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ONE of the key assumptions about the origin of life is under fire. The widely held belief that all life on Earth today originated from a single ancestor cell is being challenged by a theory that several different lineages evolved independently.

In his 鈥渄octrine of common descent鈥�, Darwin was the first to argue that all life on the planet began with a single 鈥減rimordial form鈥�, generally interpreted nowadays as the first living cell. Not so, says Carl Woese of the University of Illinois at Urbana-Champaign. In a controversial new theory published this week, he argues that the three fundamental types of cells that form the building blocks of present-day life actually evolved independently, not in an orderly succession from a common ancestor.

Woese is in a strong position to theorise. In 1977 he discovered the single-celled archaea, the last of the three basic cell types to be recognised. The other two are the ordinary bacteria, sometimes called eubacteria, and the nucleated cells of eukaryotes such as animals and plants.

Evolutionary biologists think Darwin鈥檚 primordial cell evolved and split into two types, the archaea and the eubacteria. Eukaryotes later emerged from either the archaean lineage or a mixture of the two.

Woese鈥檚 theory is that the three types of cell emerged independently from a soup of chemicals containing 鈥減roto-cell鈥� structures and simple 鈥渕odules鈥� of genetic information. Early proto-cells swapped gene modules through a process called horizontal gene transfer. But some of these cell-like structures eventually evolved to a stage at which their genes had become highly dependent on each other. New 鈥渙ff the peg鈥� gene modules could no longer be imported successfully.

At this point, which Woese calls the 鈥淒arwinian threshold鈥�, the proto-cells became 鈥渟pecies鈥� in their own right and started to evolve by mutation and reorganisation of their own genes rather than by importing gene modules from outside. Most of these proto-cells vanished without trace. But the three that survived carried on evolving into the life forms of today.