Classifying Living Things

Why scientists classify

The Catalogue of Life, a worldwide list kept by scientists, includes more than 2 million species that have been named and described. Many more are still waiting to be found. Without a system, nobody could keep track of them.

Classification means sorting living things into groups based on what they have in common. The science of naming and classifying organisms is called taxonomy. Each named group, like "mammals" or "fungi," is called a taxon (plural: taxa).

Classifying helps in three ways:

• Talking clearly. A scientist in Brazil and a scientist in Japan both know exactly which animal Puma concolor means.
• Finding things. Like books in a library, organisms are easier to look up when they are shelved in groups inside bigger groups.
• Making predictions. If you learn that a new animal is a mammal, you already know it has hair and feeds its young milk, before you have seen either.

Linnaeus and two-part names

Carl Linnaeus (1707–1778) was a Swedish naturalist. Before him, a plant's scientific name could be a long Latin description several words long. Linnaeus made a habit of giving every species a short two-part name. This is called binomial nomenclature ("two-name naming"). He used it across whole books in the 1750s, and it caught on.

Two of those books still matter today. Scientists treat his Species Plantarum (1753) as the starting point for plant names, and the 10th edition of his Systema Naturae (1758) as the starting point for animal names. That 1758 book is where our own species got its name, Homo sapiens. Sapiens is Latin for "wise."

The two parts

• The first word is the genus, a small group of very similar species. It always starts with a capital letter.
• The second word is the species name. It is always all lowercase, even when it comes from a person's or place's name.
• The whole name is printed in italics.

The coyote is Canis latrans and the gray wolf is Canis lupus. Same genus, different species, so you can tell from the names alone that they are close relatives.

Sometimes a third word is added for a subspecies, a group within a species that lives apart from the rest. The mountain gorilla is Gorilla beringei beringei, a subspecies of the eastern gorilla, Gorilla beringei. Only the genus gets a capital letter.

From domain to species

Scientists sort organisms into groups inside groups, like nesting boxes. Each level is called a rank. From the biggest group to the smallest, the main ranks are:

Domain → Kingdom → Phylum → Class → Order → Family → Genus → Species

As you move down, each group holds fewer kinds of organisms, and they have more in common. Linnaeus himself used kingdom, class, order, genus, and species. Family, phylum, and domain were added later.

Memory aid: Dear King Philip Came Over For Good Soup. Or make up your own sentence with the same first letters. Silly ones are easier to remember.

Where do gorillas and humans split?

Compare three mammals. Read down each column until the names stop matching.

The sea otter leaves the other two at the order level. Gorillas and humans stay together all the way down to the family Hominidae (the great apes) and only split at the genus. The lower two organisms share a rank, the more closely related they are.

The three domains

The highest rank is the domain. Every living thing belongs to one of three. The big difference is inside their cells: does the cell have a nucleus, a walled-off compartment that holds its DNA?

Bacteria

Single cells with no nucleus. They live almost everywhere, including inside you. Escherichia coli (E. coli) lives in the intestines of people and animals. Most kinds are harmless and even help digest food, though a few can make people sick.

Archaea

Also single cells with no nucleus, so under a microscope they look a lot like bacteria. Their chemistry and genes are very different, though. Some were first found in hot springs in Yellowstone National Park, and others live at deep-sea vents hotter than boiling water or in very salty water.

Eukarya

Organisms whose cells have a nucleus. This domain includes protists (such as the single-celled amoeba), fungi, plants, and animals, including you.

For a long time, archaea were lumped in with bacteria. In 1977, Carl Woese and George Fox at the University of Illinois compared a molecule found in every cell, ribosomal RNA, which is built from instructions in the DNA. Their results split living things into three main lines instead of two. In 1990, Woese and two colleagues proposed a new rank above kingdom, the domain, and named the three: Bacteria, Archaea, and Eucarya. Most books now spell the last one Eukarya.

How many kingdoms? It depends on the book

How many kingdoms are there? It depends on whom you ask, because there is no single official number. Kingdoms are groupings that people choose, and different scientists have chosen differently as they learned more.

On this page we use the six-kingdom system, because that is what many U.S. schools teach. If your textbook says five or seven, it is not wrong. It is using a different system. The 2015 seven-kingdom system was built as a practical way to organize more than 1.6 million species in a catalog, and its authors say plainly that it does not try to show the full family tree.

The trouble with protists

In the six-kingdom system, Protista is the "everything else" drawer for eukaryotes that are not fungi, plants, or animals. It holds single-celled amoebas and also giant kelp, which can grow taller than a house. Kelp is a large brown alga, not a plant. It makes food from sunlight like a plant, but it holds onto rocks with a holdfast instead of roots. The seven-kingdom system moves brown algae like kelp into Chromista.

Using a dichotomous key

You find an organism and want to know what it is. A dichotomous key helps you name it. Dichotomous means "cut in two." The key is a list of numbered steps, and every step gives exactly two choices. You read both, pick the one that matches your organism, and follow it to the next step. Keep going until the key names the organism.

Tips for using a key:

• Read both choices before you pick one.
• Use traits you can actually see or measure, not guesses.
• A key only works for the organisms it was written for. This one covers ten organisms; a real field guide key might cover hundreds.

Worked example. A furry animal floats on its back in the ocean, cracking open a crab. Many cells, so 1b sends you to 3. It cannot make its own food: 3b, go to 5. It eats the crab and digests it inside its body: 5b, go to 6. It has a backbone: 6a, go to 7. Fur: 7b, go to 9. Lives in the ocean: 9a. It is a sea otter, Enhydra lutris.

How DNA changed the tree

Linnaeus sorted organisms by how they looked. That works well most of the time, but looks can fool you. Kelp looks like a plant. A hippo looks more like a pig than a whale. Today scientists also compare DNA, the instructions inside cells, and they treat any family tree as their best explanation of the evidence so far. When new evidence arrives, the tree can change.

Case study: whales and hippos

Whales are mammals. They breathe air and feed their young milk. But which land mammals are they related to? For a long time, many scientists thought whales were most closely related to an extinct group of mammals called mesonychids.

In 1999, three scientists, Masato Nikaido, Alejandro Rooney, and Norihiro Okada, tested the question with "jumping genes." These are short pieces of DNA that can copy themselves and paste the copy into a new spot in an animal's DNA. When a copy lands, it gets passed down to all of that animal's descendants.

The team found jumping-gene copies sitting at the very same spots in the DNA of whales and hippos. Cows, pigs, camels, and their relatives did not have them there. The best explanation is that the copies landed once, in an ancestor that whales and hippos share, and no other living group.

Fossils agreed. In 2001, scientists described two early whales from Pakistan, about 47 million years old, that still had legs. Their ankle bones had a "double pulley" shape found only in even-toed hoofed mammals, the group that includes pigs, camels, deer, and hippos. The same week, a second team described skeletons of Pakicetus, a wolf-sized early whale from Pakistan that lived on land around 50 million years ago. Its ankle had the double-pulley shape too. That showed whales belong with that group, not with the mesonychids.

Whales did not evolve from hippos. Both groups come from a shared ancestor that lived long ago. The earliest whales lived more than 50 million years ago and walked on land. The hippo family showed up much later: its oldest known fossils, from Africa, date to more than 15 million years ago. Its liking for water evolved separately.

A mystery organism has turned up, and you have its field notes. Use the dichotomous key, one step at a time, to name it. Watch the "Still possible" list shrink as you go. Each round has five organisms picked at random from the ten in the key.

How scoring works: each organism is worth 10 points. Every wrong turn costs 3 points, but an organism never scores below 0. A perfect round is 50. Your best finished round is saved in this browser only.