Pelvic anatomy consists of four bones. They are the two innominate bones which consist of three fused segments, the illium, ischium and pubic bones, and is located laterally and ventrally. The pubic bones come together in the ventral or front aspect. These innominates also house the acetabulum that form the cup-like structure creating the socket part of the hip for the ball of the femur or thigh bone. The third bone that foms the pelvis is the sacrum consisting of three fused segments and the fourth, the coccyx consisting of three to five fused segments, although the jury is out on how many fuse and at what age. The sacrum and coccyx are considered part of the spine and are at the dorsal aspect of the pelvis. It was named a pelvis due to its resemblance to a basin or funnel type structure.
Human pelvic anatomy has many advantages when it comes to upright posture. The human pelvis specifically evolved to accommodate prolonged upright posture. Consequently, the pelvis has a backward bend in it, called the sciatic notch, not seen in other primates. It is the backward bend in the pelvis and the sciatic notch, though, that gets humans into trouble with hip and leg problems.
Most mammals have to accommodate either bipedal and quadriped styles of posture and locomotion, but as always, there are exceptions and some mammals prefer to swim rather than walk. In addition to standing and walking, the pelvis in some mammals has to accommodate sitting. For many mammals their tail gets in the way when sitting on their buttocks. Many primates and humans, however, are specially built for sitting upright and come equipped with sitting bones with some rocker action on the bottom and pads or plenty of padding as in apes or humans respectively. Even with our extra padding, however, prolonged sitting is bad for humans and compresses important vessels to the legs.
Interestingly, far from being upright some birds are excellent pseudo-bipedal walkers. Others, like penquins stand as upright as humans but waddle about rather than walk. And some birds, such as owls, like to perch and sleep in upright positions.
Over eons of time the pelvis has gone through many modifications. Even some dinosaurs walked on two feet.
The picture below is of a modern ape and a human. Notice in particular how small the human pelvis is compared to the pelvic anatomy in the ape. More than that, notice the backward bend in the pelvis that forms the sciatic notch in humans that is missing in the ape. The backward bend in the pelvis makes it easy for humans to stand perfectly upright with little effort.
Another fascinating feature, is that humans have more tail segments and a longer tail than apes and some other primates. Our fellow primates in these cases obviously didn’t feel a particular need to keep their tails. Afterall, it just gets in the way of sitting. It’s not like humans don’t sit a lot as well, but I believe humans kept longer tails to support the pelvic contents and keep them from falling out during upright posture. That’s why the tail curls under and not out. There are muscles and ligaments that attach from front to back that create a pelvic floor.