Shape is not enough to distinguish life from abiotic systems

In August 1996, US president Bill Clinton made a portentous announcement from the White House. He was talking about a rock.

‘Today, rock 84001 speaks to us across all those billions of years and millions of miles’, he said. ‘It speaks of the possibility of life. If this discovery is confirmed, it will surely be one of the most stunning insights into our universe that science has ever uncovered.’

The Martian meteorite ALH84001, picked up at Allan Hills in Antarctica in 1984, was then in the process of acquiring notoriety for revealing the Martians that never were. Researchers at NASA claimed in 1996 that inspection of the meteorite under the electron microscope revealed weird, wormlike nodules which could be ‘fossil remains of a past martian biota’.

In fact, ALH84001 had a different message for us: about the danger of inferring life from morphology. It has never become clear what processes created the peculiar microstructures of the meteorite, but there’s no strong reason to believe they involved Martian microbes, and plenty of evidence that non-living processes can generate comparable shapes and forms. Perhaps chief among these are reaction-diffusion systems, of which the periodic precipitation process discovered in 1896 by German chemist Raphael Eduard Liesegang may be a variant. It’s now widely believed that some of the oldest putative microfossils on Earth, showing filamentary structures once thought to be fossil bacteria, are also in fact inorganic.