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.