 One of my students asked me a really interesting question about how food webs start. Before we had predators, when we go back in time, there were only bacteria, archaea, microscopic eukaryotes, and of course viruses. When all the organisms were very small, there weren't these elaborate food webs that we have today. Instead, the bacteria and other organisms are just exchanging organic matter. The image that we're looking at here is from Lake Joyce in Antarctica, where glaciation has excluded most of the large organisms. There are a few little protists, there's some rhodophoids and nematodes, but this is an example of a modern ecosystem where we just have this microbial loop of resources going among those organisms. In this video, I want to talk a little bit about how those nutrients are exchanged and how when animals and predators first evolved, we might have gotten the first food web the way we conceive of it today. If we think about the microbial food loop, we start with primary productivity. This is the process that organisms use to take inorganic carbon, like carbon dioxide, and turn it into organic matter. This produces the organics that cells are made up of. This is, of course, photosynthesis. Photosynthesis creates the most organics on Earth today. We also have bacteria that can perform chemosynthesis. These bacteria use chemical energy to convert inorganic carbon into organics. From this, we end up with basically cells that consist of particulate organic matter. We have cells here. These cells can burst open when they die. If, for example, they don't have enough energy to keep their cell walls covered, they can break open. If the cells break open, they release a lot of their components. Some of it might be particulate organic carbon, and some of it is dissolved organic carbon. We have particulate organic carbon and dissolved organic carbon. These are two pools of resources that other organisms can use. These cells are the primary productivity cells, but we can also have fermenting and respiring. These organisms take the dissolved organic carbon and sometimes particulate organic carbon and use those materials to make the cells themselves. Then, of course, we have viruses. They infect cells. They can infect the primary production cells. For example, the cyanobacteria, or the very common photosynthetic cells, and the viruses that infect those. They can also infect these fermenting and respiring cells. They can either just live within cells and reproduce within them, or they can cause the cells to break open or lies to, again, create more particulate organic matter and dissolved organic matter. One of the processes is the cell's natural death, which produces these resources, and then the other is caused by viruses. There are also some cases where some cells can ingest other cells. It's also the case where maybe you have a small cell and a larger cell, and the smaller cell gets engulfed into the larger one, and then becomes part of it. This basically gets pulled in, and then it starts to break down. There's some engulfment. I can add that there are also organic molecules that can connect between the cells. This is one way that smaller cells can get energy from the larger ones. They basically suck the components out of the larger cells. That's a parasite reaction. There are some exchanges of materials among living cells that end up killing them. This is basically the status of the food web before the first predators actually evolved. If we think about what happened when we got the first predators, we have all these cells at a larger scale. This represents our photosynthetic primary productivity cells here. And then sitting within them, there were plenty of fermenting and respiring cells, and particularly lower down in the mat where there's not enough light for the photosynthetic ones. We just have the respiration. Eventually, the first grazers evolved. I'll draw a larger one. They were probably smaller than this at first. They had a mouth part, once I'm drawing anywhere, and then a gut. These organisms could graze along the mat, eating them. One of the things that could have happened is that maybe there was a smaller version here, and the larger one actually managed to ingest the smaller one, and then maybe that starts being your first set of predation. If we draw a food web here, we still have our primary producers. We still have our fermenters and our heterotrophs. We still have our organic carbon dissolved in particulate that's going between them with the viruses. Then we start having our grazers, and the grazers can eat primary producers and the heterotrophs and fermenters, and once you have the grazers, then it becomes possible to have the first what I'll call omnivores, and these can eat the grazers. They probably first started off as grazers themselves with the primary productivity and the heterotrophs, and they started ingesting the grazers, and then from there these could maybe specialize to become predators. And the grazers could specialize to become omnivores. So if we go back to another view of Lake Vanda, in some places you might have had these elaborate microbial mats, and as soon as the grazers evolved, they would eat off all of these pinnacles formed by the primary producers, and then eventually those grazers would be consumed themselves. And so we have this really interesting building up of complex food webs from something that was really very simple that lasted for billions of years on Earth, and until what we call Ediacarantime, when we have what we know are the first grazers and also the first predators, and from there we get the shelled organisms in response to the evolutionary pressures for protection from predators, and the ecosystems keep building up to be more and more complicated to what we see today. So thanks for watching.