Sorting Life

A supermarket puts thousands of products into aisles, the aisles into shelves, the shelves into rows, so you can find the yoghurt. Biologists face a much bigger job: about two million kinds of living things have been named so far. How do you sort them all?

Groups inside groups

Sorting living things into groups is called classification. The idea most biologists still use comes from a Swedish doctor and botanist, Carl Linnaeus. In 1735 he published Systema Naturae ("The System of Nature"), which put living things into groups inside bigger groups.¹

Today the groups, from biggest to smallest, are: domain, kingdom, phylum, class, order, family, genus and species. Each group holds living things that share more features than the group above it. Here is the leopard, which lives in the mountains of Iran among many other places:²

Groups drawn as boxes inside boxes. The biggest box is labelled animals. Inside it is a box for mammals, animals with fur that feed their young on milk. Inside that, a box for carnivores, meat-eaters. Inside that, the cat family. Inside that, the big cats, Panthera. In the smallest box is a leopard.

The deeper two living things share a group, the more closely related they are. A leopard and a lion share the smallest group but one, the big cats. A leopard and a house cat share only the cat family.

The three domains

Linnaeus sorted all life into plants and animals. Microscopes later showed many more kinds of tiny living things, and the groups changed. In 1990, the American biologist Carl Woese and two colleagues proposed a new top level, the domain, based on comparing the chemical instructions inside cells. Life, they said, falls into three domains:³

Bacteria - tiny single cells, found almost everywhere.
Archaea - also tiny single cells; they look like bacteria but are very different inside. Many live in hot springs or salty lakes.
Eukarya - everything whose cells have a control centre, a nucleus, inside them: animals, plants, fungi and many single-celled living things.

A tree with three main branches labelled Bacteria, Archaea and Eukarya. Small rod-shaped cells sit on the first two branches. The Eukarya branch splits into three twigs with a mushroom, a plant and a fox, labelled fungi, plants and animals.

So you, an oak tree and a mushroom are all in the same domain, Eukarya, but in different kingdoms.

A painted portrait of Carl Linnaeus, an older man in a grey wig and a dark coat, with a decoration on a ribbon at his chest.

Carl Linnaeus, painted by Alexander Roslin in 1775. Public domain, via Wikimedia Commons.

A growing list

The Catalogue of Life, a list kept by hundreds of experts, counts over two million species named so far, and tens of thousands are added every year.⁴ The next lesson, What Is a Species?, looks at the smallest group of all.

Taxonomy and the Tree of Life · Khan Academy

Try it yourself

Take ten things from a kitchen drawer or a toy box and sort them into groups, then split each group into smaller groups. Did someone else in your home sort them the same way? Biologists had to agree on rules for the same reason.

Words to know

classification - sorting living things into groups inside bigger groups
domain - the biggest group of all; there are three: Bacteria, Archaea and Eukarya
kingdom - a big group inside a domain, such as animals, plants or fungi
species - the smallest main group: one kind of living thing

Big idea

Biologists sort living things into groups inside groups, and the more groups two living things share, the more closely related they are.

Sources

University of California Museum of Paleontology, Carl Linnaeus. ↩
Mary Ann Clark, Matthew Douglas and Jung Choi, Organizing Life on Earth (Biology 2e, OpenStax, section 20.1). ↩
Carl Woese, Otto Kandler and Mark Wheelis, Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya (PNAS, 1990). ↩
Catalogue of Life, Annual release 2025 (2025). ↩