While it's not technically an organ in the traditional sense, the primitive streak is a crucial structure that establishes the body's axis (left-right, head-tail) and sets the stage for the development of all other organs.

Here's why it's considered the first step in organ development:

* Initiates gastrulation: The primitive streak is the site where cells begin to migrate and differentiate, a process called gastrulation.

* Establishes germ layers: Gastrulation gives rise to the three primary germ layers: ectoderm, mesoderm, and endoderm. These layers are the building blocks for all organs and tissues.

* Determines body axis: The primitive streak determines the head-to-tail axis and the left-to-right symmetry of the embryo.

So, while the primitive streak isn't a fully formed organ, its formation is the fundamental step in the development of all the organs that follow.