I don't know for sure, but I would imagine it would have something to do with the human pelvis needing to be strong enough to support the weight of the torso, arms, head etc being above the pelvis on a human skeleton, as opposed to four legged animals, where the weight is distributed across the broader area of the four legs. A larger pelvis may also serve to lower our center of gravity as opposed to a smaller one, which is helpful in not toppling over when we only have two stumps not four to walk on. Gives us greater stability.
Because wesit on our butts so much that they need more mass. LOL<br />
Or for easier birthing of the heads of our babies and their shoulders to fit through the birth canal.
They aren't. A full-grown African elephant has a pelvis not dissimilar to ours,and it's a lot bigger than mine! Ditto for hippos,rhinos,horses...
Man/woman has to stand upright...there's a lot of strain there...whereas a 4 legged animal has a smaller weight ratio due to it all being spread out...<br />
When you think about, the human body is relying on something that's usually less than a foot long in length, (the foot) so it needs something a little more hardy to stay upright and give support to the spine as well as the lower limbs....<br />
Sammy Jo xx
Because we have to support more weight on two legs,and they have four so they distribute more weight on each leg