The Human Knee is a sophisticated piece of engineering called a Four Bar Mechanism. It functions as a simple hinge, but has an additional function. In the context of the knee, it enables the leg to bend, but when standing up straight it also enables the leg to act as a single column. This means there is no muscular effort required to keep the knee straight. A human being can stand for hours with little effort. By comparison, a monkey’s knee really is just a hinge, so for a monkey (or other primate) to stand straight for more than five minutes is a huge effort.
Take a look at this article to see how a Four Bar Mechanism is both so simple yet also so complex. There is no half-way house between a hinge and a Four Bar Mechanism. Therefore there is no possible gradual evolutionary pathway from one to the other. The Four Bar Human Knee had to appear in one go, fully formed and fully functional. It needed a Great Engineer to both design and implement it.
There are many other biological structures for which the same reality applies. To give but one example, nobody has come up with a credible evolutionary pathway for the eye. Even the starting point that students tend to imagine, a simple light-sensitive cell on the skin, is unsupportable. This is because, as we know now, that the retina at the back of the human eye uses thirteen instantly reversible chemical reactions, enabling it to change one photon of light hitting the retina into one electron travelling up the optic nerve to the brain. Even given all those reactions ready-made, plusthe physical structures in which they can operate, the probability of getting them in the right order by chance is rather low:
13 x12 x11 x 10 x 9 x 8 x 7 x 6 x 5 x 4 x 3 x 2 = 6,227,020,800
So even with the clever chemistry and structures already in place, the probability of a light-sensitive cell working is only one in six billion. And that’s just a light-sensitive cell, not an eye! See Microbiologist Michael Behe’s book Darwin’s Black Box: The Biochemical Challenge to Evolution (1996.)
Also see the Links on the Science Page for much, much more on this.