Morphic field

The theory of morphic fields conjectures that all living beings have a field associated with them which defines their entire biology. Morphic fields provide a force that guides the development of an organism as it grows, making it take on a form similar to other species. DNA is not the source of structure itself, but rather a "receiver" that translates instructions in the field into physical form.

Morphic fields are reinforced by every successive generation and Gallifreyans were the first sentient life to form in this universe, as a result they created a morphic field for humanoids. That morphic field apparently became a template that resonated out across the universe. That made it more probable that the next race to evolve would be humanoid, and that made it much more likely that the next race would be humanoid, and so on. The majority of the non-humanoid races developed in environments where two legs, two arms and a head would have been a positive handicap.(NA: Lucifer Rising)

Behind the Scenes
The theory of morphic fields and resonance originated as a serious Real World scientific proposal. However, most in the scientifc community does not widely accept the idea and most in the scientific community regard it as a fringe science or pseudoscience.