Kaplan Nursing Entrance Practice Exam

How many domains is all life on Earth classified into?

1. Two

2. Three

3. Four

4. Five

The correct answer is: Three

All life on Earth is classified into three primary domains: Bacteria, Archaea, and Eukarya. This classification system was introduced by Carl Woese in the late 20th century and reflects fundamental differences in the cellular organization, genetics, and biochemistry of organisms. Bacteria are prokaryotic, unicellular organisms known for their diverse metabolic pathways and ecological roles. Archaea are also prokaryotic but have distinct biochemical and genetic characteristics that differentiate them from bacteria, often thriving in extreme environments. Eukarya includes all eukaryotic organisms, which have complex cells with a nucleus and membrane-bound organelles; this domain encompasses a wide variety of life forms including animals, plants, fungi, and protists. The focus on three domains highlights the evolutionary relationships among organisms and the concept of a universal common ancestor. Other classification systems, like the traditional five-kingdom model, have been used in the past but are considered less accurate in reflecting the true diversity of life, particularly with the discovery of Archaea and advancements in genetic analysis.