(A shorter version was first published on Blogcritics)
The early evolution of life on Earth is a subject I’ve always found fascinating, but it’s a couple of decades since I last revisited the subject in any depth, and having read Oxygen: A Four Billion Year History by Donald E Canfield I now know that pretty well everything I’ve ever read or been taught on the subject was wrong. The idea that gradually algae spread around the earth, pumping out oxygen in a steady-growing stream, well it simply isn’t true.
It’s not surprising my teachers were so wrong, for as I read in Oxygen, a big breakthrough in understanding early life on Earth came in only 1999, when a colleague and friend of the author, James Farquahar, found some highly unexpected results on a study of sulfur isotopes, in Archaen rocks aged from 2.3-2.4 billion years ago. That led to the conclusion that at this time there’d been interaction between UV light and sulfur dioxide gas from volcanoes. Today, that’s absorbed by ozone, of course from oxygen. Further studies on the form some molybdenum takes in rocks of this age from some parts of the world, however, show that in some places there was free oxygen – what’s come to be known as a “whiff” of oxygen.
What was happening was that by around 2.5 billion years ago, the production of oxygen by photosynthesis more or less balanced the consumption of it by volcanic gases. Sometimes the balance shifted one way, so the oxygen disappeared, sometimes the cyanobacteria were beating the volcanoes.
It was between 2.3 and 2.4 billion years ago that “the great oxidation event” (GOE) changed that. Quite what caused it is still up for grabs. Canfield has a favourite, not evolution of cyanobacteria but a less active mantle, as it gradually cooled, cutting the production of reducing gases. Seems entirely plausible to this interested amateur.
But the GOE wasn’t entirely even – it was Canfield suggests concentrated in the atmosphere, the oceans remaining anoxic and rich in sulfide, with more sulfur being weathered from the land through oxidative weathering of sulfides. This is now known as the “Canfield Ocean” – yes after the author, we’re in seriously expert hands here.
Related to that is the likelihood that for much of the Earth’s “middle ages” atmospheric oxygen levels were much lower than today’s. (Any time machine travellers would need to take oxygen cylinders.) The author’s theory is just 10% of today’s levels, others suggest 40-50%.
That’s until around 630 million years ago, when at least some deep ocean waters became oxygenated, evidence from iron and molybdenum indicate. That takes the story to Newfoundland, and fossils from the Edicaran era of some 575 million years ago, including rangeomorphs, which lived, attached to the ocean floor, too deep to be photosynthesising, without a gut or reproductive organs. These were probably thus animals, although animals with no living relatives of even the most distant kind, obtaining their oxygen by passive diffusion across their surface. One theory suggests that this evolution was driven by high rates of organic matter burial as the supercontinent Rodinia broke up. Still, however, we’re at only 15% of current-day oxygen levels.
But where this gets really fascinating is the still very speculative discussion of how animals might have actually influenced the oxygen levels, at least in different parts of the oceans. This is where the idea that the Earth even then was having its chemistry fundamentally shaped and reshaped by living things, just as we’ve more obviously created the Anthropocene today. For the oxygenation of those waters might reflect the development of zooplankton, which from the surface of the ocean produced fast-sinking fecal pellets that would decompose less in the upper layers of the ocean than earlier, slower-settling biomass, increasing their oxygenation. the timings don’t exactly add up, but indirect evidence for those zooplankton (not preserved in the fossil record) is that their potential foodstuffs, the acritarchs, believed to be the preserved casings of ancient algae, show an increase in “ornamentation” – which might well be defence mechanisms.
Plants have a role to play in the next understood period of fluctuation, the Permian and Carboniferous, when levels rose to up to around 35% (today about 21%). Plants, growing big and tall with lignin and cellulose, which are slow to decay, pumped out oxygen, and often when trapped in anaerobic environments like swamps, didn’t break down. (This of course is the source of much of the fossil fuels we’ve been burning up at a great rate – finally releasing that carbon.) Fascinatingly, some of the giant insects of these primordial forests would not be able to fly today, but the extra oxygen allowed rates of respiration that allowed the energy to keep them afloat. Similar gigantism is visible in the oceans.
That tale, told blow by blow, reflects the general approach of the book – it doesn’t just explain the conclusions, but how researchers have arrived at them. It probably isn’t for everyone; you’ll need either a bit of scientific background, or a preparedness to read and re-read, to follow the story, but if you’ve either of those this makes a gripping, fascinating story.
The difficulty of making sense of the early fossil record no doubt contributed to this still unclear picture, for as Canfield explains, really old rocks have invariably been subjected to massive forces of change, particularly heat. So the organic matter in some of the oldest rocks containing it, 3.8 billion year old samples from Isua, Greenland, have been completely converted into graphite as a result of heat. The balance of carbon isotopes in those indicates the existence of life, through what’s known as the Rubusico pathway (the name of an enzyme). And this was deposited on the floor of an ocean, as we’d expect from life in an upper ocean. But many organisms other than cyanobacteria use the pathway, so it isn’t proof of photosyntheising organisms.
What other organisms? There were oxygenic photosynthesising organisms, using bacteriochlorophyll rather than chlorophyll. It’s a more complex biosynthetic compound, but as Canfield says, it may simply have emerged first by chance, and due to lack of selection pressure, ruled for some time. There’s still a lot of questions in this area.
Still, it’s clear that we now now a lot more – at least what questions to keep asking – about the Earth’s early history. And in understanding that, hopefully we can come to understand more about how we, as the dominant animals, are causing such massive impacts on our world today. (Although as Canfield makes clear, oxygen levels aren’t anything we need to worry about in the next few million years – that’s as yet even beyond the immediate impact of human action.)