In the old days we learned that all living things were either plants or animals. Many books nowadays teach that five or six kinds of living things, or kingdoms, exist. Below is one of several representations:


(from Biology, by Peter Raven and George Johnson,
Wm. C. Brown Publishers, 1996)

  1. plants
  2. animals
  3. fungi
  4. protista
  5. eubacteria
  6. archaebacteria

If we go along with the above six-kingdom idea (A more updated version is described on our Tree of Life Page) we can say that each living thing is a member of one of those kingdoms. What's interesting for us right now is that we can further say that each kingdom is then subdivided into yet smaller groupings. And then those categories are yet further subdivided. And then those are subdivided, and then everything is subdivided again and again... Here we're talking about the system known as classification, a very important topic if you want to learn more about Earth's living things..

In the biological classification system, each level of subdivision has its own name. Here are the traditional names for those levels:



Specialists refer to yet other subdivisions not shown, such as subfamilies, superorders, subspecies and varieties, but maybe the above seven subdivisions are enough to make us happy here.


To give you a feeling for how these groupings "work," here's a simplified analysis of how they apply to the animal called the human being:

kingdom: animal
phylum: chordate (animals with backbones)
class: mammal (with hair, female makes milk)
order: primate (apes and monkeys)
family: mankind, (with Neanderthals, etc.)
genus: Homo
species: sapiens

You see, at every step down the classification ladder, the thing that we are is narrowed down. At first we're just animals. Then the phylum grouping separates us from all animals without backbones, such as sponges, insects, and worms. On down the ladder we go until we land at the species, and at that point we know that we're just talking about one kind of animal, and that animal is us.

Now let's do the same thing with House Sparrows:

kingdom: animal
phylum: chordate (animals with backbones)
class: bird (egg-layers with feathers)
order: passerine (songbirds)
family: thick-beaked birds such as finches
genus: Passer
species: domesticus

By comparing the two breakdowns, we can see that humans and birds are alike down to class level. We are both animals with backbones, but we humans sure don't belong to the animal class that lays eggs and has feathers.


OK: Here comes something important: Notice that the House Sparrow's last two pigeonhole names, the genus Passerand the species domesticus, also happen to be the House Sparrow's scientific name, Passer domesticus. It's like that with all living things. Referring back to our category breakdown for humans, you can confirm that Homo sapiens is the human animal's scientific name.

So, now it's clear that plant and animal names and plant and animal classification elegantly blend into one another.


Here's an interesting experiment: Go to NCBI Taxonomy Browser by clicking on its name, then in the search window at the top left where it says "Search for" type in the scientific name for any plant or animal and see what comes up. The resulting page will have a section beginning with "Lineage (full)" and after that you'll see the entire system of taxonomic pigeonholing for that organism. And there'll be many more subdivisions than the few outlined above.

For example, in the box I type Homo sapiens, the technical name for humans. Here's what I get with words in parentheses being my own explanations and the traditional categories being shown in red:

cellular organisms (no rank)
Eukaryota (superkingdom)
Fungi/Metazoa group (no rank)
Metazoa (kingdom)
Eumetazoa (no rank)
Bilateria (no rank)
Coelomata (no rank)
Deuterostomia (no rank)
Chordata (phylum)
Craniata (subphylum)
Vertebrata (no rank)
Gnathostomata (superclass)
Teleostomi (no rank)
Euteleostomi (no rank)
Sarcopterygii (no rank)
Tetrapoda (no rank)
Amniota (no rank)
Mammalia (class)
Theria (no rank)
Eutheria (no rank)
Euarchontoglires (superorder)
Primates (order)
Haplorrhini (suborder)
Simiiformes (infraorder)
Catarrhini (parvorder)
Hominoidea (superfamily)
Hominidae (family)
Homininae (subfamily)
Homo (genus)
sapiens (species)

When you do the above exercise with several species you'll see that the classification system of some species has more subcategories than others. That's because in the Evolutionary Tree of Life some branches diversify much more vigorously than others. Species in the faster evolving branches generate more kinds of life forms than those in the slower evolving ones, and that translates into more categories needed to represent those species' full lineage and set of relationships.

If you're unsure about the "Evolutionary Tree of Life" concept, check out our  Tree of Life Page.

Basically the Tree of Life concept reflects the manner by which the various groups and species of organisms arose during the course of evolution by branching off  ancestral species. If we're talking about birds called shrikes and we say that the whole genus has hooked bills, we're implying something profound. We're suggesting that at a certain point in bird evolution a species arose with a hooked bill and other characteristics today associated with shrikes, and this species became the ancestor of all the many shrikes that exist today, the proto-shrike, the first of its genus.


To cement in our minds the above concepts, let's practice talking about them. For instance, we can say that the above breakdown shows that all birds belong to the same class. By the same token, all mammals, from mice to humans, belong to the same class. Among plants, all flowering plants comprise the angiosperm class, and all conifers, such as pines and spruces, comprise the conifer class. In the bird world, there are only around thirty orders and one order holds the woodpeckers, another the penguins, and another all the passerines, or perching birds. In the plant world, only specialists deal with orders. Much more important are plant families, such as the Oak Family, the Grass Family, and the Sunflower, or Composite, Family.

Specialists seldom speak of "the Shrew Family" or "the moss division of plants." They use technical, Latinized words. The Shrew Family is the Soricidae, while the moss division of plants is known as the Bryophyta. Since we're beginners, however, we don't really need to wrestle with these terms.

By the way, the science dealing with scientific names is called taxonomy.