The theory of evolution postulates that small, incremental, beneficial steps propel the evolutionary process forward. It is much like a device where one component of that device is modified at a time, so as to improve the efficiency of the device in some way, while at the same time allowing the device to remain functioning without any other modifications. Once the device has settled into the fact that it now has an improved component, it then “sees” the benefit of upgrading another one. The point is that it takes these steps one at a time, sees how good that step is, and then takes another step. The device must both continue to function and improve its functionality.
But what if the upgrade requires more than one improvement at a time? Evolutionary theory cannot accommodate this. The improvement must be one step at a time, and if a component doesn’t offer an advantage to an organism (i.e., it doesn’t function), it will be lost or discarded. Are there devices occurring in nature that therefore cannot be explained by evolution? Indeed there are many, but it only takes one to disprove the theory.
We will choose one which everyone will be familiar with-the amazing human knee joint. The knee joint is unique in our bodies. It is quite unlike the ball and socket joints of our hips or shoulders and the pivot joint of our elbows. Although those are all marvels of engineering, the knee is truly exceptional. It consists of several elements, but the critical design parts are (a) the two condyles of the femur bone that rotate in (b) the matching concave grooves of the tibia, and (c and d) the two cruciate ligaments (so called because they cross over each other) that fit in the space between the condyles.
If a structure is so complex that all of its parts must initially be present in a suitably functioning manner, it is said to be irreducibly complex. The knee joint is irreducible; all four of these parts must be present for the knee to work. (The knee has other parts, but these four are essential to each other for them to function in the way they do.) Any one, two, or three of them on their own would not perform any useful function. They are all unique to the knee.
Therefore it is impossible for the knee to evolve from a simpler joint like the hip or the elbow, according to the theory of evolution. How such a device could have evolved in a gradual, step-by-step process as required by classic Darwinian evolution is an insurmountable obstacle to evolutionists. ([Footnote:] Stuart Burgess, “Critical Characteristics and the Irreducible Knee Joint,” Creation Ex Nihilo Technical Journal, Vol. 13, No. 2, 1999.)