 People have likely been trying to name and classify things around them for a very long time. Like back in the ancient times, some people tried to classify plans based on if they're poisonous or edible. Then the Greek philosopher Aristotle tried to classify living beings based on whether they had blood or if they could lay eggs or if they had four legs, stuff like that. But it was a Swedish botanist named Carl Linneus who made a breakthrough. In this video, we're going to take a walk down history lane and see how biological classification has developed over the last few centuries. Carl Linneus came up with the two-kingdom classification system. It classified all living beings into two kingdoms, plantae and animalia. Plantae had all the plants and animalia had all the animals. And he mainly classified them on the basis of locomotion and cell walls. A.k.a. the organism's ability to move from one place to another and the presence or absence of cell walls. Plants stayed fixed in a place and had cell walls so they went in one group. And animals could move around and they didn't have any cell walls so they went in another group. However, things weren't exactly right with this classification system. For one, there's no way anyone could tell which one of these organisms were prokaryotes and which one of them were eukaryotes. Not only that, a bunch of these organisms were tiny and single-celled like the amoebas and the bacteria. And they were placed with large and multi-celled animals and plants. Now let's zoom in here a little bit right over here. Yes. So amoebas were put in animals because they didn't have any cell walls. And bacteria were put in plants because they did have cell walls. It was all very chaotic to be very honest. Yet another feature that didn't make any sense was the mode of nutrition. Fungi are exclusively heterotrophic organisms. In the two-kingdom system, they were placed with plants. Plants which are strict autotrophs. So it doesn't make any sense to place fungi and plants together in the same group. And after all of this, there were still some organisms that couldn't be placed in either of these kingdoms. One of these is euglena, an organism that behaves both like a plant as well as an animal. It can photosynthesize like a plant and it can move around like an animal. Now where would you place this fellow? So that's why sometime in the 1960s, American ecologist R.H. Whitaker came up with a system that addressed most of these problems. His classification system differentiated between prokaryotes and eukaryotes and also between unicellular and multicellular organisms. It further took nutrition into consideration and rightfully gave fungi a group of their own. Even euglena found its home in a new kingdom. Whitaker also considered features which weren't taken into consideration before like cell wall composition. Now we'll understand why cell wall composition became a criterion for classification in just about a minute. Keeping everything in mind, Whitaker came up with five kingdoms. Going from the simplest to the most complex, the very first kingdom is monera. It includes the simplest organisms to ever exist, the single-celled or unicellular prokaryotes or in other words all kinds of bacteria. These organisms don't have a nucleus or organilies and while some can survive on autotrophic nutrition, others rely on heterotrophic nutrition. Most monerans also have cell walls but this is where the composition of cell walls comes into the picture as a criterion for classification. You see, unlike plants, bacterial cell walls they are non-cellululose, that means their cell walls are not made up of cellulose. Instead, they are made up of these polysaccharide chains called peptidoglycans. The second kingdom is called protista, a very diverse kingdom which houses all the single-celled eukaryotes as well as some simple multicellular eukaryotes. This is the kingdom where amoebas and euglenas finally found a place for themselves. Now the mode of nutrition varies from one protist to another. Some are autotrophic, some are heterotrophic and some are mixotrophic in nature. A mixotroph is an organism that can act both as an autotroph as well as a heterotroph. And this euglena by the way right over here, a euglena is a mixotroph. The third kingdom is where the mushrooms are, kingdom fungi. Now both fungi and plants are multicellular eukaryotes but a major difference between them lies in their cell walls. A fungal cell wall is made up of a compound called chitin. Meanwhile a plant cell wall is made up of cellulose. So you see how important the composition of cell wall is over here for this classification. Fungi are also exclusive heterotrophs and most of them get their nutrients from dead and decaying matter. The fourth kingdom is plantae which is the kingdom of plants. All plants are exclusively photosynthetic multicellular eukaryotes. As we have already mentioned once before, they also have cell walls and these cell walls are made up of cellulose. And since they're all photosynthetic, all plants are strict autotrophs. The fifth kingdom is animalia, the kingdom of animals. All animals are exclusively heterotrophic multicellular eukaryotes. They're the most complex organisms to ever exist on our planet. And as you've probably guessed by now, animals don't have any cell walls. The fifth kingdom classification was a very very well-made system back in the day. But then the 1970s rolled in and that decade came with some mind-boggling revelations. Earlier we used to think that prokaryotes and eukaryotes are completely different from each other. But then multiple studies were carried out in the 1970s and they revealed that there are some special bacteria that have genes and enzymes which are more closely related to that of the eukaryotes. Now mind you, these special bacteria are still prokaryotes. Like they have no nucleus or any membrane bound organelles. But yes, these bacteria, these special ones, they have certain things which you won't find in the other bacteria or the other prokaryotes. That makes these very special bacteria stand apart from both the eukaryotes as well as the other prokaryotes that are there. Now this whole fact, whatever we just found out, it was pointed out by this American microbiologist named Carl Woos. And he significantly changed the entire concept of Monera. He developed a new system where he introduced something called domains, which was a rank that was higher than kingdoms. He then divided all living beings into three main domains, bacteria, archaea and eukarya. Now eukarya had all of the eukaryotes present in it. Like every living organism with the nucleus in its cell went in eukarya. Meanwhile, bacteria and archaea were the two domains that had all the prokaryotes in them. So you see, not all Monerans are the same and that's the thing about classification. It's not set in stone. As we continue to find more and more organisms or more about the organisms around us, we will reevaluate our older decisions and try to accommodate all of them into a single system. Chances are that whatever we have learned today might work for now or for the next few years and then suddenly we'll find something new and reorganize all that we have known so far.