Writing in the journal Bioinspiration & Biomimetics, engineer and biomimetics expert Stuart Burgess recently analyzed the “Universal optimal design in the vertebrate limb pattern and lessons for bioinspired design.” His article demonstrates that the similarities between vertebrate limbs is best explained not by common ancestry but by intelligent design. He explains how the general vertebrate limb layout (aka architecture or plan) is the optimal design pattern for complex motion in diverse environments. He also demonstrates that six specific limb designs in five different vertebrate taxa (i.e., groups in animal hierarchy) are the best possible designs for the animals’ environment and behaviors.   

Evolutionary Assumptions

Biologists have long recognized that vertebrate limbs are built around the same basic layout (Figure 1). 

The standard evolutionary model assumes that the similarities between the limbs are best explained by common ancestry. For instance, similarities between tetrapod (four-footed animal) limbs in animals today result from the common ancestor of all tetrapods evolving an earlier version of the limb and then passing it down with evolved modifications to its descendants (Figure 2). 

The standard model predicts evolutionary processes were constrained to modify only the initial design without fundamentally changing it, so the resulting designs should often be suboptimal. This assumption misled many biologists to conclude that some tetrapod limbs represent clumsy and inefficient designs (here, here, here). 

Better Explanation of Similarities

Burgess summarizes the basic design pattern of tetrapod limbs as follows:

In the case of amphibians, reptiles, birds, and mammals, there are four particular skeletal features that are generally present in the limb:

1. Three main joints (shoulder-elbow-wrist or hip-knee-ankle)
2. Two parallel bones in the lower limb (radiusulna or tibia-fibula)
3. Network of small bones in the wrist or ankle
4. Multiple multi-jointed digits (typically five digits)

He argues that the best explanation for tetrapods consistently employing this layout is not common ancestry but how it represents the best general architecture for limbs:

A central finding of this study is that the vertebrate limb pattern is highly versatile and optimal not just for arms and legs but also for flippers and wings. … The triple-hinged layout — shoulder/elbow/wrist or hip/knee/ankle — is optimal for each application because it enables the limb to perform the whole range of limb functions of deployment, retraction, flapping, running, and leaping.

In a previous lecture Burgess illustrated how the triple-hinged layout is commonly employed in human engineering since it is the best design for products that perform complex movements. That is due to its flexibility, stability, and ability to accommodate motion in multiple dimensions. Burgess explained how the design is employed in machines as diverse as digging rigs, industrial robots, and satellite deployment mechanisms. Needless to say, no one believes the similarities between digging rigs and human limbs is due to their sharing a common ancestor. 

Humans, Whales, and Birds 

Burgess explains how the triple-hinged layout was meticulously tailored in each vertebrate tetrapod to meet the needs of that group. He details the perfection of the design of the human arm, human leg, cetacean (e.g., whale) flipper, bird wing, feline hindlimb, and frog hindlimb. 

Burgess summarizes the evidence for the masterful design of the human leg that he published previously in BIO-Complexity (here, here, here). He also details a multitude of optimized features in the human arm, including the hand, such as the exquisite design of the system of bones, muscles, and tendons. For instance, he describes the wrist as follows: 

The eight wrist bones also form a double joint in wrist flexion-extension. As with abduction-adduction, the curvature of the bones is fine-tuned to give an approximate common centre of rotation in flexion and extension. As before, this is important to give smooth and consistent motion.

He also describes how cetacean flippers are perfectly engineered for locomotion in aquatic environments. For instance, numerous mechanisms ensure that the flippers constantly adjust to maintain the ideal shape and stiffness:

Flexor and extensor tendons are inserted into each digit joint so that the muscles can maintain smooth curvature of the flipper during load-induced bending or during flexion and extension. Some whales also have wrist adductor and abductor muscles that can control in-plane stiffness and shape. The fact that flippers have many sensors to detect hydrodynamic loading and the magnitude of vortices indicates that fine-tuning of stiffness and shape is important for achieving high hydrodynamic efficiency.

The cetacean limb dramatically differs from that of other tetrapods due to its distinct environment and operational goals. 

Later, Burgess explains how bird wings are optimized for flight. For instance, the shoulder is meticulously engineered to enable wing flapping and twisting and to maximize energy conservation:

Three rotational degrees of freedom at the shoulder are needed for flight. Flapping is needed to create lift, twisting is needed to change the angle of attack and a variable sweep angle is required for manoeuvres like braking. Bird shoulder joints also utilise elastic energy storage to improve the efficiency of flight.

Here again, the limb dramatically differs from that of other tetrapod limbs due to the unique requirements of flight. 

Failure of Evolutionary Explanations

The standard evolutionary model cannot explain the perfection of design in vertebrate taxa. That is for several reasons. As mentioned above, evolution predicts that vertebrate limbs should often appear clumsily designed, which is the exact opposite of what Burgess and others have demonstrated (here, here). In addition, the distribution of similarities in animal groups is often so inconsistent with any evolutionary tree that similarities cannot be trusted as evidence for common ancestry (here, here).

Equally problematic, undirected evolutionary models predict that the manufacturing of the limbs and the entire body should look very similar. If random mutations gradually modified the limbs and other body traits of the MRCA without altering the underlying design layout, the embryology and genetics should also have remained largely intact. Again, the truth is often the opposite. Limb similarities between different taxa can be generated through different embryological processes controlled by different genes (here, here), and different vertebrates’ entire embryology can differ at every stage of development. These incongruities suggest that each animal was designed independently. 

Moreover, the time available for the evolution of many limbs is far too short for such dramatic changes to have occurred without intelligent guidance (here, here). The challenge is particularly daunting in the case of whales. The cetacean flipper differs in almost every respect from that of its proposed terrestrial ancestor, but the radical transformation to aquatic life is believed to have occurred in a timeframe less than what is required for the arrival of only two coordinated mutations. Far more coordinated mutations would have been required to evolve even one of the novel limb features that Burgess lists. This evidence, as well as other data, clearly favors the design hypothesis.