So the paper says that filter-feeding predators might have been the reason, if you are bigger than a certain size you become irrelevant to them. But how does the filter predator become so big without multicellularity? I mean: filter predators always have to be bigger than their food, no? So isn't it rather coevolution?
Cells can become quite large, the nerve cells in your back run the length of your spine for example, so it's certainly possible for a single cell to trawl through the ocean like a filter feeder and pick on smaller targets. Maybe tendril feeder would be a better description for a single cell though.
There's also caulerpa (https://en.wikipedia.org/wiki/Caulerpa) can be large but only have a single cell, googling turns up several other surprisingly large single cells.
Cells can become large, but can't a single big cell also escape a filter predator that's also a single big cell? There needs to be some reason for why in most cases, both are multicellular.
From a mechanics of materials perspective, having a larger size also means being easier to break. Multicellularity seems to be a way to have preferred breaking points that are cheaper and easier for the organism to repair.
> but can't a single big cell also escape a filter predator that's also a single big cell
Maybe if it's big enough, but there's a huge cost to being big and the cot is harder to bear for more herbivorous cells. For most organisms the better survival strategy is probably to be more numerous and accept a certain level of attrition, hence ants outnumbering humans.
