The Tree of Life

In this article we will learn about: 1

Classifying life The two domains

Organisms with a nucleus and organisms without

The seven kingdoms

2

Other ranks

Example with humans

1

---

Classifying Life

When we look at the different kinds of organisms, it's clear that we need to classify them (or put them into different groups) according to their similarities and differences. We notice that all the life forms that are moving around and eating other life forms are different to the ones growing out of the ground and remaining in place and also there are microbes which are obviously different because they only consist of a single cell. So straight away we can classify them according to these major characteristics.

Then as we look more closely we notice that we can group them according to other characteristics. For example, some animals are warm blooded and give birth to live young while others are cold blooded and lay eggs outside their body, which then hatch. Even after grouping them like this, we can still find more ways to separate them into other categories.

Therefore, we need an effective way to classify organisms. Luckily, scientists have put a lot of effort into this and done a good job of it!

Taxonomy is the scientific study of defining, classifying and grouping things; in the case of biology, this is done for organisms.

Before we get into it, I need to explain that an individual kind of organism is called a species. Basically, members of a species can only reproduce with each other and not with those of other species. You can sometimes get a member of one species able to copulate and have offspring with a member of a different, closely related species, such as a horse and donkey can produce a mule. But then either that individual that was produced cannot reproduce or its offspring can't. So, two different species can't continually reproduce with each other.

It should be noted that the work of classifying organisms is ongoing. What I lay out here is not actually considered the correct, most up to date science, but it's a simpler way to group them that will help you understand the basic idea. Unless you're at a very high level of biology education, this classification system works just fine.

So, without further ado, let's get into it.

The Two Domains

Since cells are the building blocks of life, it makes sense to look at the different kinds of cells to see if they make a notable contribution to determining which categories life forms fall into.

So to begin our taxonomy, we divide organisms up into those which have a nucleus and those which don't. The nucleus controls everything that happens inside the cell, so whether or not an organism has a nucleus in its cells is a key part of distinguishing between them. You can find out more about the nucleus and how a cell can function without one in future articles.

Prokaryotes are organisms without a nucleus. All of them are microbes with only one cell. So the whole organism is a single cell.

Eukaryotes are organisms with a nucleus. An easy way to put organisms into this domain is to consider if it is made up of lots of cells. All multicellular organisms (ones with more than one cell) are eukaryotes. But there are some unicellular (single celled) eukaryotes also.

The Six Kingdoms

Image by Brgfx

Next, we have the kingdoms.

Two of these are for the prokaryotes; so there are two main groups of prokaryotes. The first is bacteria and the second is another kind of microbes whose cells are different enough from bacteria that they form their own kingdom. These are called archaea (meaning ancient bacteria, as they are called in the image above). I won't go into what their differences are here, but one of the places you can find archaea is around places of extreme temperature, either hot or cold, including around geysers. As for bacteria, they are found all over the place: on walls, tables, the ground. Some are harmful and cause disease, but most are not.

All the other four kingdoms consist of the eukaryotes.

Animals

First we have animals, which don't need much of an introduction as you'll be familiar with a lot of them. All animals are multicellular. With few exceptions, all animals eat other living organisms, breathe oxygen, can move and reproduce sexually.

Plants

Next up, we have plants. These also don't need much of an introduction. While animals breathe and absorb oxygen, plants produce oxygen and they are one of the main sources of oxygen for animals. Note though, that they also require oxygen themselves. Okay, this may not make much sense right now, but I'll explain it in another article. Another notable feature of plants is they remain in place with their roots (usually) underground and don't move around their environment (the space around them). They do, however, move their bodies, such as when they bend towards sunlight.

Fungi

Fungi consist of mushrooms and other species like yeast, which we use to cause bread to rise, and molds, which are what cause food to go moldy. Both yeast and molds are microorganisms, but yeast are unicellular, while molds form multicellular filaments (or threads) called hyphae. Fungi eat other living organisms, often the dead remains of plants, animals or other fungi.

Protists

Protists are all the eukaryotic organisms that are not animals, plants or fungi. There is ongoing discussion about how to group these life forms. It's difficult to group protists because some are more similar to other animals, plants or fungi than other protists. However, the term "protists" is a catch all term for these organisms.

Most protists are unicellular, but some are multicellular.

Included in this group are Protozoans, which are all unicellular and feed on other microbes or organic matter and debris (bits of other organisms). Some are parasites, which live inside larger organisms and feed on them or their food. An example of a protozoan is an amoeba.

If protozoans are like animals, in that they feed on other organisms, chromists are like plants as they produce their food via photosynthesis (although some also eat other living things). Some are unicellular, while others are multicellular. They include certain types of algae.

Actual Names of Domains and Kingdoms

I gave the names of the types of organisms, but the domains and kingdoms are actually called: Prokaryota

Eukaryota

Bacteria

Archaea

Animalia

Plantae

Fungi

Protista

2

---

The Other Groups

After we sort organisms into kingdoms, we have other characteristics that we can use to separate them. This depends on the kingdom. So for animals, we look at the structure of the body and what it does (e.g. does the animal give birth to live young, like with humans, or does it give birth to eggs which then hatch?)

The best way to learn the different taxonomic groups is to first be aware of the different types of organisms and then know whereabouts they fit into the tree of life. Then your knowledge will build from there. In my opinion, it wouldn't do you much good to go through, in detail, what the different classifications are as it won't make much sense to you if you don't know much about the individual species.

Here is an example with humans:

Kingdom: Animalia

Phylum: Chordata

Superclass: Tetrapoda

Class: Mammalia

Order: Primates

Suborder: Haplorhini

Infra-order: Simiiformes

Family: Hominidae

Subfamily: Homininae

Genus: Homo

Species: H. sapiens

Now, this may be a daunting list of ranks. We'll briefly go through what some of them mean.

We've already covered Kingdom: Animalia. So next there's Phylum: Chordata. What we're looking at for a phylum is a set of features that make up the general structure of the organism. In simple terms, we can think of chordates as animals with a backbone. There are other structures to consider to include all chordates, but just to keep it simple, we'll leave it at that. If it has a backbone, it's a chordate.

Superclass: Tetrapoda tells us that the animal has four limbs. Sometimes all four are legs, but there could be two legs and two wings or, in our case two legs and two arms.

Next we have Class: Mammalia. There's actually a type of jaw that all mammals have which no other type of animal does. But usually, we think of mammals as animals being able to give birth to live young and feed them milk. As you should know already, only females can do this.

Then we have Order: Primates.

Think monkeys and apes, and you've got yourself some primates. Also included are lemurs, lorises and tapiers. Primates have the largest brains compared to their body weight of all terrestrial mammals (mammals that live on land). Most species live on trees and are able to grasp branches with their hands and feet. They have eyes on the front of their faces and are very good at seeing things at very high detail.

Homonidae

These are what we call the great apes or hominids. They include humans, gorillas, orangutans, chimpanzees and bonobos.

Homoninae

Since orangutans are significantly different to the other hominids, we have a further division for the great apes, called the hominins or ultra apes. So this list includes humans, gorillas, chimpanzees and bonobos.

Homo

The only species currently alive today in the genus homo is homo sapiens, the modern humans. But there were other homo species that lived in the past. Homo Sapiens These are humans, like you and me (unless you're another intelligent organism that is able to read this, like a being from another planet perhaps).

So here we have a rundown of the tree of life. I've given quite a lot more detail than I normally would for Green and Orange level but I think it's for the best as you get a clear picture of the different organisms that exist on Earth.