Primarch's, Space Marine's and Halo's; Master Chief are really not that far fetched of an idea, if you know you know.

The Primarch's and Emperor of Mankind make more sense if you think about it from a different perspective.

In the Warhammer 40,000 universe, the process by which a human becomes a Space Marine, also known as an Adeptus Astartes, is one of the most grueling and transformative procedures imaginable. This process involves both genetic and surgical enhancements, turning the aspirant into a superhuman warrior, far surpassing any normal human in strength, resilience, and combat prowess.

Selection and Testing:
Potential Space Marines, often referred to as aspirants or initiates, are usually selected at a young age from the worlds that owe allegiance to the Imperium. They are chosen for their physical prowess, mental fortitude, and potential for enhancement. These aspirants undergo a series of rigorous tests and trials, which may include tests of strength, endurance, combat skill, and psychological stability.

Gene-Seed Implantation:
The key to the transformation into a Space Marine is the implantation of gene-seed organs that are derived from the genetic material of the Primarchs – the original superhuman commanders created by the Emperor. Each Space Marine Chapter has its own gene-seed, which is a resource closely guarded. The gene-seed comprises several genetically engineered organs, each of which provides the Space Marines with their superhuman abilities. These organs are implanted into the aspirant's body in a series of complex surgeries.

Initiation:
Once the process is complete, the newly created Space Marine is inducted into his Chapter, taking his place among his brethren in the defense of the Imperium of Man.

What else is hidden from us?

Doctors May Have Found Secretive New Organs in the Center of Your Head
https://www.nytimes.com/2020/10/19/health/saliva-glands-new-organs.html

Scientists discover new organ in the throat
https://www.livescience.com/new-salivary-gland.html