As all children learn, everyone poos . But when did this begin?
Authors
- Russell Dean Christopher Bicknell
Post-doctoral researcher in Palaeobiology, Flinders University
- Julien Kimmig
Head of Palaeontology Division at the Natural History Museum Karlsruhe
And what evolutionary and environmental changes accompanied the dawn of dung? Going further, is the evolution of excrement the reason we exist?
Our new research suggests the evolution of fecal matter helped fuel one of the most important events in the history of life on Earth: the Cambrian explosion.
And poo really might be the reason animals were such a huge evolutionary success.
A hidden clue to the Cambrian explosion
Around 540 million years ago, the Cambrian explosion transformed life on Earth. During a relatively brief interval in geological time – somewhere between 13 million and 25 million years – most of the major animal groups appeared.
Around the same time, the first marine ecosystems developed and became more complex. Marine food webs began to look like those of modern oceans.
Scientists have long debated what drove this diversification. Rising oxygen levels, evolutionary innovations and increasingly complex predator-prey interactions have all been proposed as factors.
However, much research on ancient and modern ecosystems steps around an important substance: feces. Scientists do look at fossilised feces, known as coprolites, but usually only for evidence of ancient diets and to figure out which creatures ate each other.
Our study considered these fossils as part of a much bigger picture. We argue they also record the emergence of a fundamental ecological process.
As animals became more abundant and evolved more sophisticated digestive systems, they were able to produce more fecal matter. More feces meant more organic material and nutrients were distributed through the oceans. This in turn sped up the development of ecosystems.
From simple guts to complex ecosystems
The earliest animals appeared around 60 million years before the Cambrian explosion began. However, until the Cambrian there is little or no evidence of complex digestive systems, nor any confirmed coprolites.
The fossil record of guts and coprolites shifts dramatically during the Cambrian. The first fossilised feces appear right at the onset of the Cambrian and become increasingly abundant.
As we move forwards in time, more diverse coprolites appear. Evidence from dozens of deposits across the globe shows an array of different shapes and sizes. The fecal matter ranges from microscopic pellets through to centimetre-scale coprolites containing crushed shells and other animal fragments.
Associated with this shift, there was an increase in very well-preserved fossils with complex digestive systems, particularly among early arthropods. These animals had evolved specialised foreguts and digestive glands that could process a wider variety of food.
Together, these fossils show animals were beginning to process and then redistribute organic matter using entirely novel pipelines.
Nutrient cycling on an evolutionary scale
The ecological significance of this innovation becomes clearer when viewed alongside modern oceans.
Today, fecal pellets are an important component of the biological pump – the process that transports organic carbon and nutrients from surface waters into deeper marine environments. As fecal pellets sink, they carry nutrition to support life well beyond the sunlit surface ocean.
Our research suggests the widespread appearance of fecal matter during the Cambrian may represent the evolutionary origin of this process.
As animal diversity and biomass increased during the Cambrian, so too would have the production of fecal pellets. This would have meant more sources of energy and nutrients sinking to deeper marine habitats, and making food more widely available in shallower waters.
More complex marine communities
The emergence of widespread fecal production had an important ecological feedback. Greater animal diversity resulted in greater production of fecal matter, increasing the redistribution of organic carbon and nutrients through marine ecosystems.
In turn, improved nutrient availability likely supported larger populations, more complex food webs and the colonisation of increasingly diverse marine habitats. Those expanding ecosystems would then have generated even greater biological productivity.
Our observation here complements other hypotheses explaining the Cambrian explosion. Oxygenation, ecological interactions and evolutionary innovations undoubtedly played critical roles as well.
