Some science is simply descriptive. Whether exploring the human body or searching jungles for new plants and animals, discoveries need to be described. This accumulation of knowledge about the world is an important part of science. Not all science is done by experiments. Classification of organisms is a descriptive science. It helps make sense of the varied life forms in the world.

Why this is important…Understanding diversity helps us better understand disease and nutrition. Living beings vary tremendously, each with its own unique abilities and weaknesses. When it comes to fighting disease-causing organisms, it is important to know what weaknesses to exploit. As for nutrition, eating plants has different health benefits than eating animal or fungus-based foods. And some bacteria in food can make us sick.

Foundations

Every culture has its own traditions for describing other living beings. Some view other living creatures as mere objects, while others believe that plants and animals have souls. Aristotle’s classification of living beings (from the 300’s BC) was based on the combination of vegetable, animal and rational souls that he thought organisms possessed. Only humans possessed all three, setting them apart from all other life.

Carl Linnaeus stands out, among many others, as the first to establish a purely rational system to organize and classify other living beings. This system is still with us today, but the discovery of new species has increased the complexity of what we know. Linnaeus’s system has seen many changes over nearly 300 years, but its core principles remain in use.

Key terms

Species – a group of organisms of the same type; they usually look alike and are able to breed with one another

Prokaryote – organism whose cells lack a nucleus or internal organelles (pro = before, karuon = kernel) (adjective: prokaryotic)

Eukaryote – organism whose cells have a nucleus and internal organelles (eu = true, karuon = kernel). (adjective: eukaryotic)

Details

The period from about 1400 to 1700 was one of intense global exploration. One famously obsessive cataloger of living beings from around the globe was Carl Linnaeus. The tradition at that time was to publish scientific works in Latin – the language of the educated elite - including descriptions of species. Linnaeus proposed a system that shortened the names to just two Latin words, which we call the species’ scientific name (also known as the Latin name or binomial). This name is made of the genus and species names. Humans are called Homo sapiens. The common puncture vine is named Tribulus terrestris, Latin for “a tribulation of the land” – an appropriate description if you have ever stepped on one of their sharp seeds.

When you see scientific names in print, they are italicized or underlined. This “official” name is the name scientists agree to call an organism. Common names vary from place to place, and language to language. Scientific names are fairly stable and give us a way to access scientific information about species, even Nostoc azollae, which has no common name.

Pyramid depicting hierarchy. Domain is broadest at top. Species is most specific and narrow at bottom. Other classifications shown in between. Fox used as an example.

Figure 1-5. Classification hierarchy. Categories shown from broadest (domain) to narrowest (species). The fox is categorized as an example. ("Taxonomic Rank Graph" by Annina Breen is licensed under CC BY-SA 4.0)

Working under the assumption that species are unique and unchanging, Linnaeus developed a system to organize species. This classification started with two broad categories – plants and animals. Plants could be divided into narrower groups like mosses, ferns, conifers and flowering plants. Each of these had smaller groups within them. This is known as a hierarchical system of classification (Figure 1-5)

Video 1c: Classification of living beings

After watching this video, you should be able to:
list levels of classification from broadest (domain) to narrowest (species)
format a scientific name correctly
describe the value of scientific classifications

Stop & Think #4: Look up the scientific classification for humans. List each group, from Domain to species.

This system is still used today, but it has changed a lot. One recent change is the addition of an even broader classification category – the domain (Figure 1-6). There are 3 domains:

Bacteria – single-celled organisms with very small cells and no internal compartments, such as organelles or nucleus
Archaea – physically similar to bacteria, but with differences in biochemistry, such as how they make cell membranes and arrange their DNA
Eukarya – organisms with cells that have internal compartments (nucleus and organelles); this is subdivided into four groups – animal kingdom, plant kingdom, fungus kingdom and protists

Photo examples for each major group. Grey, oval cells for archaea and bacteria. Green algae for protist. Yellow flower for plant. Mushrooms for fungi. Furry marmot for animals.

Figure 1-6. Major life groups. Examples of three domains are represented along with the subcategories of the domain Eukarya. ("Halobacteria " by US Government is in the Public Domain, CC0 || "Salmonella typhimurium" by Volker Brinkmann, Max Planck Institute is licensed under CC BY 2.5 || "Euglena" by Deuterstome is licensed under CC BY-SA 3.0 || “Arnica in flower” and “Mushrooms on tree trunk” and “Yellow-bellied marmot” by J. Buegge are licensed under CC BY-SA 4.0)

Another way to describe organisms is by whether or not they have a nucleus. Members of both Bacteria and Archaea domains do not have a nucleus. Their DNA is loosely lumped in the middle of the cell. We refer to these organisms as prokaryotes. The organisms in the domain Eukarya have cells with a true nucleus. They are eukaryotes. These distinctions between cell types will come up a lot.

Organisms in the Eukarya domain are probably the most familiar. We will learn more details about these in the chapters to come, but it would be useful to have a brief description of each subgroup of Eukarya to start.

Animal Kingdom – these are large (usually many-celled), eat their food, lack cell walls, usually reproduce sexually, and move around using muscles (EX: humans, dogs, crabs, fish, ants, worms, jellyfish)
Fungus Kingdom – most are large (usually many-celled) but are arranged as thin filaments, they absorb their food from surroundings; their cells have walls, they reproduce both sexually and asexually, and move only by growing (EX: mushrooms, molds, yeast, Penicillin)
Plant Kingdom – large (many-celled), make food powered by sunlight, have cell walls that help them stand up, reproduce sexually and asexually, typically live on land and move mainly by growing (EX: pine trees, ferns, moss)
“Protists” – diverse group that includes anything with complex cells that is not animal, fungus or plant. These typically live in water. Most protists fall into one of the following groups.
- Algae come in single and many-celled varieties. They have cell walls and some can swim. Their colorful appearance (greens, reds, and browns) comes from pigments that allow them to make food powered by sunlight (EX: kelp and the green haze in pools and lakes)
- Protozoa are single-celled organisms often without cell walls. They consume or absorb food from their surroundings. Most can swim through fluids. (EX: paramecium, amoeba, malaria)

Stop & Think #5: Which Eukarya group do you think each of the following belong in? (a) spiders, (b) ringworm, (c) green “pond scum”, (d) seaweed, (e) pollen

You might wonder where viruses belong. Viruses are not living organisms so they are not classified in this system. They are important, but we will wait until the final chapter to learn about them.

Learning objectives

Describe each of the Domains of life, as well as major groups within Eukarya
Recognize and write scientific names in the correct format
Differentiate prokaryotes and eukaryotes

Stop & Think Questions Explained

4. Starting with domain: Eukarya, Animalia, Chordata, Mammalia, Primates, Hominidae, Homo sapiens. (The last two are the genus and species, which I formatted as a scientific name.)

5. Answers: (a) spiders are in the animal kingdom, (b) ringworm, despite its name, is actually a fungus that infects the skin, (c) green “pond scum” is probably a mix of protist and bacteria (both groups have algae), (d) seaweed are many different kinds of protists, (e) pollen comes from plants