However, in terms of synapomorphy-based reasoning, this similarity is largely based on retention of plesiomorphic features below the level of Dinosauria, in combination with pedal digitigrady and with bipedality
Nevertheless, orangutan footprint morphology does not need to be exaptive for bipedality
for the purposes of our model.
Was it the adoption of upright posture and bipedality
that caused a shift in the poise of the head on the vertebral column?
Birds have a unique collection of traits, such as feathers, flight, bipedality
(walking on two legs), a wishbone, and an unusual respiratory system, most of which are rare or nonexistent in any other living animal group, making it challenging to determine their closest relatives.
Why, exactly, bipedality
developed remains a contentious issue, and the author frankly admits that the sort of "pelvic adaptations" the earliest hominids had in order to accommodate bipedality
remains a mystery.
Human birth was constrained by the need to pass a head that was enlarging over geologic time through a pelvis that was being extensively modified by the requirement for bipedality
, such that cephalopelvic disproportion is not an uncommon cause of both maternal and fetal death today where Cesarean sections are not readily available.