Deacon = Ultramorph. It's evolution

"What defined the evolution of the Deacon?" If you ignore the Trilobite for a moment, the birthing of the Deacon is akin to what happened to Kane in [i]Alien[/i]. That is, the host is firstly smothered by an intermediate vector, directly impregnated with what we assume is some kind of embryonic form, and gives rise to a resulting creature that takes on attributes of the host in addition to the basic xeno blueprint. This is quite a different process to the ingestion of black goo which, as we see, is quite flexible in what it can do. I'm not going to get into the origin/purpose/history of the black goo itself, as there are no concrete answers to be found about this at this time. However, if one accepts the premise that a 'traditional' impregnation from a facehugger variant simply mixes up a xeno blueprint with attributes of the host, then the question of Deacon evolution disappears, because no evolution as such has taken place.

"dessicated trilobyte beside him" remember, I've only seen Prometheus once and as far as I can remember the trilobyte did a disappearing act like the fachuggers of Alien lore. Forgive me if my memory banks are all corrupted by the acid of temporality.

The trilobite is dead, laying over the engineer's feet when the Deacon emerges.

That concept looks a lot like the Orginal Xenomorphy concept Necronom 4 it looks like a cross between Necronom 4 and the Aliens Xenomorph. I am not sure if that Humanoid is supposed to be Human or Engineer. Or indeed if the Xeno is to scale... If it is roughly and its a Human next to it well that means we are looking at a 13ft tall Xeno.... If its a Engineer and we assume the Engineers was intended to be 9ft then we are talking a 20ft Xeno.... The traditional Xeno Queen stood 16-18ft I guess the idea could be that as the Chest Buster was larger that it would grow larger in that original draft.... Unless the image is not to scale.... this would show however that they was trying to redesign the Xeno to be more like the Original Necronom 4 Concept and back to its roots.