Growth Could Slow

Human history has seen accelerating growth, via a sequence of faster growth modes. First humans grew faster than other primates, then farmers grew faster than foragers, and recently industry has grown faster than farming. Most likely, another even faster growth mode lies ahead. But it is worth remembering that this need not happen. For a very concrete historical analogue, the Cambrian Explosion of multi-cellular life seems to have resulted from an accelerating series of key transitions. But then around 520 million years ago, after life had explored most multi-cellular variations, change slowed way down:

In just a few tens of millions of years – a geological instant – almost every major animal group we know made its first appearance in the fossil record, and the ecology of the planet was transformed forever. …

Scientists have struggled to explain what sparked this sudden burst of innovation. Until recently, most efforts tried to find a single trigger, but over the past year or two, a different explanation has begun to emerge. The Cambrian explosion appears to have been life’s equivalent of the perfect storm. Instead of one trigger, there was a whole array of them amplifying one another to generate a hotbed of animal evolution the likes of which the world has never seen before or since. …

The first sign of multicellular animals is in rocks about 750 million years old, which contain fossilised biomolecules found today only in sponges. Then another 150 million apparently uneventful years passed before the appearance of the Ediacaran fauna. This enigmatic group of multicellular organisms of uncertain affinities to other lifeforms flourished in the oceans up to the beginning of the Cambrian. Then [110 million years later] all hell broke loose. … Studies of “molecular clocks” – which use the gradual accumulation of genetic changes to estimate when particular evolutionary branches diverged – suggest that animal complexity emerged before the Cambrian. …

Two huge ecological innovations that make their debut in the Cambrian fossil record. …The first is the ability to burrow into the sea floor. … The second innovation was predation. … What else were these early creatures waiting for? One intriguing possibility is that they were waiting for fertiliser. Geological evidence suggests that rising sea levels during the Cambrian could have increased erosion, boosting levels of nutrients such as calcium, phosphate and potassium in the oceans. …

Atmospheric oxygen levels crept up gradually. … The crucial threshold seemed to be between 1 and 5 per cent of present oxygen levels. Geochemists’ best guess at when the ancient oceans reached this point is about 550 million years ago – just in time to kick off predation and its resulting ecological feedback. …

Precambrian oceans were full of single-celled algae and bacteria. When these small cells died, they would have started to sink, decomposing quickly as they went – and because decomposition consumes oxygen, this would have kept ocean waters anoxic. Filter-feeding sponges, which evolved sometime before the Ediacaran,then started clearing these cells out of the water column before they died and decomposed. The sponges themselves, being larger, were more likely to be buried in the sediment after death, allowing oxygen to remain in the water. Over time, this would have led ever more of the ocean to become oxygenated. (more)

So it remains possible that growth will slow down now, or after the next transition, even if a new series of accelerating transitions lies far ahead.

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