Although protozoa evolved from eukaryotes about 2 billion years ago, it was 1674 that humanity saw them for the first time. That’s when Antonie van Leeuwenhoek, using his meticulously crafted microscopes, discovered protozoa—the first microscopic observation of single-celled organisms. His detailed observations and descriptions of what he called “animalcules” in a drop of pond water opened a completely new world to scientific study, fundamentally transforming our understanding of life on Earth. This discovery not only marked the birth of microbiology as a science but also challenged existing views on the complexity and diversity of life, laying the groundwork for the exploration of biological processes at the cellular level. Little did Antonie know, he was looking at the descendants of the ancestor to plants, animals, and fungi! Later, in the 1990s, scientists put together the following timing:

• 1.7 billion years ago: Plants diverge from the common protozoa ancestor.
• 1.5 billion years ago: Fungi and animal branch emerges.
• 1.3 billion years ago: Fungi diverge from the common fungi-animal ancestor.

Analysis: The discovery of protozoa brought to light the vast, previously invisible world of microorganisms, reshaping biological and medical sciences by introducing the concept that life extends far beyond what is visible to the naked eye. This revelation had profound implications, leading to advances in disease control, understanding ecological interactions, and even prompting discussions on the origins of life.