CRYSTALS that look just like fossilised bacteria have been grown in the lab, disproving the idea that such structures are too complex to form in the absence of life.
The conclusion has serious repercussions for the debate about the emergence of life on Earth, and the search for life on other planets. The world was stunned in 1996 by news that a Martian meteorite, ALH84001, appeared to contain fossils of alien nanobacteria. Similarly, controversy has dogged the interpretation of microscopic worm-like structures embedded in Australian rocks, ever since their discovery in the 1980s. William Schopf, now at the University of California, Los Angeles, says they are fossilised bacteria. He claims to have identified 11 different species, including some that photosynthesise. But the rocks they are embedded in are 3.5 billion years old. If the structures are bacteria, they would be the oldest signs of life yet discovered. Many scientists doubt that such complex life forms could exist so early in Earth鈥檚 history.
Several groups have been trying to explain how else such complicated shapes could have formed. Last year, Martin Brasier of the University of Oxford suggested that the 鈥渟hells鈥� of the Australian structures could be volcanic glass, ejected from a hydrothermal vent while the rock settled as sediment.
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But no one had demonstrated how such structures might arise. Juan Manuel Garcia-Ruiz from the University of Granada, Spain, was studying the way that crystals grow in gels when he noticed that they took on shapes similar to primitive organisms. Then he realised that the Australian 鈥渕icrofossils鈥� are found in a particular type of rock known as chert, which forms when water is squeezed out of silica-rich sediment. This sediment would behave like a gel.
To recreate what might have happened in the rocks, Garcia-Ruiz added barium and carbonate to his gel, as these minerals are also found in chert. Objects emerged from the mixture that look identical to the worm-like shapes (see Graphic). Each filament consists of a twisted core of barium carbonate enclosed in a skin of silica (Science, vol 302, p 1194).
Garcia-Ruiz鈥檚 colleague, Stephen Hyde of the Australian National University in Canberra, bathed the crystals in simple organic molecules such as phenol and formaldehyde, which stuck to the silica skin. After gentle heating, the filaments produced an organic signature under laser light. Schopf detected a similar signature in his structures, which he interpreted as further evidence of their biological origin (快猫短视频, 9 March 2002, p 5).
He says the research is 鈥渋nteresting and ingenious鈥�, but he remains convinced that the shapes have their origin in life. The work doesn鈥檛 prove that they aren鈥檛 bacteria, but it does show that non-biological processes can create much more complicated structures than thought. And although the researchers were trying to recreate the Australian 鈥渕icrofossils鈥�, they say their creations may support the argument that the Martian structures aren鈥檛 biological either. 鈥淭he morphologies of the ALH84001 beasties are very similar to ours,鈥� says Hyde. 鈥淭he whole issue of nanobacteria is possibly related.鈥�
Earth scientist Owen Green, who works with Brasier in Oxford, says the lab-made crystals have all the elements used to classify microscopic structures as signs of life. He wants researchers to judge them much more strictly before concluding they were made by bacteria.
Meanwhile, Hyde suggests that we might need to change our ideas about what is life and what isn鈥檛. 鈥淭he take-home message is that the conventional distinction between living and dead matter is probably naive,鈥� he told 快猫短视频. 鈥淭here is a fuzzy zone between biological and inorganic morphology, and between biochemistry and inorganic chemistry.鈥