The putative fossils formed just a few hundred million years after Earth itself
SCIENTISTS have a pretty good idea of how the Earth formed: it condensed, around 4.6 billion years ago, from the same cloud of dust and interstellar gas that gave birth to the sun and the rest of the solar system. They are less sure how and when life got going. Last year a group of researchers found evidence for stromatolites—small, layered mounds produced by photosynthesising bacteria—in rocks from Greenland that are 3.7 billion years old.
react-text: 401 Now, though, the date of life’s debut may be pushed back even further. As they report in /react-text Nature react-text: 403 , a group of researchers led by Dominic Papineau from University College London have found what they think is the signature of living organisms in rocks from Quebec that date back to between 3.8 and 4.3 billion years ago. Intriguingly, the sort of life that Dr Papineau and his colleagues think they have found is very different from the sort that built the stromatolites. This suggests that even very early in its existence, Earth was hosting several different kinds of living organism. /react-text
The rock in question is a 3-kilometre-long swathe on the eastern shores of the Hudson Bay called the Nuvvuagittuq Greenstone Belt. It is mostly composed of pillow-shaped basalt, a type of rock formed when lava cools rapidly in seawater. When Dr Papineau visited the formation in 2008 he found unusual reddish-coloured outcrops of jasper, a type of quartz formed from compressed volcanic ash, that contained odd-looking veins and nodules. Closer examination revealed rings, between 50 and 100 microns (a millionth of a metre) across. That made him sit up: similar rosette-shaped features have been found in younger, but still ancient, rock formations from Biwabik, in Minnesota, and Løkken, in Norway. They are thought to have been formed when micro-organisms decayed and were fossilised.
But that evidence was not quite conclusive. Similar-looking structures can also be formed by non-living, geological processes. So Dr Papineau gave the rock samples to Matthew Dodd, his PhD student, to look at. Within the veins and nodules of the jasper that intrigued his boss, Mr Dodd found hollow tubes between 2 and 14 microns in diameter and up to 0.5mm long made of haematite, a mineralised form of iron oxide. Some of these filaments form networks anchored to a lump of haematite; others are corkscrew-shaped.
The team contends that these bear more than a passing resemblance to the networks of bacteria that live in hydrothermal vents—towering, crenellated structures that form in the deep ocean above the boundaries between tectonic plates, where superheated mineral-laden water spurts up from beneath the seabed. Well-preserved fossil remnants of these microbes have been found at many sites younger than Nuvvuagittuq, and they closely resemble the coiled and branching tubes that Dr Papineau and his colleagues have found.
Such a find is doubly intriguing because hydrothermal vents are seen as a plausible candidate for the cradle of life. Microscopic pores in the rock might have served as natural cell walls, and the chemistry of the water could provide exactly the sort of energy gradient that a primitive living cell would have needed to go about its biochemical business. Although the sorts of bacteria apparently found by Dr Papineau and his colleagues are too complicated to reveal much about the very earliest organisms, the suggestion that hydrothermal vents have played host to life for so long is a strike in the theory’s favour.
Bacteria to the future
The find—which will face fierce scrutiny from other palaeobiologists—has other implications, too. Most living organisms, including those that built the stromatolites, ultimately derive their energy from photosynthesis, the process by which plants and some micro-organisms convert sunlight into sugar. The creatures that live around hydrothermal vents are fundamentally different: no sunlight penetrates so deep into the oceans, so the food chains of such ecosystems are based on reactions between the dissolved chemicals that well up from the crust.
If Dr Papineau’s fossils are as old as he thinks, that implies that Earth was, within a few hundred million years of its formation, already playing host to very diverse sorts of life. One of the biggest questions in science is whether life is an inevitable and common consequence of the laws of chemistry, or a lucky one-off confined to Earth alone. If life got going on Earth so quickly, and was able to diversify so rapidly, it suggests the same might have happened elsewhere, too.