The first bacteria may well have infested Earth more than 3 billion years ago, but they left no fossils behind to prove it. In April a team of geologists at the University of Bergen in Norway proposed a new type of evidence: microscopic tubules 3.5 billion years old. Harald Furnes and his colleagues found the tubules in South African volcanic rock that formed when lava bubbled up from the ocean. Tubes are typically three micrometers wide and look like those in modern volcanic rock, which are thought to be made by microbes that consume the rock’s nutrients.
Furnes found that the carbon-rich rock surrounding the tubes had an unusually high ratio of carbon-12 isotopes—which microbes release as waste in CO2. The tubules and isotopes together add up to a solid case, he says. “We think we have found a clear indication that life was present.”
Paleontologist Martin Brasier of Oxford University isn’t convinced. He and his colleagues have studied similar ancient rocks from Australia, and they have another explanation for the tubules: As the lava flowed, volcanic gases propelled metallic grains through it. “The trails left behind can be astonishingly like microtubules,” Brasier says. Furnes needs proof, he adds, that there aren’t metals in the South African lavas as well as additional isotopes: “Without these lines of evidence, it might turn out to be wrong.”