Here's why:

* Self-sufficiency: These organisms are complete and independent, carrying out all essential life functions (like metabolism, reproduction, and response to stimuli) within a single cell.

* No tissues or organs: They lack the complex organization of multicellular organisms, which have specialized tissues and organs working together.

* Examples: Bacteria, archaea, protozoa, and some algae are all examples of single-celled organisms.

While they are simple, these organisms are incredibly diverse and have a huge impact on the planet's ecosystems.