How Many Kingdoms?
For a long, long time it appeared to man that the world was divided neatly into two kinds of organisms - plants and animals, each in their own Kingdom.
Once biologists discovered micro-organisms and found out more about other living beings, a simple division into plant and animal kingdoms was clearly inadequate. Today most biologists use a system based on The Five Kingdoms of Life.
Four of the Kingdoms consist of eukaryotic organisms (those whose DNA is packaged in chromosomes within a nucleus). The members of the Kingdoms Animalia and Plantae have many cells during most of their lives. The Kingdom Fungi contains both multi-cellular forms and single celled yeasts.
A large number of single celled eurkaryotes are arbitrarily gathered into a Kingdom called the Protista which also contains the algae, all of which are single-celled during important parts of their lives.
The fifth Kingdom, the Monera, contains only bacteria, prokaryotic organisms which are vastly different from all other living organisms. Bacteria are the most ancient of organisms, and for many, many millions of years, they were the only life form on this planet. All are fundamentally single-celled. They have no nucleus, nor chromosomes, and their DNA lies free in the cytoplasm.
[< Go Back]