Morphic field

Morphic fields are an interesting, but obscure offshoot of biology, first mentioned in Lucifer Rising.

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

Humans are the way they are because there's a morphic field for humans, just as there's a morphic field for Alpha Centurians, Arcturans and Daleks, and just as there is one for sparrows and for sheep. The fields are but a minor part of a singular universal morphic field encoding the "basic pattern" of all life on every planet.

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 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.

The mathematics of morphic fields is too complex for a computer and requires a living mind.