TINY tubes in lava 3.5 billion years old may be the world’s oldest trace fossils, pushing back hard evidence for life by around 800 million years. Hints of life from even further back have been seen in isotope traces, but till now the oldest undisputed fossil evidence has dated from only 2.7 billion years ago.
Harald Furnes of the University of Bergen in Norway is basing his claim on the similarity between tubes in ancient rock and holes drilled by microbes in modern seafloor lavas. But the evidence does not convince Martin Brasier, the University of Oxford palaeobiologist who two years ago cast doubt on claims of fossil bacteria dating back 3.5 billion years (Â鶹´«Ă˝, 9 March 2002, p 5).
Furnes and his team studied pillow lavas – which are formed when they erupt on the seafloor – in the 3.5-billion-year-old Barberton greenstone belt in South Africa. In last week’s Science (vol 304, p 578), they describe a multitude of tubes 1 to 9 micrometres across and up to 200 micrometres long, stretching away from cracks where water once flowed.
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The tubes look very much like those dug by bacteria in modern pillow lavas. When Furnes and his group looked at samples of these, they found that bacteria and other microbes seep into water-filled cracks, and then dissolve holes in glassy parts of the rock. They extract chemical energy from iron in the rock by oxidising it or making it react with sulphur.
Calcite in the ancient tubes contains anomalously low levels of the isotope carbon-13. Living cells also have low levels of this isotope, so this backs up the idea that a microbe was involved. “Who did it we cannot tell, but someone did it,” says Furnes.
Furnes is convinced that the tubes are almost as old as the rock because some of them are broken into segments. This probably happened when the rocks were altered by heat and pressure shortly after solidifying.
Martin Fisk of Oregon State University in Corvallis says the South African find is the best evidence yet for very early life on Earth. But Brasier doubts Furnes’s conclusions. He says he has “photos which look absolutely identical” of tubes in Australian rocks from the same period, which he says are not directly produced by organisms. “They are a product of chemical reactions in volcanic glass,” he says.
Brasier thinks the tubes were formed when mineral grains were pushed through the molten glass. He isn’t sure what pushed them, but adds, “we should assume it’s not biological until we have rejected all possible mechanisms that are not biological”. He has yet to examine Furnes’s South African samples.
If Furnes’s claims hold up, researchers could go on to look for similar patterns in rocks from Mars. Fisk has already seen a few short tube-like structures in Martian meteorite samples, but he recognises that calling them evidence for life would be premature.