Read more: Cosmic accidents: 10 lucky breaks for humanity
Early life had to ride an oxygen rollercoaster – until humble symbioses of algae and fungus put an end to boom and bust
The world of the first complex cells was very different to today’s. The early efforts of photosynthesising bacteria had raised atmospheric oxygen levels to 2 per cent, barely one-tenth of current levels. Suffocating air and stagnant seas were the leitmotifs of the next period of our planet’s history, an environmental stasis dubbed the “boring billion”. An extraordinary coincidence of geology and biology shook Earth from its slumber.
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If eukaryotes were ever going to get where they are today, they needed oxygen‘s chemical muscle. It powers the process of aerobic respiration that keeps not just us going, but almost all animal and plant life.
Around 800 million years ago, things began to look up for oxygen. Rifting and volcanism associated with the break-up of the supercontinent Rodinia increased weathering and flooded the oceans with nutrients, causing photosynthesising cyanobacteria blooms to undergo something of a boom.
Happy ending, with abundant oxygen for all? Not in the first instance: increased photosynthesis sucked up carbon dioxide, and as the blooms died and sank to the ocean floor, this important greenhouse gas leached out of the atmosphere. By 720 million years ago, the planet had been plunged into a glaciation that reached to the equator – a “snowball Earth“.
As so often before, seeming catastrophe was a chance for life. “When CO2 levels decrease, it drives biological innovation,” says geochemist of University College London. In this instance, it bounced new types of complex cell into leaving the marine ghetto and colonising the land.
Faced with a largely icebound planet, these pioneering terrestrial life forms – green algae and lichen – initially did little. But CO2 levels gradually recovered and, some 635 million years ago, the glaciers receded to the poles, revealing land that was primed to go green as never before.
Lichen, symbioses of algae and fungus, have roots known as hyphae that anchor them to the rocks beneath. These created new avenues for physical and chemical rock weathering, and the oceans were flooded with nutrients once more.
This time, though, the result wasn’t just bloom and bust. The lichen kept eating away at the rocks, and a constant stream of nutrients kept photosynthesising bacteria in the seas on the perpetual up. Slowly, atmospheric oxygen levels rose to the heights of today.
Soon afterwards, about 580 million years ago, the first of our animal ancestors nosed their way into the fossil record, followed by the leafy plants – lucky beneficiaries of some unlikely biological heroes.