On the largest scale, the tale of life on Earth, is governed by just two things. One of them is a slow decline in the amount of carbon dioxide in the atmosphere. The other is the steady increase in the brightness of the Sun.
Why is carbon dioxide becoming so scarce? In a word—weathering. New rock thrust up to become mountains is swiftly eroded. Erosion sucks carbon dioxide out of the atmosphere. The rocks are ground to dust which is, eventually, buried on the seafloor.
In the earliest days of the Earth, the entire surface of the planet was covered with ocean. There was little or no land to erode. Over time, however, the proportion of land has increased, and, with it, the potential for weathering. Slowly, the amount of carbon dioxide removed from the atmosphere has increased, relative to the rate of its replenishment through, say, volcanic eruptions.
One of life’s first challenges occurred during the Great Oxidation Event, between 2.4 and 2.1 billion years ago. A spike in tectonic activity led to a sharp increase in the burial of carbon. The world, no longer benefiting from the greenhouse effect, was tipped into an ice age that lasted 300 million years—its first and greatest ‘Snowball Earth’ episode. The severity was exacerbated by the fact that the Sun produced less heat than it does nowadays.
Life responded by an increase in complexity. Individual bacteria, living in loose associations, pooled their resources, each individual concentrating on the one aspect of life it did best. The result was the eukaryotic cell.
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