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Sci Tech
Fresh light on cellular theory
LIFE DID not begin with one primordial cell. Instead, there were initially at least three simple types of loosely constructed cellular organisations. They swam in a pool of genes, evolving in a communal way that aided one another in bootstrapping into 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, has been horizontal gene transfer, in which acquisition of alien cell components, including genes and proteins, work to promote evolution of recipient cellular entities.
Woese presents his theory of cellular evolutionin the Proceedings of the National Academy of Sciences.
His theory challenges the Darwinian assumption known as the Doctrine of Common Descent — that all life on Earth has descended from one original primordial form.
The three primary divisions of life now comprise the familiar bacteria and eukaryotes, along with the Archaea identified by Woese in the late 1970s. Archaea thrive primarily in extremely harsh environments.
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 organisation 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 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, according to him, was the invention of symbolic representation on the molecular level — that is, the capacity to `translate' nucleic acid sequence into amino acid sequence.
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