Is the human knee joint “irreducibly complex”?

This is a re-post, in slightly modified from, of one of my Talk Origins Archive feedback responses (from back when the Archive had a feedback section, specifically March of 2002)


Luke asked:

Please email me your response if possible. I don’t want to categorize myself as a evolutionist or creationist. I was visiting the other website I think it was creationist or similar, your guys arch enemies; anyways I was trying to find proof to support modern man evolving from ape, and they had an extensive article written about the human knee and how it has sixteen parts and minus just one and its useless. Apes have non locking knees and because of their makeup you aparently can’t evolve it into a locking one you have to start from scratch. Evolution and mutations from what I’ve read only allow for small changes no mutation can allow for the formation of a complex organism with sixteen moving parts? The knee would have to be built all at once it couldn’t evolve or it would have no use. How do you suggest that apes dumped their knees and immediately mutated new ones with sixteen brand new parts? I would like to believe it could happen just seems far fetched?

This argument originates from an article (Critical Characteristics and the Irreducible Knee Joint) published in the Creation Ex Nihilo Technical Journal (Vol. 13, No. 2, 1999) by a British engineer named Stuart Burgess.

From my reading of the article it seems to be highly flawed, especially in its almost total lack of discussion on the comparative anatomies of living non-human apes (gorillas & chimps etc.), extinct hominins (australopithecines, early Homo) and modern humans. This lack of attention to comparative anatomy (and physiology) is typical of anti-evolutionists and it leads them to continually talk about the anatomy/physiology of various organisms as if they exist in a vacuum (examples: The woodpecker or The bombardier beetle). They focus on some extreme example of organ or system in a particular species as if it is totally unique to that species. The fact is that when one looks at other closely related species one usually finds that there are variations on the extreme example that the anti-evolutionists have focused upon.

For instance the bombardier beetle that anti-evolutionists often cite is just one species of a whole group of beetles (“Ground beetles”, Family Carabidae) many of which have some variation on a chemical defense mechanism, using the same basic chemicals (which exist in many beetles in varying amounts), but used in differing ways. The specific example that anti-evolutionists cite sprays an explosive mixture out of its abdomen in a fairly well aimed stream at its attackers, however there are other Carabid beetles that spray with less accurate aim, and others that merely excrete bad tasting chemicals out of their abdomens when attacked. There is a whole spectrum from fairly simple to fairly complex defense mechanisms. Anti-evolutionists only talk about the more complex variant.

This discussion of the human knee is another example of this sort of argument in a vacuum.

While I am not an expert in the comparative anatomies of the living non-human apes and humans, as far as I am aware there is no material difference between them. That is, every bone, muscle, ligament, tendon, and cartilage in the human knee has its corresponding representative in the knee of chimpanzees and the other great apes (and presumably in the knee of their concestor). Yes they are shaped somewhat differently. Yes they are proportioned differently. But as far as I know all the same parts are there (if there are any primatologists or physical anthropologists out there, please correct me if I am wrong). [Note: see the 2nd Lovejoy quote provided by Adam Benton in the comments section below.]

As for fossil hominins, the knees of more derived types like Homo erectus (which are either “fully human” or “just apes” depending on what anti-evolutionist you talk to) seem to be virtually identical to those of H. sapiens. As for the knees of the more basal species of Homo (H. habilis) and the australopithecines these become increasingly like those of living non-human apes the farther back in time one goes. Exactly the sort of thing one would predict if humans evolved from an “ape-like” ancestor. The knee of Australopithecus afarensis (which most anti-evolutionists say is “just an ape”) retains a number of “ape-like” features but also has characteristics like those of later hominins including H. sapiens. In other words it is an intermediate form in this regard.

The knee bones of a modern human, an australopithecine and a chimpanzee.

See The ICR and Lucy: Bearing False Witness Against Thy Neighbor for more comparative photos, or refer to any good text on human evolution for comparative illustrations.

