doctrine of Common Descent."
"Neither it nor any variation of it can capture the tenor,
the dynamic, the essence of the evolutionary process
that spawned cellular organization," Woese writes in his
paper.
Going against traditional thinking is not new to Woese,
a recipient of the National Medal of Science (2000), and
holder of the Stanley O. Ikenberry Endowed Chair at
Illinois.
In the late 1970s, Woese identified the Archaea, a group
of microorganisms that thrive primarily in extremely
harsh environments, as a separate life form from the
planet's two long-accepted lines -- the typical bacteria
and the eukaryotes (creatures like animals, plants, fungi
and certain unicellular organisms, whose cells have a
visible nucleus).
His discovery eventually led to a revision of biology
books around the world.
The three primary divisions of life now comprise the
familiar bacteria and eukaryotes, along with the
Archaea. Woese argues that these three life forms
evolved separately but exchanged genes, which he
refers to as inventions, along the way.
He rejects the widely-held notion that endosymbiosis
(which led to chloroplasts and mitochondria) was the
driving force in the evolution of the eukaryotic cell
itself or that it was a determining factor in cellular
evolution, because that approach assumes a beginning
with fully evolved cells.
His theory follows years of analysis of the Archaea and
a comparison with bacterial and eukaryote cell lines.
"The individual cell designs that evolved in this way are
nevertheless fundamentally distinct, because the initial
conditions in each case are somewhat different," Woese
writes in his introduction. "As a cell design becomes
more complex and interconnected a critical point is
reached where a more integrated cellular organization
emerges, and vertically generated novelty can and does
assume greater importance."
Woese calls this critical point in a cell's evolutionary
course the Darwinian Threshold, a time when a
genealogical trail, or the origin of a species, begins.
From this point forward, only relatively minor changes
can occur in the evolution of the organization of a
given type of cell.
To understand cellular evolution, one must go back
beyond the Darwinian Threshold, Woese said.
His argument is built around evidence "from the three
main cellular information processing systems" --
translation, transcription and replication -- and Woese
suggests that cellular evolution progressed in that order,
with translation leading the way.
The pivotal development in the evolution of modern
protein-based cells, Woese says, was the invention of
symbolic representation on the molecular level -- that
is, the capacity to "translate" nucleic acid sequence into
amino acid sequence.
Human language is another example of the evolutionary
potential of symbolic representation, he argues.
"It has set Homo sapiens entirely apart from its
(otherwise very close) primitive relatives, and it is
bringing forth a new level of biological organization,"
Woese writes.
The advent of translation, he says, caused various
archaic nucleic-based entities to begin changing into
proteinaceous ones, emerging as forerunners of modern
cells as genes and other individual components were
exchanged among them.
The three modern types of cellular organization
represent a mosaic of relationships: In some ways, one
pair of them will appear highly similar; in others, a
different pair will.
This, Woese says, is exactly what would be expected
had they individually begun as distinct entities, but
during their subsequent evolutions they had engaged in
genetic cross-talk -- a commerce of genes.
[Contact: Jim Barlow]
18-Jun-2002
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