A trophic framework for animal origins

Metazoans emerged in a microbial world and play a unique role in the biosphere as the only complex multicellular eukaryotes capable of phagocytosis. While the bodyplan and feeding mode of the last common metazoan ancestor remain unresolved, the earliest multicellular stem-metazoans likely subsisted on picoplankton (planktonic microbes 0.2-2 μm in diameter) and dissolved organic matter (DOM), similarly to modern sponges. Once multicellular stem-metazoans emerged, they conceivably modulated both the local availability of picoplankton, which they preferentially removed from the water column for feeding, and detrital particles 2-100 μm in diameter, which they expelled and deposited into the benthos as waste products. By influencing the availability of these heterotrophic food sources, the earliest multicellular stem-metazoans would have acted as ecosystem engineers, helping create the ecological conditions under which other metazoans, namely detritivores and non-sponge suspension feeders incapable of subsisting on picoplankton and DOM, could emerge and diversify. This early style of metazoan feeding, specifically the phagocytosis of small eukaryotic prey, could have also encouraged the evolution of larger, even multicellular, eukaryotic forms less prone to metazoan consumption. Therefore, the first multicellular stem-metazoans, through their feeding, arguably helped bridge the strictly microbial food webs of the Proterozoic Eon (2.5-0.541 billion years ago) to the more macroscopic, metazoan-sustaining food webs of the Phanerozoic Eon (0.541-0 billion years ago).