Kingdoms of Life

Kingdom of Life Facts

Kingdoms of Life ['kiɳ-dᵊmz-ᵊv-līf]

How things on Earth are classified into groups so scientists can study them

Coral Reef Red Sea Egypt Marsa Alam

The world is filled with living things of all kinds: insects, fish, humans, trees, mushrooms, and much more. The diversity of life is one reason Earth is such an interesting place! How many different living things can you name?

Vibrio Cholerae Bacteria

Scientists have identified 1.7 million different species on Earth, but they estimate there are at least 10 million species yet to be discovered. Every year scientists identify and name more new organisms. Living things include everything from the blue whale, which is as long as three school buses, to tiny bacteria that you cannot even see. How can anyone keep track of so many living things? For as long as humans have lived, people have tried to classify organisms or sort them into groups, based on their similarities and differences. Taxonomy is the scientific study of how living things are grouped together, the ever-changing process of classifying life forms.

What Is Alive?

A cute teen boy stands on a country plot on a cold winter day, holds an icicle in his hand, winks

Before we can decide how to classify living things, we have to decide whether something is alive in the first place. What is the difference between living and nonliving? You might think that's easy: you might say that if something moves, it is alive. But a cactus plant doesn't move, so is it alive? Can a forest fire or water in a river move? How about a car, or clouds, or lava? They all can move, but they are not alive! You might say that if something grows, it is alive. An icicle can grow a little longer each day, but is it alive? You might say a stick of wood is not living, but is it a piece of a tree that is alive? What about a seed that looks like a little pebble? Is it alive? This isn't as easy as it seemed at first!

Living Things all have these characteristics:

In science, "living" is used to describe anything that is or has ever been alive (dog, flower, seed, log). "Nonliving" is used to describe anything that is not now nor has ever been alive (rock, glass, spoon, car.) Some scientists prefer to use three categories: Living, Nonliving, and Dead (or Once-living.)

Cells, the Building Blocks of Life

Wilson Fig2

Every living thing is made of cells. Cells are the basic unit of all organisms. Cells are like little factories inside all living things that have specific jobs to do, working each day to keep the organism alive and functioning. Some creatures are made up of just one cell, while others are made up of trillions of cells. Human beings are made up of many different types of cells, including skin cells, nerve cells, brain cells, blood cells, and muscle cells. Cells are tiny, but they can be seen with a microscope. Scientists study cells of all kinds to learn more about how life works.

Plant Cell

There are two main kinds of cells that make up living things: prokaryotic and eukaryotic. Eukaryotic cells, such as those in animals and plants, are known as eukaryotes (you-carry-oats). Eukaryotic cells are larger and more complex. They contain smaller components called organelles, such as mitochondria which make energy for the cell, and ribosomes which make proteins. They have a cell nucleus which contains the cell's DNA - all the instructions that tell the cell what to do. You can learn more about DNA at the Science Trek Heredity page.

Prokaryotic cells, such as those making up bacteria, are known as prokaryotes (pro-carry-oats.) Prokaryotic cells are small, simple cells that contain no nucleus or organelles. They have a membrane on the outside, with cytoplasm and DNA on the inside.


Plant and animal cells are both eukaryotic, but they are different in structure. For example, only plant cells have rigid cell walls and contain chloroplasts, which make food for the plant in a process called photosynthesis. Animal cells have a flexible cell membrane, but no cell wall.

Taxonomy: The Science of Classification


Organisms were first classified by Aristotle, who lived in ancient Greece over 2,000 years ago. He divided all living things into two main groups: Plants and Animals. He sorted plants into smaller groups such as Small, Medium, and Large, and sorted animals into Land, Water, and Air. You can probably see that this system wasn't very useful. In such a system, birds, bats, and bumblebees were grouped together, simply because they all fly.

Much later, in 1758, a Swedish scientist named Carl Linnaeus developed a new way of organizing living things. If you think about where you go to school, a way of classifying with different levels, might be: District - School - Grade - Class - Student. Linnaeus's classifying system had seven levels, starting with two broad groups he called "kingdoms": Plantae (plants) and Animalia (Animals). He then used descending subgroups based on more specific characteristics. Linnaeus is called the "Father of Taxonomy" because we still use his classification and naming system today.

Biological Classification

The classification categories look like this:
Kingdom - Phylum - Class - Order - Family - Genus - Species

Humans are classified like this:
Animalia - Chordata - Mammals - Primates - Hominids - Homo - Sapiens

At that time, Plants and Animals were the only two kingdoms. However, as microscopes were improved, other life forms were found that didn't fall into either category. A Dutch scientist named Anton van Leeuwenhoek first observed microorganisms through a magnifying lens. He is known as the "Father of Microbiology." He was amazed to see thousands of small creatures in a drop of water, calling them "wee beasties" and "animalcules." In 1866, these one-celled creatures were recognized as being neither plants nor animals, but a third kingdom known as Protista. In the 1960's, the kingdoms of Fungi (mushrooms and molds) and Monera (bacteria) and were added.

This five-kingdom model is still used in some places. In 1990, many scientists agreed that kingdom Monera should be divided into Eubacteria and Archaea, making six kingdoms. Today, the exact way that kingdoms should be divided is still a matter of disagreement among some biologists.

In the past, classification was made on the basis of appearance and similar body parts. Today, scientists classify living things based on their cell structure, their DNA sequences, their method of getting food, and the way they reproduce. As scientists continue to learn more about cells, genetics, and relationships between living things, the way they classify organisms may continue to change.

