Now, the difference between the human and primate knee is as important. That's one of the reasons why they are classified as they are. - Vidqun
So how do you explain hominin fossils of knee joints millions of years old that are designed for real upright walking like modern humans but come from species with cranium capacities of primates and show a mosaic of primate and human traits?
For example, teeth, jaws, wrist joints, hands, hip joints, skull features such as muscle attachment, the location of the foramen magnum, anatomy of the feet etc etc etc
You didn't answer my question. What is the most significant difference between the human and primate knee joints? You are a professional anatomist so this should be easy.