This work is licensed under
CC BY-SA 4.0
: also see my full Copyright statement.
Vertebrates evolved from a shrimp-like animal
The pelvis and lungs are relatively minor adaptations(!)
The shrimp-like baseline biochemistry (ATP, neurotransmitters, etc.) and anatomy (the three-compartment body plan along with its peripheral nerves) is so efficiently evolved that it remained as a basic template for all "higher" creatures. Any subsequent evolutionary changes after about 550 million years are really footnotes and embellishments. Cetaceans (dolphins, whales) found the pelvis to be superfluous and discarded it.
The addition of a pelvis and hind legs about 375 MYbp created a necessity for additional Sacral nerves. There has been some recent debate as to whether the Sacral Plexus is "sympathetic" or "parasympathetic". But that debate somewhat misses the point of the three-segment body. The pelvis is a functional subset of boththe visceral abdomen and motor thoracic compartments. So the Sacral Plexus is primarily Pelvic (i.e. it doesn’t have to conform to a division relevant to the other pre-existing compartments), with features that necessarily link to the functions of the other three compartments.
Birds and mammals (and dinosaurs) probably share the same warm-blooded ancestor (180 MYbp) at which point animals had sufficient control of their metabolism to be capable of spreading out into far more ecological niches. Life likes to fill space – trees fill space externally with roots and branches, bodies fill space internally with lungs and vasculature, and species fill space by constant invention, self-remodelling, and collaborative symbiosis.
