All living things on Earth are linked with each other. Scientists believe all life as we know it has evolved from a common ancestor. This ancestor, often known by the term LUCA (last universal common ancestor), was believed to be alive around 3.5 billion years ago. Since this point, life has become varied through evolution to the wide and beautiful array of life we see all over the planet.
Nobody knows exactly how many different species exist on Earth. We have discovered just around 1.3 million species, but scientists predict that there are several million more out there that we haven’t discovered yet. New species are constantly being discovered and added to the ever-growing list. It has been a huge challenge for scientists to catalog and organized these different types of organisms. The classifying of organisms is known as taxonomy. The system most commonly used is based on the characteristics of living things and this taxonomy was first formalized by Carl von Linné (Carolus Linnæus) in the eighteenth century. The system involves sorting living things into groups and breaking those groups into multiple subgroups.
The largest groups of life are domains, of which there are three: Bacteria, Archaea, and Eurkarya. Bacteria are all around us, but are difficult to see without a microscope. Archaea are unicellular organisms that are the only life able to survive in the most extreme conditions. All the organisms in the Eukarya domain have a cellular nucleus in common, and are what we typically think of as "life".
The three domains are then be broken down into kingdoms. There are six kingdoms that all have distinct characteristics. The six kingdoms are Animals, Plants, Fungi, Protists, Bacteria, and Archaea. Bacteria and Archaea are considered both domains and kingdoms. The Eurkarya domain contains the other kingdoms: Animals, Plants, Fungi, and Protista.
| Kingdom | Examples | Properties |
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| Animals |
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| Plants |
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| Protist |
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| Fungi |
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| Bacteria (a.k.a Eubacteria) |
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| Archaea |
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These kingdoms are then split up into groups known as phyla (singular: phylum). Phyla are then divided further into classes. Classes are then broken down into smaller groups known as orders. Orders are broken down into families. Within families there are subgroups known as genera (singular: genus). Finally, the genera are split into categories known as species. The definition of species is a group of organisms that can reproduce and make fertile offspring. In order to remember the order of the naming convention, can use the following mnemonic device: Keep Ponds Clean Or Fish Get Sick. Which refers to Kingdom, Phylum, Class, Order, Family, Genus and Species.
Let's look at an example of how a living thing is classified.
This long list of names is normally shortened down to the last two names, Homo sapien, which are the genus and species, known as binomial nomenclature. A lot of the names are written in Latin as historically the first names were written in Latin by Carl von Linné. This agreed naming system allows scientists all around the world to communicate with each other. The kingdom Animalia contains all animals. Animals in the phylum Chordata all have one thing in common: a support rod up their back and in the case of humans, that means our spine. Animals in the mammalia class of which we are a part, all breath air, are warm blooded, and give birth to live young. Other animals in the class include dolphins, dogs, and bats. The order Primates contains animals such as monkeys and gorillas. Primates are categorized as having larger brains for their size than other mammals. The genus Homo contains human beings, but also a number of extinct species closely related to humans.
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