I tempered the other week’s repost on some rather impressive 1.5 billion year-old black smoker chimneys, and the fossilised microbes found within them, with some words of caution about the ‘clues to the origin of life’ spin that the discovery was being given:
the first evidence of life in the geological record comes in beyond the 3.5 billion year mark, meaning that there is more distance between the first replicating organisms and these black smoker microbes as there is between the black smoker microbes and us.
In other words, the scientists who speculate on a possible connection between hydrothermal vents and the earliest life are doing just that – speculating – until they find much older equivalents of the Chinese palaeo-smokers, and fossil microbes associated with them. Unfortunately for them, such things are rather hard to come by. Hydrothermal systems are most commonly found in oceanic crust, usually associated with mid-ocean ridges, and most oceanic crust gets subducted after a few hundred million years rather than hanging around for billions like continental crust can.
It seems strange, then, that no sooner does one report of an ancient hydrothermal system appear in the literature, than another one shows up just behind it, courtesy of PhD student Lawrence Duck and his colleagues (including Suzanne Golding) at the University of Queensland. Their study of 3.24 million year-old massive hydrothermal deposits on the Pilbara craton in western Australia provides strong evidence for a flourishing bacterial ecosystem around hydrothermal vents almost 2 billion years further back in Earth’s past.
In these older deposits, you don’t find any fragments of the smoker chimneys, just the plumbing – a widespread, extensively mineralised system of faults and fractures in the the crust beneath where the vent field would have been, forming a ‘massive sulphide deposit’. Microscopic examination of the sediments that host this deposit not only shows that they are full of hydrothermally precipitated silica (quartz) and sulphides, but also reveals numerous bands of dark organic matter. A lot of this seems to be in the form of bundles of tubes and filaments, about 0.1 mm long and 0.001 mm wide (top picture). Another interesting, although slightly less common, morphology is clusters and chains of hollow carbon spheres, with a much smaller diameter (of the order of a few nanometers, or 0.000001 mm) but some of which appear to be budding (arrow in the bottom picture).
In addition to the suggestive morphologies seen under the microscope, signs of life are also found in the ratios of carbon isotopes found in the filaments: the increased proportion of light carbon-12 with respect to carbon-13 are exactly what you would expect if life, and its lighter isotope-favoring enzymes, had been involved. So it seems reasonably likely that these filaments and spheres are some sort of bacteria, and the fast and early growth of minerals precipitated from hydrothermal fluids around them, which appears to have aided their preservation, strongly associates them with the vent system. Of course, in the absence of any preserved vent chimneys, you cannot prove a linkage as intimate as the one seen in China, but the much greater age of these bacterial traces, and their association with possibly the oldest black smoker deposit left on the Earth’s surface, makes this discovery just as exciting. Only a few hundred more million years further back to go…