(Spoilers) Question about those bursters

What do you mean by serpent e chest burster? I remember all the aliens bursting out of the chest to be small versions of the adult aliens. They have tiny arms and legs with a elongated oval head and a long whiplike tail.

A retreading of the Abrahamic faiths associating paradise with the Garden of Eden and the forbidden fruit that is knowledge is the underlying theme here.

David is not human and so falls into the domain of Seraphim which means he could very well be the original right hand of god fallen from grace.

Add this to temptation and the sexual divide between men and women, which Giger so dedicated his works to illustrating, and you have David creating the fall of mankind into his idea of heaven via a snake.

It's working backwards and without Giger's input (And it shows!) but that's roughly what Scott is scrambling for I reckon.

I think that since they have black goo as their origin, they are just constantly mutating. Every generation mutates so you never see exactly the same alien twice. They pick up some DNA from their hosts and they also mutate to have different life cycles and different methods of reproduction.
Sometimes you need a queen, or you might just need airborne spores. giant eggs, small fetuses, deacon, neomorph, xenomorph, squid beast, flying bumblebees... their mutations are limitless.
It's not a linear evolution, it's just constant mutation.