Morphic field

Morphic fields are an interesting, but obscure offshoot of biology.

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 and Arcturans 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, so that human babies look the way they do because the human morphic field influences the development of the embryo in the womb.

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

Gallifreyans were the first sentient life to form in this universe, and as a result they established the morphic field for all humanoid species throughout this universe, making these far more likely to evolve via parallel evolution. Non-human races only evolve in environments that would be hostile to humanoids.