Three Domains of Life

Domain is a level that has been added above Kingdom. Domain is the most inclusive level of organization. All living things can be classified into three domains: Archaea, Bacteria, and Eukarya.

Image of acinta microbe

Archaea are prokaryotes, meaning that they are single-celled organisms that do not have a nucleus. They are said to be "extremophiles," meaning that they live in harsh, extreme environments where other organisms could not survive, such as hot springs, deep ocean volcanic vents, sewage treatment plants, and the insides of some animals. They can be found in boiling water, very salty water, and environments with lots of acid or no oxygen. Archaea are pretty tough little creatures!

Kingdoms of Life S20 Enterobacteriaceae Bacteria Family

Bacteria (or eubacteria) are also unicellular prokaryotes, but their DNA structure is different from archaea. They are microbes or microorganisms, meaning they are so small they can only be seen with a powerful microscope. One pinch of dirt, or one drop of water, contains millions of bacteria. They reproduce by splitting themselves in two, and they come in many shapes: spirals, spheres, and rods. Bacteria are found everywhere - soil, air, water, and on and in plants and animals. In fact, in your own body, there are more bacteria than human cells! There are many different kinds of bacteria. Some bacteria in your body can make you sick, so fortunately we have antibiotic medicines which fight the "bad guys." Other bacteria live in your digestive system and help keep you healthy. Some bacteria live in the soil and decompose dead plants or animals. Some bacteria live in food and help to make yogurt, cheese, and pickles. Learn more and see that while some bacteria are harmful, most are not. Bacteria have an important role in Earth's ecosystems and in human survival. Check out National Geographic's amazing photographs of bacteria, magnified under powerful microscopes.

buffalo bulls grazing in savannah at africa

Eukarya is the third domain, and it includes all organisms on Earth whose cells have a nucleus. Eukarya contains four kingdoms. These are the living things we are most familiar with because we can see them with our own eyes.

Six Kingdoms of Life

The category Kingdom is the level of classification just below Domain. The six kingdoms are: Animal, Plant, Protist, Fungi, Bacteria, Archaea . Bacteria is both a domain and a kingdom. Archaea is also both a domain and a kingdom. Within the Eukarya domain, there are four more kingdoms: Animal, Plant, Fungi, and Protist.

Diagram of the Kingdoms of Life
Huge hippopotamus walking in the Serengeti ecosystem, Africa

Animals are multicellular organisms with eukaryotic cells. This kingdom includes elephants, grasshoppers, dogs, earthworms, octopi, and humans. The animal kingdom is the largest, with over 1 million known species. Animals are heterotrophs, which means they must find and ingest their own food. All animals can move on their own at some point in their life cycle. Most animals reproduce by combining the DNA of two living organisms to create a new one. The animal kingdom is divided between vertebrates, which are animals with backbones, and invertebrates, which do not have backbones. Vertebrates include fish, birds, reptiles, amphibians, and mammals (that's you!). Invertebrates include insects, worms, snails, jellyfish, and clams. 97% of known animal species are invertebrates, with many more yet to be discovered. Learn more about animals at Science Trek's Zoology site.

Close-up shot of premna serratifolia plant on a leaf background

Plants are also multicellular and eukaryotic, but they are autotrophs, which means they make their own food. They use sunlight, air, and water to make sugars and oxygen in the process of photosynthesis. Unlike animals who often have skeletons, plants need their rigid cell walls to support and hold them up. Plants are often divided into vascular and nonvascular plants. Vascular plants have roots, stems, and leaves that move water throughout the plant, while nonvascular plants absorb water through their surface cells. All animals eat either plants or other animals that eat plants, so plants are essential for animal life. Learn more about plants at Science Trek's Botany site.

Hypholoma dispersum fungi on dead wood

Fungi is the kingdom that includes mushrooms, yeasts, molds, and mildew. They are eukaryotic and mostly multicellular. Like plants, they have cell walls, but unlike plants, they do not make their own food. They have to absorb nutrients for energy. Fungi are important in many ways: they are decomposers that help to recycle nutrients in ecosystems. Some important antibiotic medicines, such as penicillin, come from fungi. Yeast is an organism that makes dough rise into delicious loaves of bread. Some fungi, like mushrooms, are tasty to eat. However, certain kinds of fungus can cause diseases in plants and animals. Fungi range in size from too small to be seen (microscopic) to the largest organism on Earth: a fungus in Oregon that covers more than 3 miles! Learn more about different types of fungi.

Green algae

Protists are a very diverse kingdom, full of organisms that don't really fit anywhere else. Some protists are animal-like, some are plant-like, and some are fungus-like. Protists are mostly unicellular, but not always. Some are heterotrophs, and some are autotrophs. Some have cell walls, while some do not. Protists are mostly harmless to humans, but some protist diseases can be carried by mosquitoes, flies, or water. Some of the better-known protists are amoeba and algae. Most protists are microbes, which cannot be seen without a microscope. Learn more about microbes, which include bacteria, archaea, most protists, and some fungi.

Six Kingdoms of Life

Close up photo of little girl using microscope

New species are being discovered every day, and as scientists study their characteristics, they are classified into Kingdoms of Life. But it is estimated that there are millions more species left to discover. The most commonly discovered kinds of organisms are insects and microbes, but even new species of mammals are sometimes found. This chart will help you see how many more species scientists estimate each kingdom may contain. Maybe you will be a biologist who discovers a new species someday!