by Piter Kehoma Boll
Thanks to the appearance of the first photosynthetic bacteria, the so-called Cyanobacteria or blue-green algae, our atmosphere started to accumulate oxygen. As you may know, photosynthesis is a process by which plants and other photosynthetic organisms convert water and carbon dioxide into oxygen and organic compounds.
Oxygen is a very reactive element, so it can easily interact with other compounds and is great to burn organic matter to release energy. Without oxygen, heterotrophic life, such as animals, would not be able to use large quantities of energy and therefore would have never been able to achieve large size.
As you may also know, animals most likely appeared in the oceans and only much later conquered the land. However, oxygen produced by photosynthesis accumulates mainly in the atmosphere and not in the oceans. Today, only 1% of the global oxygen is found in the oceans, and it was even worse during the first million years of multicellular life. Do you know why?
The most primitive animals alive today are sponges, which are quite different from other animals. They usually have a hollow body with several pores, which function as tiny mouths through which water carrying small planktonic organisms and other organic matter is pulled inwards and later released by a large opening on the top of the body. So the main thing sponges do is mixing water and extracting a small amount of organic matter from the water column. Their feces, when returning to the water, are not very different in size from the organic matter they initially ingested.
Thus, in a sponge-only world, the water column was possibly always crowded with dissolved organic matter. This was a feast for bacteria, which are always eager to decompose organic matter and, while doing so, they consume large amounts of oxygen. Therefore, water with high amounts of organic matter increases bacterial activity and turns the environment anoxic, i.e., without oxygen. As a result, there was no oxygen available to allow animals to become large.
Despite not growing very much, animals were still evolving, of course, and eventually the bilaterian animals appeared. Bilaterian animals have a bilateral symmetry and, the most important feature in this story, a gut. It means they ingest food, digest it, process it and later eliminate the rests as… poop! In the gut, feces become compact as fecal pellets and sink much quickly to the bottom of the ocean, cleaning the water column from organic matter and drastically reducing bacterial activity. With no bacteria decomposing in the water column, the oxygen levels rapidly started to increase, allowing animals to grow and things like fish to evolve.
If animals had never started to poop, we most likely would have never been able to arise in this world. Long live the poop!
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Holland, H. (2006). The oxygenation of the atmosphere and oceans. Philosophical Transactions of the Royal Society B: Biological Sciences, 361 (1470), 903-915 DOI: 10.1098/rstb.2006.1838
Turner, J. T. (2002). Zooplankton fecal pellets, marine snow and
sinking phytoplankton blooms. Aquatic Microbial Ecology, 27, 57-102