Here's why:

* Single Cell: A fertilized egg is a single cell formed by the fusion of a sperm and an egg.

* Complete Genetic Information: It contains the complete genetic blueprint for the development of an entire organism.

* Capable of Independent Existence: Though it relies on the environment provided by the mother, the zygote can undergo cell division and development on its own.

However, it's important to note that the zygote rapidly begins to divide and differentiate, quickly transitioning from a single cell to a multicellular organism.