Class 9 BiologyChapter 12NCERT 12.5
Biological classification systems over time
Every new tool for seeing life forced a new way of sorting it.
Think it over
- Aristotle put animals in groups by where they live. Where would a frog, a duck and a flying fish go?
- An amoeba moves like an animal but is a single cell. Is it a plant, an animal or neither?
- If scientists keep changing the system, does that mean the old ones were mistakes?
In one minute
- Aristotle (4th century BCE): animals by habitat (land, water, air) and by appearance. An artificial system, based only on easily seen features.
- Two kingdoms (Linnaeus, 1758): Plantae make their own food and stay put; Animalia move and depend on others.
- Three kingdoms (Haeckel, 1866): Protista for microscopic single cells such as Amoeba and Paramecium.
- Four kingdoms (Copeland, 1938): bacteria, which lack a true nucleus, go to Monera.
- Five kingdoms (Whittaker, 1969): Fungi separated, because they absorb food instead of making it.
From habitats to five kingdoms
Each classification system was built on what scientists could see at the time. Scroll down the line: each block row shows how many kingdoms that system had.
- 4th century BCE Aristotle
Animals sorted by where they live (land, water, air) and by how they look. An artificial system: it only used features you can see.
LandWaterAir - 1758 Carolus Linnaeus
Two kingdoms. Plantae stay put and make their own food; Animalia move and depend on others for food.
PlantaeAnimalia - 1866 Ernst Haeckel
Three kingdoms. Protista added for microscopic single-celled life that fits neither plants nor animals.
PlantaeAnimaliaProtista - 1938 Herbert F. Copeland
Four kingdoms. Bacteria, with no true nucleus, move out of Protista into a new kingdom, Monera.
PlantaeAnimaliaProtistaMonera - 1969 Robert H. Whittaker
Five kingdoms. Fungi leave the plants: they make no food and absorb it instead.
PlantaeAnimaliaProtistaMoneraFungi - 1977 Carl Woese
Three domains above the kingdoms (Bacteria, Archaea, Eukarya), based on comparing DNA. Ready to go beyond.
BacteriaArchaeaEukarya
Aristotle's scheme was easy to use but judged only by outward look and home, so very different animals ended up side by side. In the 18th century the two kingdom system split all life into Plantae, organisms that stay in one place and make their own food, and Animalia, organisms that move about and get food from other organisms.
Quick check
Who proposed the three kingdom system, and which kingdom did it add?
Show the answer
B. Haeckel, Protista Haeckel's 1866 system gave single-celled microscopic life its own kingdom, Protista.
The misfits that broke each system
Every system cracked on organisms it could not place. Follow an amoeba, a bacterium and a mushroom as the kingdoms multiply.
Around the 4th century BCE: land, water and air. Easy to see, but an amoeba ends up beside fish, and a microscopic bacterium has no place at all.
- 1Aristotle: by habitat
Around the 4th century BCE: land, water and air. Easy to see, but an amoeba ends up beside fish, and a microscopic bacterium has no place at all.
- 2Linnaeus: two kingdoms
1758: Plantae stay put and make food; Animalia move and eat. The amoeba moves, so it is filed with animals. Bacteria and mushrooms go with plants.
- 3Haeckel: three kingdoms
1866: Protista is added for single-celled microscopic life. The amoeba and the bacterium both move in.
- 4Copeland: four kingdoms
1938: better microscopes show the amoeba has a true nucleus and the bacterium does not. Bacteria get their own kingdom, Monera.
- 5Whittaker: five kingdoms
1969: a mushroom makes no food; it absorbs it, often from dead matter. Fungi become the fifth kingdom.
The amoeba problem. Amoeba and Paramecium move about and feed on other organisms, like animals, yet each is a single cell, while true plants and animals are multicellular. A third kingdom, Protista, was made for such microscopic single-celled life.
The nucleus problem. Better lenses then revealed a deeper split: inside an amoeba sits a nucleus wrapped in a membrane, while a bacterium has nothing of the kind. Two single-celled organisms that different could not share a kingdom, so bacteria moved to Monera. That made four: Plantae, Animalia, Protista and Monera, with Protista now holding mostly single-celled organisms that do have a true nucleus.
The mushroom problem. Fungi such as mushrooms do not move, which made them look like plants, but they make no food. Instead they soak up nutrients from outside: most from rotting matter, a few from living partners, and some by feeding on a host. So they got a kingdom of their own: Fungi. Monera, Protista, Fungi, Plantae and Animalia make up the five kingdom classification.
Quick check
Why were fungi given a separate kingdom?
Show the answer
C. They are heterotrophs that soak up food They stay put like plants, yet they cannot photosynthesise; they take in food by absorption.