No record at the Olympics was broken quite like this one. The discovery of 3.7 billion-year-old microbial structures in Isua, Greenland, extends the fossil history of Earth by 220 million years.
In Nature, the University of Wollongong's Professor Allen Nutman describes a set of wavy lines 1 to 4 centimeters (0.4 to 1.6 inches) high found in metamorphic rocks in southwest Greenland. Nutman identifies these as stromatolites, the planet's most ancient type of fossils. The site is one of two where the oldest rocks on Earth survive; the specific site has been dated to 3.7 billion years old.
Molecular clocks tracking the time since living species shared a common ancestor suggest life is older than 4 billion years. Nutman has previously found evidence of carbon dioxide sequestration, suggestive of life, in rocks of the same age. Nevertheless, until Nutman's new discovery, the oldest fossils were from the 3.48 billion-year-old Dresser Formation in Western Australia. The find is powerful confirmation for the theory life appeared almost as soon as Earth was cool enough to support it.
“We know very little about the life forms that made these at this point,” Nutman told IFLScience. “These aren't fossils in the way most people think. They are constructs made by microscopic life, communities of single-celled organisms.” He added that he is working on a project that “may tell us something about the metabolism” of the microbes involved, but this is yet to produce results.
In the meantime, Nutman's confidence that the lines are a product of life, rather than a geological formation, rests on several pieces of evidence. “We examined the discovery with a stromatolite expert, who has a series of criteria to tick off,” Nutman said. “These include the general shape, the relationship with adjacent rocks, and the chemistry.”
Close up of the Isua stromatolite (a) and (b) compared to specimens from Western Australia (c) and (d). The lens cap in blue at the bottom is 4cm (1.6in) in diameter. Nutman et al/Nature
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.