I did. The physical differences between humans and chimps is due to the mutations that have accumulated in each lineage since we shared common ancestry.
Whether bipedality is an advantage or not is a much larger question. No difference between species is automatically assumed to be advantageous. As to bipedality, it does appear to be quite advantageous for modern humans who live in a savanna-like environment. The pelvis and lower girdle that we have is very good for moving about land, and pretty efficient at jobbing pace. For an ape species moving from an arboreal environment into a savanna environment, it is easy to see why a human-like pelvic girdle would be advantageous. Comparatively, chimps walk around on their two leggs like bow-legged cowboys. They arenât good walkers or joggers, but their pelvic girdle is well suited for living amongst trees.
Mammals didnât evolve from fish. We evolved from reptile-like amniotes. Modern reptiles already have adaptations for separating the the oxygen rich and oxygen poor blood in the form of a three chambered heart with a restriction between the ventricles that prevents complete mixing. Amphibians donât need a separation between the ventricles because they get about 30% of their oxygen by absorption across their wet skin, much like fish extract oxygen across gills. Some amphibians have no lungs at all and depend entirely on their skin for respiration.
