I think the Mural is misunderstood.
I could be wrong but when they are in the mural room- It is when David touches the black goo urns & remarks, ' Organic' that the 'Paintings in the ceiling begin to change. Shaw notices this and thinks they've 'contaminated the environment in the room'
Holloway soon after shines a light on the mural for the first time
Therefore I hypothesise that the mural is in fact a form of 'Black goo mirror' or a form of readout of the shape of reaction the organisms in the room will turn into as a result in contact with the substance. The shapes we see are what is to become...
The Deacon we see at the end of the film is essentially a Proto Human Facehugger & Engineer, this is just a consequence of that combination. The Deacon is a one off aberration, the black goo leaves scope for even more horrific creatures to exist. Ultimately the black goo is the root.
The origin of the classic Xenos is I believe going to remain the result of David, personally in terms of the bigger picture I love this decision & thus the spirit of Gigers original bio tech vision is in place & respected in order to develop further in the future.
