Here's why:

* Humans are multicellular organisms. This means we are made up of trillions of cells that work together.

* We start as a single cell. A fertilized egg (zygote) is the first cell of a human, but it rapidly divides and develops into a complex organism.

* Specialized cells form tissues and organs. As the embryo grows, cells specialize into different types, forming tissues like muscle, bone, and nerve tissue. These tissues then form organs, and multiple organs work together to create organ systems.

So, while we begin as a single cell, we become incredibly complex and diverse in terms of cell types and structure.