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Thursday, June 20, 2002
How life began: redux
By ROWAN HOOPER
What was the force driving the evolution of life on earth? This question, the answer to which has profound implications for our world view, was neglected for most of the 20th century, not because it was outside science, but because scientists didn't have the technical means to address it. Since the advent of molecular biology, however, the question has become tractable -- and has been recast. The question now is about the origin of cells -- how did modern cells evolve?
The orthodox Darwinian model says that life began with one primordial cell. But this week, Carl Woese, a microbiologist at the University of Illinois at Urbana-Champaign, proposes a new theory. Instead of one "mother cell," there may initially have been three simple types of loosely organized molecule groups. These protocells lived in a kind of genetic commune, says Woese, a slime of RNA, where genes -- and hence new evolutionary tricks -- could be shared freely. The process is known as horizontal gene transfer. Whole chunks of genetic material, RNA, could pass between different protocells, in some cases completely reworking their genetic makeup.
It is horizontal gene transfer, says Woese, that was the engine of cellular evolution. Woese has come up with radical new theories before. In the late 1970s he transformed biology when he proposed an entirely new domain of life, the Archaea.
Until Woese and his colleague, Ralph Wolfe, identified the Archaea, all life was thought to belong to one of two lineages -- the eukaryotes (containing animals, plants, fungi and some unicellular organisms) and the prokaryotes (bacteria). But Woese's work on RNA sequences showed that the Archaea, living in extreme conditions such as hot springs and deep in the sea, were a totally distinct form of life.
Most school textbooks now agree there are three primary divisions of life -- the Eucarya, the Bacteria and the Archaea. But the big question -- according to Woese, the most important in biology today -- is did these three divisions have a common ancestor? Traditional Darwinian thinking says yes, but Woese disagrees.
"We cannot hope to explain cellular evolution if we stay locked in the classical Darwinian mode of thinking," Woese said in the journal Proceedings of the National Academy of Sciences. "The time has come for biology to go beyond the Doctrine of Common Descent."
Before we get carried away (and before the creationists and intelligent design brigade get their trumpets out), let's just qualify Woese's statement. He doesn't mean Darwin was wrong -- he's not saying that we didn't evolve from apes in Africa, nor that humans, plants and fungi don't all have a common ancestor, nor that natural selection is not the process that shaped us and all forms of life in the world. What Woese is saying, however, is that the old idea that all life on Earth descended from one original primordial form needs rethinking.
Until now, says Woese, the problem of cellular evolution has been tackled by taking the eukaryotic cell as a starting point. It's been convincingly shown that the cellular powerhouses of the eukaryotic cell, the mitochondria, were once free-living bacteria, but became absorbed into larger cells and gradually lost the ability to survive autonomously (though they retain their own DNA).
Chloroplasts, too, which convert sunlight into energy in plants, were once free-living. The success of the "absorption" hypothesis (scientists call such cellular fusion "endosymbiosis") at explaining the existence of mitochondria and chloroplasts in modern cells has meant that few other explanations for cell evolution have been entertained. Norman Pace, professor of molecular, cellular and developmental biology at the University of Colorado, calls this "eukaryote chauvinism."
But microbial genomics -- the study of the genetic relationships among microorganisms -- has shown that the domains of life, Archaea, Bacteria and Eucarya, do not form a hierarchical tree. There was no "upward" evolution from primitive bacteria to complex eukaryotes. Instead, Woese argues that cellular evolution was interconnected, that protocells could easily share evolutionary inventions.
"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. "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."
By "vertically generated novelty," Woese means genetic differences that occur when two individuals mix their genetic material and make a new one; when genetic information travels vertically, to the next generation.
Woese labels the point when vertically generated novelty becomes important as the "Darwinian Threshold," because, he says, from this point on, the horizontal transfer of genes is unimportant, and only after this point is it possible to speak of species as we know them.
"The Darwinian Threshold truly represents the Origin of Species, in that it represents the origin of speciation as we know it," writes Woese.
It remains to be seen how Woese's latest theory will be received by other scientists -- much will depend on their eukaryote chauvinism.
E-mail Rowan Hooper at email@example.com