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No Early Earth Life?
We know a lot less about early Earth life than I’d thought:
Two decades ago [Schopf] … was investigating … 3.5-billion-year-old rocks that are among the oldest on Earth. In 1993, he announced that they contained 11 different types of “microfossil” that looked for all the world like modern photosynthesising cyanobacteria. … Other 3.5-billion-year-old Australian rocks contained rippling structures that looked like fossil stromatolites. … Astonishingly soon after our planet formed some 4.6 billion years ago, photosynthesising bacteria were widespread. This emerging consensus lasted only until 2002, when palaeontologist Martin Brasier of the University of Oxford unleashed a barrage of criticisms. …
New lines of evidence mean that the oldest undisputed signs of cyanobacteria are now fossils found in rocks from the Belcher Islands in northern Canada dating from just 2.1 billion years ago. … In this “great oxygenation event” of around 2.4 billion years ago, levels rose from around 1 per cent of today’s levels to perhaps 10 per cent. Our best guess is still that cyanobacteria were around some time before this event. Persuasive evidence is converging on a date around 2.7 billion years ago. … The modern nitrogen cycle kicked off around this time. … The great oxygenation event … seems to have been the forerunner to a “snowball Earth”…. And yet the great oxygenation was impermanent. … By 1.9 billion years ago, levels of breathable oxygen in Earth’s atmosphere were back down to the merest trace. We don’t know why. …
Oxygen levels in the atmosphere soon recovered again … [at] a new equilibrium, at about 10 per cent of present-day levels. …. This time, the oxidative weathering of sulphides on land filled the oceans with sulphate. That in turn fuelled a hardy group of bacteria that … [turned oceans] into stinking, stagnant waters almost entirely devoid of oxygen. … An extraordinary stasis in Earth’s environment … dubbed the “boring billion” [years followed]. … What happened to the oxygenic utopia in which life supposedly grew and prospered, evolving the complex cells that went on to make up animal and plant life? … It probably never existed. … [Those] stinking oceans were the true cradle of life. Evidence behind this idea includes the fact that mitochondria, the powerhouses of all complex, oxygen-respiring “eukaryotic” cells today, were once far more varied, sometimes “breathing” sulphur or nitrogen instead of oxygen. ….
Earth’s anoxic stasis was broken in the end by a dramatic series of snowball Earths, indicating bursts of oxygen, beginning about 750 million years ago and recurring over the following 100 million years. They broke the eternal loop: soon afterwards, oxygen levels shot up and never looked back. Animal life soon exploded onto the scene. What made the difference this time? One intriguing possibility is that it was down to the organisms that had evolved in a leisurely way during the boring billion: terrestrial red and green algae and the first lichens.