New cellular evolution theory rejects single cell beginning
Jim
Barlow, Life Sciences Editor
(217) 333-5802; b-james3@illinois.edu
6/17/02
CHAMPAIGN, Ill. —
Life did not begin with one primordial
cell. 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.
The driving force in evolving cellular life on Earth, says Carl Woese, a microbiologist
at the University of Illinois at Urbana-Champaign, has been horizontal gene
transfer, in which the acquisition of alien cellular components, including genes
and proteins, work to promote the evolution of recipient cellular entities.
Woese presents his theory of cellular evolution, which challenges long-held
traditions and beliefs of biologists, in the June 18 issue of the Proceedings
of the National Academy of Sciences.
Cellular evolution, he 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 said. "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 wrote 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 wrote 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 he 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
said, 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 wrote.
The advent of translation, he said, 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 said, 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 – they had indulged in a commerce of genes.