It appears that even in the Paleozoic, feces played a fertilizer role that enriched marine ecosystems, as they do now. The claim is that excretions from primitive marine animals spread an unprecedented amount of organic matter through the oceans, causing an explosive increase in animal diversity. This is similar to how the excrement of animals such as whales and penguins today contributes to nutrient cycling in marine ecosystems.
Russell Bicknell, a professor at Flinders University in Australia, and Julien Kimmig, a curator at the State Museum of Natural History Karlsruhe in Germany, said in a paper published on the 4th (local time) in the journal Trends in Ecology & Evolution that "evidence from sedimentary layers around the world shows that during the Cambrian period, both the size and complexity of digestive tracts and fecal fossils increased."
Starting about 540 million years ago in the Cambrian period of the Paleozoic, countless animals appeared on Earth at an unprecedented pace. This is the Cambrian explosion. Trilobites are a representative example. Over 20 million years, most major animal groups on Earth emerged. Animal body plans also became more complex. Eyes and claws and teeth appeared, and antennae, fins, and tails also emerged at this time.
◇Feces grew larger and digestive systems became more complex
So far, scientists have not yet determined exactly why animals surged during the Cambrian. Some have said it was thanks to a sharp increase in marine oxygen concentrations, while others have argued that climate stabilization or competition over resources accelerated evolution. The researchers inferred that the marine ecosystem at the time became rich in organic matter, triggering the explosion of life.
They considered the source of the organic matter to be the excrement of primitive marine animals. They noted that deep-sea creatures today rely on fecal matter falling from the sea surface. To test the hypothesis, the team examined coprolites found in strata from that time. Coprolites are fossils of animal excrement that has hardened into rock, called coprolite in English. Examining coprolites can also reveal the development of digestive systems.
The study found that in more than 35 sedimentary layers worldwide, both the size and complexity of digestive systems and coprolites increased. Bicknell said, "Coprolites range from grains smaller than a millimeter to centimeter-scale pieces that contain shells or fragments of other animals," adding, "Along with this, the digestive systems of early arthropods became more complex, and digestive glands and guts evolved to process a variety of foods."
Animals first appeared in the Ediacaran period 600 million years ago. After that, fossils of complex digestive systems and excrement began to appear in the early Cambrian. As digestive systems developed, animals ate a wider variety of foods, and the organic content in their feces also increased. Bicknell said, "The fossil record makes it clear that the evolution of feeding strategies is closely linked to the production and distribution of organic matter and nutrients, creating an environment akin to today's use of fertilizers in food production."
Of course, this is also a theory. Some scientists criticize that it is difficult to determine whether the increase in feces came first or the rise in animal diversity did. But given the impact of animal excrement on today's marine ecosystems, some say it can be seen as a sufficiently plausible scenario.
◇Whale and penguin feces that revive the sea
The iron contained in animal excrement serves as a fertilizer that enriches today's marine ecosystems. A joint team from the University of Vermont and Harvard University reported in 2010 in PLoS ONE that whales act as pumps that draw nutrients from the deep ocean to the surface.
Whales feed on krill, crustaceans at the bottom of the food chain, on the seafloor and then rise to the surface to excrete. At this time, the iron in the krill's bodies spreads across the surface through whale excrement. Phytoplankton absorb the iron and increase rapidly. Krill and small fish feed on the plankton and grow, and they in turn become food for whales and larger fish.
The fact that whale feces sustain marine ecosystems was demonstrated inversely through the decline in whale populations. From 1910 to 1970, 1.5 million baleen whales in the Southern Ocean fell victim to whaling ships. People thought krill, their prey, would increase if whales disappeared, but the result was the exact opposite. Krill populations have also plummeted by more than 80% since the mid-20th century. As whale feces disappeared, the ocean's productivity declined.
The Southern Ocean has faced a crisis due to a decrease in penguin feces. In 2023, the Spanish Institute of Marine Sciences of Andalusia said that the disappearance of penguin feces had pushed the Antarctic ecosystem into crisis. According to a paper in Nature Communications, as chinstrap penguins declined to half their 1980 levels, the amount of iron they delivered to the Southern Ocean fell by the same amount.
A drop in whale and penguin feces also reduces the ocean's absorption of greenhouse gases. Phytoplankton absorb carbon dioxide through photosynthesis and, after death, sink to the seafloor where it is sequestered. The world's oceans absorb 30% of the carbon emitted by humans each year in this way. If whale or penguin feces decrease, plankton cannot grow properly, and carbon uptake can also fall. In the past and now, feces are precious to the sea.
References
Trends in Ecology & Evolution (2026), DOI: https://doi.org/10.1016/j.tree.2026.06.013
Nature Communications (2023), DOI: https://doi.org/10.1038/s41467-023-37132-5
PLoS ONE (2010), DOI: https://doi.org/10.1371/journal.pone.0013255