While the organisms of the Plantae lineage have chloroplasts as a result of what we'll call primary endosymbiosis, meaning they engulfed a cyanobacterium, which then turned into the chloroplast. There are lineages of protists that actually have these quadruple membraned chloroplasts. Right? So remember that bacteria have that double membrane structure to them. Right? So, like mitochondria, chloroplasts have that double membrane. But in certain protists that have chloroplasts, they have this quadruple membrane chloroplast. People take this as evidence of what's called secondary endosymbiosis. So basically, this eukaryotic cell would engulf a photosynthetic eukaryotic cell, as we see happening here. It's going to engulf it, and then this photosynthetic eukaryotic cell will eventually become this quadruple-membraned chloroplast structure, and this will account for the origin of chloroplasts in other eukaryotic lineages outside the Plantae lineage. So within Plantae, it is what we see going on right here. Outside of Plantae, it is going to be this situation here. Right? They engulfed an organism that already had undergone endosymbiosis, and then there was another endosymbiosis, which we call secondary endosymbiosis.

Now, let's actually get to a little bit about protists. What defines them? Well, not a lot, unfortunately. Protists, again, they're a grouping of convenience. Right? They're basically everything that does not belong to fungi, Animalia, and Plantae. Right? Fungus, animals, plants. Every eukaryote that is not in one of those three groups is a protist. That means that protists are a paraphyletic group. Right? They are not monophyletic, unlike fungi, animals, and plants, which are monophyletic. Protists are a paraphyletic group. So, as a group, they do have some shared features, but many protists will display unique characteristics. In a later video, we will actually go through and look at some of those characteristics. So most protists are unicellular, but some are colonial, meaning they'll live in colonies, and others actually are multicellular. And these are, like brown algae, red algae, and slime molds. Those are some examples of multicellular protists.

It is worth noting that multicellularity actually arose independently in many eukaryotic lineages. There wasn't some original multicellular organism from which all multicellular organisms came, instead, multicellularity has cropped up various times in various lineages. Right? Right? So you might remember, when we talked about evolution, we mentioned that the eye evolved independently over 16 different times. So, just like that, multicellularity arose independently many times in eukaryotic lineages. Now, most protists undergo asexual reproduction, which will include mitosis, unlike binary fission. Right? Remember, protists are eukaryotes, so they're going to have a nucleus, which means they're going to need to perform mitosis when the cells divide. However, they generally, sorry, while protists undergo asexual reproduction, some of them have evolved various forms of sexual reproduction. Remember, sexual reproduction is a major event in evolution because it allows for the introduction of tons of genetic variation, which is super important for evolutionary purposes.

Now, it's worth looking at this little mock-up of a phylogenetic tree, and I really want to emphasize "mock-up." This is not a very nice phylogenetic tree. I'm really just trying to show one little point here, and that is how many different lineages there are outside of plants, animals, and fungi. So here, Archaeplastida: this is plants, basically. That's not to say that there aren't protists in this group, but that's where land plants are. We have fungi over here, and it's I should note that this whole branch is coming off of this one right here. And so fungi right there, and then you can see down here we have animals. So look at all these other lineages that exist outside of plants, animals, and fungi. Right? These are all the protists. Big group there. Big group. And as you can see, lots of distantly related cousins, so to speak. That's what I meant by this is not a monophyletic group, but a paraphyletic group. There are all these phyla parallel to each other in this taxonomy.

And with that, let's flip the page.