Burgess does mention living apes briefly but only to dismiss them as being poor bipedal walkers. However this is a problem for his argument for irreducible complexity (IC), at least as I understand Michael Behe‘s (the person responsible for the recent popularity of this term) definition of the term, in that while the knees of living non-human apes are slightly different in form, and are not as efficient for use in bipedal walking as those of humans, they do work, and they can walk bipedally.

Note: please remember that evolutionary theory does not postulate that humans are descended from other living apes, like chimps or bonobos, rather we share a common (ape) ancestor with those species.

So, assuming that knee joints of the ancestor of later hominins was essentially the same as those of the living non-human apes and could, like them, walk in a bipedal manner at all, then it would be possible for there to be a selective advantage for any slight modifications in their descendants which lead towards an increase in efficiency of bipedalism.

The human knee seems to me to be a poor example of an IC structure.

Some of Burgess’ other arguments just seem nonsensical to me. For example he states:

The knee joint presents a major challenge to the evolutionist because it is unique, and because there are no intermediate forms of joint between a condylar joint and the other two limb joints found in animals and humans – the ball and socket joint and the pivot joint. (Burgess, 1999)

I fail to understand Mr. Burgess’ challenge here. Knee joints did not evolve from elbow, shoulder, or hip joints. Rather knee joints have been knee joints since their origin in the first tetrapods. The same applies to the other types of joints. So why would we expect to find “intermediate forms” between them? That Burgess even posses this as a supposed problem for evolution demonstrates a significant lack of understanding about evolutionary theory and the fossil record.

Ichthyostega; an early tetrapod and its hind leg bones. Its “knee” was the joint between the femur and the tibia and fibula.

It is a curious thing that Behe’s principle of IC as an argument for design turns the traditional argument from design on its head. It used to be argued that those features of organisms that seem perfectly sculpted to suit their needs, or seem well designed from an engineering point was evidence for design. Now, under Behe’s IC principle of design, it doesn’t matter how clunky, ungainly, and poorly designed from an engineering point of view something is, it only matters that it is supposedly irreducibly complex.

Apparently the “Designer” under this new design “theory” is a (supernatural) cosmic Rube Goldberg.

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11 thoughts on “Is the human knee joint “irreducibly complex”?

  1. I thought you might find this interesting

    The BHBK [bent-hip-bent-knee] gait of Pan and Gorilla, however, is not a function of limitations imposed by hip or knee anatomy, but is instead a direct consequence of an absence of lumbar spine mobility. African apes are unable to lordose their lumbar spines, and therefore must flex both the hip and knee joints in order to position their centre of mass over the point of ground contact
    -Lovejoy, C. O., and M. A. McCollum. 2010. ‘Spinopelvic Pathways to Bipedality: Why No Hominids Ever Relied on a Bent-hip-bent-knee Gait’. Philosophical Transactions of the Royal Society B: Biological Sciences 36:3289–3299.

    Lovejoy cites an earlier article of his discussing the knee of primates and comparing humans A. afarensis to them. Whilst I haven’t had time to read through the whole article, in his conclusion he does overview the evolutionary history of some of the unique attributes of the human knee.

    The modern human knee enjoys the benefits of a long history of natural selection acting on its function in habitually upright walking and running. The characters that distinguish it include (1) a distinct valgus position during stance phase, whose origin is most likely an effect of differential forces acting at the growth plate and joint cartilage; (2) a genomically specified mechanism of patellar retention achieved by elevation of the lateral condylar lip; (3) elongation and changes in the form of both femoral condyles that provide tibial dominance, which both reduces cartilage stress in full extension and further modulates external rotation (beyond simple differences in the radii of curvature of the two condyles) of the tibia as the joint approaches full extension; (4) anterior prolongations of both condyles that functionally increase the moment arm of the patella and thus reduce the quadriceps force required during knee extension. Three of these adaptations were already present in the ancestral species, A. afarensis, whereas the last subsequently appeared in the human clade more recently.
    -Lovejoy, C. Owen. 2007. ‘The Natural History of Human Gait and Posture: Part 3. The Knee’. Gait & Posture 25(3): 325–341.

