While there is some debate about the exact first person to observe living organisms with a microscope, Anton van Leeuwenhoek is generally credited with this discovery.

Here's why:

* Early microscopes: While others like Robert Hooke had developed microscopes before Leeuwenhoek, they were relatively crude and limited in magnification.

* Leeuwenhoek's improvements: Leeuwenhoek was a skilled craftsman who developed his own powerful microscopes, capable of much higher magnification than anything available at the time.

* Extensive observations: Leeuwenhoek was a meticulous observer who spent years examining a wide variety of materials, including pond water, scrapings from his teeth, and blood. He was the first to describe and document microorganisms like bacteria, protozoa, and even sperm cells.

However, it's important to note:

* Others may have seen them earlier: It's possible that others may have observed living organisms under microscopes before Leeuwenhoek, but they didn't document their findings or their observations were not as extensive.

* Leeuwenhoek's impact: Regardless of who saw them first, Leeuwenhoek's detailed descriptions and drawings of these previously unknown organisms had a profound impact on the development of biology and microbiology. His discoveries revolutionized our understanding of the microscopic world and paved the way for future scientific advancements.