In 2024 I published a Review paper in the journal of Bio-inspiration and Biomimetics published by the Institute of Physics. It was published In September 2024. The title is:
Universal optimal design in the vertebrate limb pattern and lessons for bioinspired robotics
The link to the paper is:
https://iopscience.iop.org/article/10.1088/1748-3190/ad66a3
One of Darwin’s iconic arguments for evolution is the homology of the vertebrate limb. He argued that the striking similarity of the skeletal layout of the limbs of land mammals, marine mammals, birds and amphibians can be explained only by evolutionary inheritance.
Darwin made the assumption that the vertebrate limb was not universally optimal. i.e. Darwin assumed that the vertebrate limb pattern was not a good concept for some animals such as whales and birds. However, he did not justify that assumption. Since the time of Darwin, biology textbooks have used the homology argument as evidence for evolution. Like Darwin, these textbooks have not justified the claim that the vertebrate limb is not universally optimal. Biology textbooks commonly state that the whale flipper is not what would be expected by optimal design.
The whale flipper in particular has always been assumed to have a strange design with the elbow, wrist and digits assumed to be a poor ‘concept’ for the flipper application. Of course, the ‘poor concept’ is assumed to be adapted for the needs of whale locomotion.
However, my review paper shows that the vertebrate limb pattern is far more versatile than previously thought. The above paper reviews the optimality of six vertebrate limbs: the human arm, whale flipper, bird wing, human leg, feline hindlimb and frog hindlimb.
The vertebrate limb pattern is shown to be a remarkably versatile design solution that can be fine-tuned to produce the whole range of shapes and motions required for all types of vertebrates. The vertebrate limb is so versatile that it is highly optimal for flippers and wings not just arms and legs. This contradicts Darwin’s basic assumption that the vertebrate limb is not universally optimal.
The segmented design of wrist/ankle/phalanges is particularly effective at enabling morphing of shapes whilst maintaining smooth curvatures – a feature that is particularly important for flippers and wings. The segmented design is also effective at enabling multifunctioning of kinematic and structural functions with compact layouts – a feature that helps explain why animal limbs are superior to robotic limbs. Another key feature of the vertebrate limb pattern is that of linkage mechanisms that fine-tune motions and mechanical advantage – a feature that helps explain the advantage of parallel bones in the lower limb.
In the case of the whale flipper, the wrist and digits are shown to be fully functional muscle-actuated joints that enable fine-tuning of stiffness and shape of the flipper which is very important for agile and efficient swimming. Flipper stiffness and shape are fine-tuned through tensioning of the wrist and digit muscles. Far from being a strange feature, the whale ‘fingers’ are an effective and efficient design.
For example, as a whale swims faster, the drag increases on the flipper and therefore the wrist and digit muscles need to apply more force to maintain the flipper position and shape. If the flipper deforms significantly, controlling swimming becomes more difficult. In addition, the lift-to-drag ratio of the flipper hydrofoil decreases and the energy required to swim increases.
When the whale does need to change flipper shape, the segmented design of wrist/ankle/phalanges is particularly effective at enabling morphing of shape whilst maintaining smooth curvatures. Even the elbow joint of the whale contains optimal structural features – such as shear keys – for producing efficient load paths through the limb.
In the case of the bird wing, the elbow, wrist and digits all perform important wing movements. The elbow enables wing retraction as well as wingspan adjustment. The wrist muscles are used to move the primary feathers in supination-pronation and flexion-extension (Table 2). The first digit muscles move the alula feathers whilst the second digit muscles flex and extend the wing tip feathers.
The optimality of the vertebrate limb has important applications to robotics because engineers need to know which animal limbs are optimal when producing bio-inspired designs. Engineers also need to know why animal limbs are so compact and efficient so they know what to copy. The paper recommends that engineers treat all vertebrate limbs as highly optimal.
The paper seriously challenges the homology argument and shows that intelligent design is a better explanation to the homology of the vertebrate limb.
Professor Stuart Burgess 