    As you can see, it is not irreducibly complex. Lucy et al. was perfectly capable of walking upright, despite lacking one of these key features. Further, these changes simply make the knee more efficient/robust. The basic structure would work perfectly well without them. If you lacked point 3 you wouldn’t topple over but would probably be more likely to damage your joint, for example.

    • Thanks Adam. Your second Lovejoy quote seems to confirm what my impression was, that the human knee, while modified by selection, isn’t different in kind from those of other apes (specifically in the quote australopithecines, which creationists dismiss as “mere apes”).

  2. One might pose the question: “if the human knee is so perfectly designed, then why do we humans have so much trouble with out knees”. Tetrapod knees were never “designed” to be held in a straight and locked position —– think about other mammals (not just apes), they all walk with bent knees.

    Note that a particular problem in humans is the tendency to dislocate the patella/tear cruciate ligaments because, with our knock-kneed (valgus) posture the quadriceps complex of muscles (that insert by a common tendon onto the patella but originate, in part, from the pelvic girdle) is now pulling on the patella at an angle (rather than in a straight line as in other mammals). Women, with their wider pelvic girdles, have a greater angle of pull of the quadriceps and so are more likely to experience knee dislocation.

    Or perhaps that’s just all part of Eve’s curse. Eat the wrong apple— tear your cruciate ligaments.

    • Ah but remember, under Michael Behe’s view it doesn’t matter how poorly designed from an engineering point of view a structure is, it is weather or not it is “irreducible complexity” (take any part away and it ceases to function) that demonstrates that it is of divine design.

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  5. Once again, the “irreducible complexity” folks forget about routine medical conditions. You can surgically remove a meniscus – knee still works; not as well, but still functional. Tear an ACL? Knee still works. Tear a PCL, MCL, LCL? Knee still works. Remove the patella? Knee still works. Etc, etc, etc. “The stupid…it burns!”

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  7. When you remove the ACL, PCL, MCL, or LCL of the knee, the stability of the knee is greatly diminished and you would not be able to perform at the same level as before (ie hunt). The problem doesn’t arise from apes and humans having a similar knee, one would expect them to as it has been proposed many times that they evolved from a common ancestor. The problem arises when we examine how this common ancestor came to have the knee.

  8. All placental mammals have a patella (i.e., kneecap) — marsupials have something similar, but not identical (a cartilaginous pad rather than a distinct bone). Birds also have a patella-like structure.

    The patella is a “sesamoid” bone — that is a bone formed within a ligament, in this case the ligament of the combined quadriceps muscles. The quadriceps arise from the femur (the 3 vasti muscles) or the ventral ilium (the rectus femoris). Their combined tendon inserts onto the tibia, and these muscles act to extend the lower leg on the femur (as when kicking a ball). In all mammals there are ligaments that anchor the patella to the tibia, although they are perhaps not as complex as in humans (I could go and look this up in a veterinary manual, don’t have time right now).

    The need for a patella to basically act as pulley/leverage for the quadriceps tendon arises from the hind leg of mammals (and birds) now being positioned under the body, rather than out to the side. In this posture the body is not rotating over a stationary foot around the knee, but rather knee is under the body and the lower leg needs to be extended rapidly to return the foot to the ground when swinging the leg forwards.

    All vertebrates that have their limbs under the body, and a very different type of locomotion from the basic tetrapod type (i.e., lizard-like) have a patella-like structure, with a ligamentous attachment to the tibia for stabilization. So, whatever the specializations of the human knee (and the ape knee) it’s simply a modification of the basic mammalian condition, which is a bit jerry-rigged because it’s been modified from the condition in a sprawling animal.

    As has been noted before, creationists live in a vacuum of understanding of the basic variety of form in nature, and of the anatomy of other animals besides Homo sapiens.

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