Rock structures that have previously been proposed to be the earliest-known remnants of life may be non-biological in origin, reports a study published online this week in Nature. This finding not only adds to our understanding of early life on Earth, but also highlights the importance of integrated, three-dimensional analyses in the search for similar life elsewhere, such as on Mars.
Earth’s oldest rocks, which are found in Greenland, are prime targets in the search for the earliest signs of life. This hunt is complicated by the fact that the original compositions and textures of the rocks having been largely obliterated by the bending, stretching and heating processes of metamorphism.
In 2016, researchers writing in Nature (https://urldefense.proofpoint.com/v2/url?u=https-3A__doi.org_10.1038_nature19355&d=DwICJg&c=vh6FgFnduejNhPPD0fl_yRaSfZy8CWbWnIf4XJhSqx8&r=_nFMf8QbHMB7ni-_C0Yw0VKx5qA48tnCdCMvwPMyE2Q&m=G27Y0m2BRhRhd9eicLERXPGJt7Q2QO7fNMNggPp-kp4&s=KWutV4KLbJ2lJmWwpdFDM0OVC835lPQP49HVxfP2gRA&e=) described 3.7-billion-year-old conical structures, each 1-4 cm high, in rocks from southwest Greenland. These, they argued, were stromatolites (layered sediments formed in shallow waters by mats of single-celled microbes), which suggested that the fossil record began 200 million years earlier than previously thought.
Abigail Allwood, Joel Hurowitz and colleagues analyse the three-dimensional shape, orientation and chemical composition of the alleged fossils. They find that, unlike stromatolites, the structures have no internal layers, lack chemical signatures of microbial activity, and are - when viewed three-dimensionally - actually ridge-shaped, not conical. Instead, they argue, the features are better interpreted as non-biological - the result of metamorphic alteration and deformation of marine sediments long after burial.
“The biological input to ancient stromato￢lites is a long-standing controversy”, comments Mark van Zuilen in an associated News & Views. “Future research might lead to a firm understanding of the primary versus second￢ary processes that shaped this rock.”