Classification is defined as identification, naming, and grouping of organisms into a formal system based on similarities such as internal and external anatomy, physiological functions, genetic makeup, or evolutionary history. .
Classification determines methods for organizing life on Earth. It is a process that reflects the very nature of organisms, which are subject to change over many, many generations in the process of evolution. .
Since life first appeared on Earth 3.5 billion years ago, many new types of organisms have evolved. Many of these organisms have become extinct, while some have developed into the present world. Extinction and diversification continue nonstop, and scientists are often encountering fluctuations that may affect the way an organism is classified.
When classifying organisms, scientists study a wide range of features, including those visible to the naked eye, those detectable only under a microscope, and those that can be determined only by chemical tests. .
Scientists compare the external shapes and sizes of organisms as well as the anatomy and function of internal organs and organ systems, such as the digestive or reproductive systems. .
Biochemists study and compare the molecular interactions within an organism that enable it to grow, make and store energy, and reproduce. The early stages of an organism's development, as well as an organism's behavior are also useful in grouping organisms. Even the role an organism plays in its habitat can help place it in a particular group. Scientists use the fossil record to learn how certain animals have changed and evolved through Earth's history, which may provide clues for classification.
There are five large groups of classification called KINGDOMS. Two of the largest are Plants and Animal. With the invention of the microscope and further studies of life, three more kingdoms have been recognized, Fungi, Protista and Monera.