Final answer:
Organisms are classified into three domains of life: Archaea, Bacteria, and Eukarya. Archaea and Bacteria consist of unicellular prokaryotes without a nucleus, while Eukarya includes complex life forms with nuclei, such as plants and animals.
Step-by-step explanation:
All living organisms belong to one of three overarching categories known as domains: Bacteria, Archaea, and Eukarya. Archaea and Bacteria are unicellular prokaryotic organisms, which means they do not have a nucleus or any membrane-bound organelles in their cells. These two domains include organisms that are often found in extreme environments and are genetically distinct from each other.
On the other hand, the domain Eukarya contains organisms that have cells with a nucleus and other membrane-bound organelles. The Eukarya domain includes more complex life forms such as plants, animals, fungi, and protists. This domain is characterized by a greater structural and functional complexity compared to the Archaea and Bacteria domains.
The concept of the three domains was introduced in the 1970s when scientists discovered that certain 'bacteria', now recognized as Archaea, were fundamentally different from other bacteria and eukaryotes. These discoveries have led to significant changes in how we understand the tree of life and the relationships between various forms of life on Earth.