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IF FOSSILS look like bacteria, are they bacteria? That鈥檚 the question that divided researchers over tiny structures in the famous Martian meteorite ALH84001, and it鈥檚 now at the heart of fierce debate over imprints in ancient terrestrial rock. The fossil furore could rewrite the textbooks on the early evolution of life on Earth.

At issue are microscopic patterns of filaments embedded in 3.5-billion-year-old rock from Western Australia. William Schopf of UCLA, who first described them over a decade ago, says they represent 11 different bacterial species, including photosynthetic cyanobacteria. This would make them the oldest fossils ever found.

Schopf has now studied the rocks using a technique called laser Raman analysis, which involves bouncing a laser off the rock surface. This produced a scattering pattern identical to that created by organic molecules found in other fossils. Schopf is convinced this proves his case.

If he鈥檚 right, early life must have diversified incredibly quickly. The first bacteria are thought to have been 鈥渃hemotrophic鈥� species that lived in the dark near underwater hydrothermal vents, feeding on chemicals released there. For a range of photosynthetic species to have evolved as early as 3.5 billion years ago, the first life must have appeared almost as soon as the last asteroid bombardment ceased 3.9 billion years ago (see Total meltown), and then evolved rapidly. That suggests life may also have had time to evolve on a young, wet Mars.

But challengers of this view, led by Martin Brasier of Oxford University, say the imprints Schopf found have no direct connection to life. They claim the rock containing the fossils is actually volcanic glass from a hydrothermal vent that formed tens of metres under water鈥攖oo deep for photosynthetic bacteria to live. When the melted rock recrystallised, organic contaminants got rearranged to look like cells.

That would imply life took much longer to get started. According to Brasier, the first clear evidence of cyanobacteria is just 2.7 billion years old, while the diversity of species that Schopf claims to have found didn鈥檛 appear until 2.1 billion years ago.

Most early life researchers take a view between the two extremes. Jack Farmer of Arizona State University thinks Schopf鈥檚 objects are fossils, but he鈥檚 not convinced they are cyanobacteria. He says they could be chemotrophic bacteria from hydrothermal vents, similar to the earliest forms of life. If that鈥檚 the case, life needn鈥檛 have diversified quite so fast after all.

Critical analysis like Brasier鈥檚 will also be important in answering questions about life on Mars. But until researchers have more sophisticated techniques for analysing such tiny and ancient structures, the implications for early life鈥攐n Earth or Mars鈥攕eem to be largely a matter of philosophy.

Brasier remains a firm sceptic. 鈥淪cientists should take the null hypothesis, that structures are not biogenic, until all abiogenic possibilities have been exhausted,鈥� he says. Farmer, meanwhile, takes a more open view. Although he accepts that it鈥檚 possible for non-biological processes to self-organise into structures that look like cells, 鈥渋t鈥檚 easier for me to accept the simpler explanation that they are fossils.鈥