In this essay I will explore how the human foot can be used as an example of perspectives and approaches that are used in biological anthropology. The perspectives and approaches will include macroevolution, microevolution, the comparative approach and the biocultural approach. I will define each approach and give examples in relation to the evolution of the foot and bipedalism.
The biocultural approach combines both biological anthropology and social/cultural anthropology when seeking to understand questions raised by anthropology. It is the notion that humans are understood on both biology and culture. Daniel J. Hruschka defines the biocultural approach as “a critical and productive dialogue between biological and cultural theories and methods in answering key questions in anthropology”. However, it is really a dynamic space because of the everchanging definitions to both culture and biology. The evolution of the foot towards bipedalism became more advantageous when environmental changes pushed heavily forested environments to become open grassland, moving the viable food sources to the ground. Bipedalism means hominins could focus on rearing young or seeing over high grasses to locate food thus increasing the chance of survival. Also, freeing up the hands led to the development of tools, however, tools were not the cause of bipedalism but rather an advantageous consequence of it. This led to being able to specialize in certain tasks and therefore contributing to social interaction and, as a consequence, the evolution of culture within the early hominins.
The comparative approach is endeavoring to understand something by understanding what the thing is and also what it is not. In primatology, by studying our closest relatives and ancestors we can decipher what behaviors and biology is fundamentally ‘primate’ and where the similarities end and thus finding out what is fundamentally human. In human, the hindlimbs are used for propulsion. The entire torso weight of the body is pushed through one foot at a time, thus the foot must be very robust to accommodate that weight and also be able to provide toe-initiated propulsion. The hallux (big toe) is much more robust compared to other primates. The calcaneus (heel bone) is also more robust and larger than other primates. These both help with stability and absorbing forces created when walking.
The shape of the calcaneus provides a base for ligaments to attach to, creating a double arch system to help absorb shock when the foot hits the ground. Humans have a non-opposable big toe or what is called a fully adducted hallux. This is because humans have a flattened surface of the medial cuneiform. Therefore, lateral movement is restricted, and the big toe is held parallel to the other toes. Human toes are also shorter in length when compared to the other primates and they cannot grasp with their toes. Toe length is most likely to be an adaptation of obligate bipedalism and it reserves more energy because the longer toes and divergent hallux of other primates impede bipedalism. Macroevolution is evolution above species level, however no one can come to an agreement about whether macroevolution occurs due to an accumulation of small changes that result from microevolution or whether it is its own separate event. Obligate bipedalism is an example of macroevolution because it is only present at hominin species level. Obligate bipedalism and the evolution of the foot in order to become solely bipedal was not the acquisition of a new way of moving but rather the choice to solely concentrate on bipedalism, something it had already been doing. Obligate bipedalism evolved from facultative bipedalism. John Marks comments that “the evolution of bipedality was not the change from a quadrupedal state to a bipedal state but rather a constraint on the locomotor systems of the ancestor who evolved from something that could be bipedal to something that could be nothing but bipedal”.
Microevolution is evolution below species level, on a small scale. It is change within a species as a response to natural selective pressures or if the change became advantageous to the species. Due to said selective pressures or advantages, the foot became more stable, but the tradeoff was flexibility. The big toe adjusted for weightbearing rather than grasping. The anatomy of the ankle varies between primate species which suggests that there are variations of walking styles between different species and therefore between the early hominin species. It is unclear whether the differences in the ankle anatomy are just simply anatomical nonsense that doesn’t really have any relevance to walking style or whether they have meaning. Human locomotion is hugely varied and is not a universal concept. There are subtle differences in gait, style, speed and endurance between all humans.
There are a few models that combine the comparative approach and microevolution leading to macroevolution. The foot of the last common ancestor to modern humans and the great apes was argued by Dudley Morton to be an intermediate form between Pan and Hylobates with smaller toes. He also argued that the hypothetical early hominin foot was an intermediate between gorillas and modern humans. Morton also hypothesized that the ‘pre-human’ foot also had some sort of grasping ability in the form of an opposable hallux but overall had shorter toes than the gorilla. The foot would have also had an enlarged heel bone for bearing weight and absorbing shock. It would also lack a longitudinal arch. Morton suggested that modern human ancestors were more similar to gorillas rather than closely resembling Pan. However, he didn’t work with any fossils, he just had comparative material.
Other models of bipedalism evolution are Lewis (1989) and Kidd (1999). They differ in that they go into the anatomical features of the evolution from ape-like feet with an opposable hallux to bipedal human feet with an adducted big toe which is parallel with the lateral toes which is therefore more stable and acts as a propulsion lever.
Lewis’ model challenges the ‘normal’ model of the evolution from the ape-like foot to the human-like foot. He argues that the toes stayed tightly packed together and the hallux stayed close to the lateral toes and therefore the rest of the foot realigned towards the hallux. The traditional model depicts the 1st tarsometatarsal adducting in line with the axis of the foot. The foot then everts making the sole of the foot flatter on the ground. Lewis argues that this results in the 1st tarsometatarsal becoming more unstable because it results in the toes being more loosely held together in position. However, he fails to consider that when the hallux adducts and remodels the joint morphology also remodels as well to be able to reach maximum stability.
The overall function of closely packed toes differs between great apes and humans. In great apes it helps with grip wherein humans it helps transfer weight when propelling off the toes.
Kidd based his model of the calcaneus, talus, cuboid and navicular of a Homo habilis foot. He suggests that the talus and navicular is basically ape-like while the calcaneus and cuboid is essentially human-like. Kidd argues that because there is no medial longitudinal arch and the fossil has an opposable toe it is ape-like and it has human-like characteristics in the lateral column. Thus, he suggests that the first part of the foot to evolve was the lateral column because it aided with stabilization. However, Kidd’s model is only based on one fossil and his study failed to include an analysis of the medial cuneiform therefore his model can only be counted as an untested hypothesis.
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