Classification of Living Things

What is life? Scientists have found and described approximately 1.75 million species on Earth according to the Global Biodiversity Assessment made by the United Nations. Plus, new species are being discovered every day. From tiny bacteria to yeasts to starfish to blue whales, life's diversity is truly impressive! With such a diversity of life on Earth, how does one go about making sense of it all?

One way to make sense of it is by classification. Scientists put species into groups that are based on similarity so that those millions of species do not seem so overwhelming. People rely on their knowledge of classification to understand what different species are like. For instance, let’s say that a friend of yours tells you that he saw an egret last weekend. You have never heard of an egret before, but if he tells you that an egret is a type of bird, you should have some idea of what it is like.

Living things are divided into three broad groups based on their genetic similarity.
The three groups are:
Archaea: very primitive forms of bacteria.
Eubacteria: More advanced forms of bacteria.
Eukaryota: All life forms with eukaryotic cells including plants and animals

These three groups are called domains. The figure at the left shows the three domains of life. The distance between domains and the groupings within them indicates the amount of relatedness between them. So, groups that are close together, like plants and animals, are much more closely related than groups that are far apart, like plants and bacteria! Do you see how the two types of bacteria, Archaea and Eubacteria, are about as similar to one another as they are to animals? Recent studies have found that bacteria are far more diverse than anyone had suspected.

The Eukaryota domain is divided into several groupings called kingdoms.
Kingdom Protista – Organisms with just one eukaryotic cell
Kingdom Fungi – Including mushrooms and other fungus
Kingdom Plantae – Including trees, grass and flowers
Kingdom Animalia – From snails to birds to mammals like you!

Within each kingdom, species are further classified into groups based on similarities.
Very closely related species belong to the same genus.
Closely related genera (the plural of genus) are grouped into the same family.
Related families are grouped into the same Order.
Related Orders are grouped into the same Class.
Related Classes are grouped into the same Phylum.
Phyla (the plural of Phylum) are grouped within a Kingdom.

These Archaea species live in extreme heat near deep sea vents.

Archaea was originally thought to be just another form of bacteria, but archaea is a much simpler form of life, simpler than a single-celled organism, which nevertheless contains DNA, the gene-code of life. If it weren't found at the bottom of the sea or buried inside of rocks, Archaea might resemble blue-green algae.

Archaea may be the oldest and oddest form of life. Most live in extreme environments. These are called extremophyles. Other Archaea species are not extremophiles and live in ordinary temperatures and salinities. Some even live in your guts!

Some extremophile species love the heat! They like to live in boiling water, like the geysers of Yellowstone Park, and inside volcanoes. They like the heat so much that it has earned the nickname "thermophile", which means "loving heat", and it would probably freeze to death at ordinary room temperature. Other extremophile Archaea love to live in very salty, called hypersaline, environments. They are able to survive in these extreme places where other organisms cannot. These salt-loving Archaea are called halophyles

For energy, Archaea does not require sunlight as do plants on Earth, neither does it require oxygen as do animals. Archaea absorbs CO2, N2, or H2S for food, chemically transforms them, and gives off methane gas or sulfur as a waste product. An example for one of the relationships is given below. Notice that free oxygen is not involved. This is among secondary pathways for photosynthesis employed by these early bacteria.

Excess sulfur, as produced in this relationship was found in Earth's early atmosphere and ocean. Planets which contain an environment wherein archaea might survive include Venus, the past environment of Mars, Jupiter, Saturn, and Jupiter's moon Io.

This photograph shows what Eubacteria look like under a microscope.

Eubacteria, also know as “true bacteria”, are microscopic organisms that have prokaryotic cells. Because of their prokaryotic cells, they have a rigid cell wall but no mitochondria or other large organelles, and they have a single chromosome that is not within a nucleus. Most reproduce asexually.

Cyanobacteria, also called blue-green algae, are Eubacteria that have been living on our planet for over 3 billion years. This bacterium grows in mats and mounds in the shallow parts of the ocean. Today it is only common in certain regions, but a few billion years ago, there was so much of it that, through photosynthesis, it was able to add enough oxygen to Earth’s primitive atmosphere for animals that require oxygen could survive.

Some Eubacteria can cause problems for human health. For instance, Streptococci bacteria cause strep throat. If Staphylococci bacteria gets into a cut in your skin is can cause an infection that is called a staph infection. Bacteria such as E.coli and Salmonella are sometimes found in undercooked meat and eggs and can make people sick. Other bacteria are beneficial to human health, such as those found in yogurt.

People have found that some types of Eubacteria can be very useful. Many forms are able to breakdown waste and are used at wastewater treatment plants and in septic systems. Eubacteria are also used to ferment grapes to make wine and to ferment milk to make certain cheeses.

Eukaryotic life forms are very diverse!

From trees, to mammals, plankton such as forams to mushrooms, Eukaryota domain contains some pretty diverse organisms. You are a member of the Eukaryota domain too! In fact, all plants, animals, protists, and fungi, are members of the domain.
These groups might seem pretty different, and they are, but they all have a few key aspects in common including their biochemistry and eukaryotic cell structure, which is unique to the group. All members of the domain Euraryota have eukaryotic cells. Eukaryota is the only domain whose members have this cell type. Eukaryotic cells contain a special membrane bound organelle called a nucleus that contains genetic material (DNA) within packages called chromosomes.