Carbon-rich filaments within the 3.46 billion-year-old Apex chert (a fine-grained, sedimentary rock) from the Pilbara, had been likened to forms of bacteria and claimed to be the oldest microfossils to be found. Although doubts had been raised as to their organic origins, it is only now, with new high-spatial resolution transmission electron microscopy (TEM) data, that it has become clear that the ‘Apex chert microfossils’ are in fact stacks of clay minerals. Carbon adsorbed onto these minerals from circulating hydrothermal fluids gave the false impression that they were fossilsed cell walls. The image shows TEM elemental maps of a filamentous pseudofossil from the the Apex chert (left) and a confirmed microfossil from the 1.9 billion-year-old Gunflint chert (below). The real microfossil has a semi-continuous ring of carbon (yellow) corresponding to the cell wall, whereas the Apex pseudofossil has a chaotic distribution of carbon (yellow) associated with iron (red) and sheets of an aluminium-rich mineral (green).
While this latest research does not really move the goalposts for when life first originated on Earth – there are robust microfossils only a few million years younger than the Apex material – it emphasises that not everything that looks like life really is life. This level of detailed analysis will provide a new tool for analysing possible signs of life, on Earth and beyond.
Their work was published in the Proceedings of the National Academy of Sciences.
April 24, 2015