Titanium and potassium concentrations of the stromatolites are around a tenth of those in the rocks around them, suggesting a radically different formation process. Similarly, the isotopes of carbon and oxygen show abundances consistent with being the products of living things.
We have only occasional examples of sedimentary rocks laid down more than 3.5 billion years ago, all of which have been through deformation events such as mountain formation. These are normally sufficient to destroy any traces of life, but Nutman’s rocks have experienced temperatures no higher than 500 to 550ºC (930 to 1,020ºF), preserving the precious evidence. Such finds are so rare that Nutman said he was not surprised we haven't found any specimens from 3.6 or 3.5 billion years ago, even though life presumably became more common as time went on.
Despite the evidence life first appeared more than four billion years ago, it is not clear whether we will ever be able to find fossils substantially older than the ones Nutman has identified. The early Earth was mostly made up of oceans, with only small areas of continental crust, so the shallow ponds in which these stromatolites lived were rare.
There is much debate among the biologists who investigate the origins of life whether the first living things appeared in a "warm little pond" as Darwin thought, or if a more likely location was around a hydrothermal vent at the bottom of the ocean. Although continental crust dating back this far is rare, it at least exists. Oceanic crust is replaced approximately every 200 million years, so any traces would have been erased many times over. We also don't know how long it took from the time the first life forms appeared to reach the communities capable of leaving a legacy in rocks large enough for us to see.
Outstanding as the discovery is for science, the way it came about is yet another cause for alarm. The stromatolites are encased in rocks that, until recently, were permanently covered by snow. Nutman told IFLScience he has been visiting the area for 30 years, and as the ice retreats, he has targeted newly exposed rocks.