Did poop enable the evolution of complex animals?

“The advent of fecal pellets,” wrote Kimming and Bicknell, “provided an additional source of organic carbon, as well as iron, nitrogen, and phosphorus, to deeper waters. … In this framework, fecal pellets significantly boosted support for marine animal life and possibly supported the diversification and increased Cambrian biomass.”

How defecation changed the world

The first animals on Earth were mostly simple creatures that lived in the sunlit upper layers of the ocean or on the shallowest parts of the seafloor. Depending on which set of scientists you ask, those early animals were similar to either sponges or jellyfish. Organs hadn’t been invented yet, so “eating” just meant waiting for tiny particles of food, like bits of plankton or bacteria, to float into the only opening in the animal’s body. Its cells would then engulf the food particle and break it down into its constituent molecules.

What these early animals released back into the ocean were mostly just chemical byproducts of their cells’ metabolism, not nutrients most other animals could use. So the deeper reaches of the ocean were largely unoccupied, because they didn’t offer enough nutrients to keep animals alive. Digestive tracts and poop changed everything.

By around 580 million years ago, some animals had started to evolve simple organs. Suddenly, these animals could swallow macroscopic chunks of food and digest them in their very simple—but still revolutionary!—guts, outside their cells, which probably really scandalized the sponges. That meant animals could eat more at a time, to support more complicated bodies, but it also meant they needed to dump the parts they couldn’t digest or absorb. Hence the advent of poop.

So far, paleontologists have unearthed Cambrian coprolites from about 35 sites around the world, spanning a period from around 540 to 494 million years ago.

“They range from microscopic to several centimeters in size and include forms such as elongated, cylindrical, ellipsoid, circular, and ‘exploded’ fecal ‘carpets,’” wrote Kimming and Bicknell, rather evocatively. Some of the larger coprolites contain bits of shell or exoskeleton, revealing a bit about the diets of whatever produced them. The sheer diversity of the size and shape of all that fossilized feces suggests that the animals that produced it must also have been diverse (although scientists haven’t yet managed to match very many Cambrian fossil animals to the corresponding coprolites).