A new theory suggests that in the earliest years of life on Earth, dozens of types of loosely organized protocells engaged in an active swap meet as they exchanged and tested what are now the basic building blocks of the cell. The scenario differs somewhat from the classic tenet, proposed by Charles Darwin, that all of today's life evolved from a single ancestral form.
Biologists have struggled for years to explain how modern cells arose. The three basic groups--bacteria, single-celled microbes called archaea, and eukarya such as plants and us--each contain machinery that deciphers genetic blueprints and produces critical proteins. But the cells' machinery varies fundamentally across the three basic groups of life. Evolutionist Carl Woese of the University of Illinois, Urbana-Champaign, who proposed the now-accepted classification of the three branches of life (Science, 2 May 1997, p. 699), says it has been unclear how such distinct cell types could have evolved from a single ancestral cell.
To come up with an explanation, Woese spent several years comparing genes from many of the 63 microbes whose genomes have been sequenced since the mid-1990s. He focused on cellular components that were likely to be around during early evolution: the building blocks of the cell's ribosomes, which assemble proteins, and the enzymes that ferry amino acids to the ribosome. Billions of years ago, gene transfer between microbe species was much more common than it is today, Woese claims the data show. He speculates that in the distant past, a variety of basic cell types freely exchanged genes and components, such as DNA-replication enzymes. As cell components became more interdependent, the organisms became unable to try out new designs. After that critical point, which Woese calls the "Darwinian threshold," the organisms began to evolve largely independently of one another.
"I think it's a really coherent and poetic vision," says molecular biologist W. Ford Doolittle of Dalhousie University in Halifax, Nova Scotia, who claims that other evolutionary biologists continue to accept a simpler vision, that a single common ancestral cell led to all modern life. But the data don't exclude other scenarios, he says, including one in which species readily exchanged genes after clear lineages formed.