The evolutionary boom that led to the emergence of most major animal groups and complex ecosystems during the Cambrian Explosion, approximately 540 million years ago, may have been significantly driven by the widespread evolution of animal feces, according to a new study.
While rising oxygen levels and predator-prey dynamics have been proposed as catalysts for this period of rapid diversification, the international research team suggests that the importance of ancient ecosystems' waste products has been largely overlooked. The study, published in the journal Trends in Ecology and Evolution, analyzed fossilized feces, known as coprolites, from 35 global sites.
The researchers observed a distinct transformation in the size, shape, and composition of feces at the onset of the Cambrian period. These ancient droppings ranged from microscopic pellets to centimeter-scale specimens containing fragments of shells and other organic material. This diversity in fecal matter coincided with the development of more complex digestive systems, particularly among early arthropods.
These organisms evolved specialized foreguts and digestive glands capable of processing a wider variety of food. As their diets became more varied and their digestive capabilities increased, they produced more nutrient-dense waste. This enriched waste is believed to have funneled more organic matter and nutrients into ancient oceans, fostering the growth of more complex life forms and facilitating the development of intricate food webs.
"As we move forwards in time, more diverse coprolites appear," the study's authors wrote. "Evidence from dozens of deposits across the globe shows an array of different shapes and sizes."
Study author Russell Bicknell, an evolutionary ecologist at Flinders University, noted that "the importance of feces in ancient ecosystems is often overlooked" alongside other factors like rising oxygen levels. He concluded that this "fecal feedback loop" likely played a crucial role in fueling the exponential ecosystem growth observed during the late Ediacaran and early Cambrian periods.
The study also draws parallels to modern ecosystems, noting that the deposition of waste products by surface-dwelling organisms continues to nourish life on the ocean floor. Similarly, animal dung remains a vital component of fertilizer for modern agriculture, demonstrating the enduring significance of waste in supporting life.