Taxonomy and the kingdoms of life

0:00- [Instructor] What do a starfish, a jellyfish, and a clownfish have in common? Well, they're all fishes, right? Surprisingly, no. Turns out only the clownfish is actually a fish, and the starfish and the jellyfishes aren't fish at all. In day-to-day life, we like to group organisms based on simple observations, like, what do they look like?

0:21Or where do they live, for example. So over here, the starfish, the jellyfish, and the clownfish, they all live in water, so it's easy to think of them as fish. Now, that might be good enough for everyday conversations, but in science, we need a more reliable way to group living things.

0:42So instead of just appearances or habitats, scientists like to group organisms by comparing their traits. This system of grouping is called taxonomy. So think of taxonomy as a system that scientists use to group and name organisms by using their shared and unique traits.

1:05But wait, there are millions of different organisms on Earth. How in the world do we organize them all? Well, some common traits we use to organize are the cell type, the number of cells, or how organisms consume food and gain energy. This helps us create a hierarchy, or levels, and that helps us a lot.

1:31It'll make a lot of sense as we dig into it, okay? So let's start. The broadest level is called a domain. There are three domains, bacteria, archaea, and eukarya. Every living thing on Earth belongs to one of these three. And guess what? The domain classification is based on the cell type.

1:53So the bacteria and archaea cells, they don't have a nucleus. In other words, they are prokaryotes. At first, scientists grouped them together because they both look similar. They both lack a nucleus. But later, they discovered that the cells of archaea were fundamentally different from that of bacteria, because of which many of these could survive in extreme environments where bacteria couldn't.

2:18So they were given their own domain. And eukarya include organisms whose cells do have a nucleus. So these are eukaryotes. Every organism having a nucleus, which includes plants and animals, you and me, we all belong to eukarya. Okay. Now, we can go one level down in the hierarchy, this level is called kingdom.

2:44The kingdom classification is mostly on the cell number and how organisms gain energy. Now, bacteria and archaea are mostly unicellular, so there isn't much to classify, and therefore they each get just one kingdom, eubacteria and archaea. But the domain eukarya is very diverse.

3:04And so, it is divided into four kingdoms. We have plantae, which are basically multicellular organisms that can create their own food through photosynthesis, so they're autotrophs. Then we have animalia. Again, multicellular organisms, but they cannot produce their own food, so they get food by eating other organisms, or they are heterotrophs.

3:28We also have fungi like mushrooms, which we initially thought were just plants. But unlike plants, fungi don't make their own food. So they are heterotrophs. But unlike animals, they don't ingest their food. Instead, they absorb nutrients directly from their surroundings.

3:47And many fungi also play an important role as decomposers, breaking down dead material and recycling nutrients. Finally, we have protista. What are these? Well, some eukaryotes don't quite fit into plants or fungi or animal kingdoms, right? For example, look at seaweeds.

4:05They may look like plants, but unlike plants, they don't have leaves or stems or roots, so they cannot be classified as plants, or fungi or animals for that matter. Think about amoeba, which get their energy by eating other organisms, kind of like animals, but unlike animals, they are unicellular.

4:24So, again, they're neither animals nor fungi or plants. So see, there are a lot of these organisms that don't really fit into any of these three. And so, scientists grouped all of those together under the kingdom protista. This ensures that every single organism on the planet is classified into six kingdoms, the six kingdoms of life.

4:51Each kingdom plays an important role in keeping our ecosystem running. For example, plants produce food for most organisms on land. And a lot of protists do something similar in the oceans, they form the foundation of many aquatic food webs, and they both release oxygen, essential for almost all living things.

5:16Animals help keep ecosystems balanced by pollinating plants, dispersing seeds, controlling populations, and building habitats. And when organisms die, it's the fungi and the bacteria that break them down, returning those nutrients back into the ecosystem. And many bacteria also help digest food inside other organisms.

5:38In fact, your own gut is full of them right now. And archaea recycle important elements, like carbon and nitrogen, often extreme environments where almost nothing else can survive. But as far as classification is concerned, kingdom is still a very broad group.

5:54I mean, think about it. Animalia, for example, includes everything from tiny insects to elephants. So we keep dividing organisms into smaller and smaller groups. Each kingdom is divided into many phyla. Phyla is plural. Phylum is singular. Each phylum is divided into many classes.

6:16Each class is divided into many orders. Each order is divided into many families. Each family into many genera. Again, genera is plural. Genus is singular. And finally, each genus into many species. As we move down this hierarchy, the groups become smaller and the organisms within them have more and more in common.

6:39And the species is the most specific classification group in taxonomy, which means organisms in the same species are very closely related and share the most characteristics. As a result, a species refers to one particular type of organisms such as, say, modern humans, homo sapiens.

6:59So now, let's go back to our original question. What do these three have in common? Well, all their cells have a nucleus. So all of them belong to the domain eukarya. But that's not all. They're all multicellular organisms and they get their energy by eating other organisms, which means they all belong to the kingdom animalia.

7:18But after that, their paths begin to separate. Clownfishes, for example, have a backbone, so they're grouped under a phylum called chordata. We don't need to memorize this name right now. What matters is that since clownfishes have a backbone, they are grouped in this phylum.

7:35But a starfish doesn't have a backbone. In fact, it has spiny skin and it has a unique five-part body plan and it has suckers, and so on. Organisms with these traits are grouped together into a different phylum. And again, we don't need to memorize this phylum name right now either.

7:52The key idea is that starfishes have a different set of traits, so they are classified in a different phylum. And what about jellyfishes? Look, they don't have backbones either, and they don't have spiny skins as well. Instead, they have specialized stinging cells that are used to catch prey.

8:09Organisms with these traits are grouped together into yet another phylum. This is how scientists classify organisms. As they compare more and more traits, they keep dividing organisms into smaller and smaller groups until each one ends up with its own unique species.

8:28That's the real purpose of taxonomy. It tells us exactly how organisms are related and exactly where those relationships begin to differ.