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
Was it the adoption of upright posture and bipedality
that caused a shift in the poise of the head on the vertebral column?
Michaud begins by detailing the evolution of bipedality
starting with the earliest ancestors of mankind, and explores theories proposing why humans transitioned from quadrupeds to upright walkers.
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.
For the most part, bipedality
has always been related to fast locomotion and to predator avoidance.
As noted, bipedality
freed our hands to become highly dexterous because of imitation.
In clinical practice, we are familiar with the consequences of a trade-off between our supposed quadrupedal past and (relatively) recently acquired bipedality