Human balance, the evolution of bipedalism and dysequilibrium syndrome
Introduction
Humans are biologically novel in several respects including language, dexterity and complex culture. One trait which tends to get overlooked is bipedality. This is perhaps because two legged locomotion is biologically common. However, the manner in which humans stand, walk and run only occurs in our own species.
Briefly, the human body is arranged vertically such that the head, trunk, legs and feet and their links in the neck, spine, pelvis, knees, and ankles dynamically balance together to form an upright “antigravity pole”. Since these segments and their points of articulation are not fixed, and the downward force of gravity never stops, the erect body exists always an inch or two away from falling. There is no locomotive use of holds, as with other primates. That the body is constantly upright is due to the unending skeletomuscular adjustment of posture (in the manner of a segway scooter or iBOT wheelchair). The habitual stance of no other animal has this total dependence upon the maintenance of vertical balance.
Here I propose a new theory about the nature and evolutionary uniqueness of the balance faculty responsible for human erectness, and the etiology of a rare autosomal recessive condition, dysequilibrium syndrome, in which this balance faculty is developmentally defective. The core of my theory concerns the evolution of a novel addition to the primate balance processes that enables humans to engage in their proficient bipedality: the internal models of the nongravitational z-axis body image.
The separate processing of gravitational and nongravitational information by the human balance faculty has already been established by neurologists [1], [2], and by those studying the biomechanics of human balance [3]. Building upon this, I propose that:
- (a)
the z-axis body image arose from the unique geometrical situation of terrestrial bipeds. In such bipeds, the constant perpendicular uprightness of body axis to the ground support (egocentric geometry) is put in direct alignment with the gravitational vertical (allocentric geometry);
- (b)
that the advantages of this alignment only came to be fully exploited for balance when hominid brain size expanded with the evolution of Homo erectus;
- (c)
that it has a close integration with higher cognition. As a result, the bipedal balance faculty has the same species specific relationship to humans as other higher cognition dependent faculties such as language or dexterity;
- (d)
that it is a defect in this balance faculty that underlies the rare autosomal recessive balance disorder of dysequilibrium syndrome [4], [5], [6];
- (e)
and that the cognitive, neurological, and genetic impairments of dysequilibrium syndrome and other developmental balance disorders provides a window with which science can study the information processing, neurology and evolution of human bipedality.
Section snippets
Human bipedalism
Bipedalism, as a descriptive term for the use of two legs for standing and locomotion, can be applied to a variety of animals. These include, as an occasional method of locomotion, primates (such as macaques, chimpanzees, bonobos, gorillas), birds (in general, and as a specialization in flightless ones such as penguins, ostriches and emus), extinct reptiles (Tyrannosaurus rex), certain lizards and even cockroaches [7]. However, human bipedal posture and anatomy are unique in four fundamental
Internal models
If the body’s center of mass goes outside its base of support, body instability starts within 100 ms [21]. Since it takes longer than 100 ms for the brain to detect an instability and then make compensating changes to posture, well-organized balance corrections must be initiated in advance. The nature of such forward feed corrections will invariably be complex. Consider an arm lifted from the side of the body to reach an object. This will need the skeletomuscular system to slightly shift back the
Dysequilibrium syndrome
Given the central role of the balance faculty in human biped standing and locomotion, it might be expected that it could be impaired during developmental. The impairment of this faculty would, however, not underlie all individuals with balance problems since balance can also be affected by dysfunction to sensory input or the motor coordination needed to make balance adjustments.
Only one developmental condition exists that can be directly attributed to the central impairment of the balance
Is bipedally primary an anatomical adaptation?
The anatomy of primates, particularly apes, can readily adapt to bipedal locomotion, in spite of its evolution for quadrupedalism. As noted above, great apes as part of their locomotive repertory occasionally walk bipedally, and can even do this permanently following forelimb injury or developmental defect. In the opposite direction, the recent Turkish cases of dysequilibrium syndrome demonstrate that the human skeletomuscular framework though evolved for bipedality retains the ability to
Neurology
At present, acquired brain lesions have allowed neurologists to locate the gravitational and nongravitational components of human balance respectively to (i) the cerebellar-cerebral circuits centered upon the vestibular area in the posterior insular cortex, and (ii) the ventral posterior and lateral posterior nuclei of the posterolateral thalamus and the associated cortical projections [2]). MRI scans of the brain abnormalities of those with dysequilibrium syndrome will be able to confirm this
Acknowledgements
The author thanks Nick Humphrey, Rob Seymour and the past and present directors of CoMPLEX (LSE) and CPNSS (UCL) for their support and collaboration.
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