Instead of one universal evolutionary tree, picture a three-trunk stand sharing a communal root system. A new theory of cellular evolution published in the current issue of the Proceedings of the National Academy of Sciences rejects Charles Darwin¿s Doctrine of Common Descent—the idea that all organisms are derived from a single primordial ancestor. Instead, Carl Woese of the University of Illinois-Champaign proposes that the three cell types that comprise life on earth arose from three forms of proto cells that swam together in a dense genetic soup, freely sharing their DNA.

Indeed, such DNA swapping was the driving force in the evolution of unicellular organisms, Woese argues. Biologists have traditionally credited this so-called horizontal gene transfer with just a minor role in cellular evolution. But Woese asserts that only by sharing their genes—or evolutionary inventions, as he calls them—could simple cellular organizations have given rise to more complex cell designs. In the beginning, he says, primitive cells "did not have stable genealogical records." But eventually, these lines—including the three that spawned all extant life forms—reached what Woese terms the "Darwinian threshold," the point at which a lineage matures to genetic stability. Here the cellular organization became fixed, leading to a traceable cell line via reproduction. "Crossing a Darwinian threshold leads to a more solidified, organized cellular design," he explains.

The idea could overturn conventional cell evolution wisdom. Instead of the individual, "it is the community as a whole, the ecosystem which evolves," Woese remarks. "We can¿t expect to explain cellular evolution if we stay locked in the classical Darwinian mode of thinking," he adds. "The time has come for biology to go beyond the Doctrine of Common Descent."