Theory of Asynchronous Evolution


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Evolutionary Theories of Asymmetrization of Organisms, Brain and Body (III):
The relationship between brain hemispheres and sides of the body

Why nerves are crossed?
Why corpus callosum and ipsilateral nerve connections were developed?

Lateral asymmetry in Phylogeny appears in placental mammals (Karamian, 1970). If we compare structures of possum (bilaterally symmetric) and human (triaxially asymmetric) can help us understand role and mechanisms of lateralization. Bilaterally symmetric possum has simple contra lateral nerve connections between hemispheres and sides of the body (Fig). The reason for the cross over is unclear (Walker, 1980). Contrary to the nerves, energy connections (blood vessels) aren’t crossed (ipsilateral schema).


Legend: l, r - left, right; H - hemispheres of the brain; h - hands;
x – contra lateral nerve connections;          - carotid arteries.

Morphological and related functional features in human
 Phylogeny during lateral asymmetrization

Morphological features. 1. Corpus callosum — the largest white matter structure in the brain, consisting of 200-250 million contra lateral axonal projections connecting different regions of brain hemispheres. The corpus callosum has been reported to be larger in males (Bishop, 1997) and slightly larger in left-handed people than right-handed people (Witelson, 1985; Driesen, 1995). 2. Ipsilateral connections—direct nerve pathways connecting each hemisphere with the same side of the body (Fig).

Functional features. 1. Asymmetry and hemisphere specialization manifested in dominance of one hemisphere over the other on different phases of functions’ evolution. 2. Asymmetry of hands and other organs. 

Legend: l, r - left, right; H - hemispheres of the brain; h - hands;
x – contra lateral nerve connections;  
        - carotid arteries;
≡ - corpus callosum; | | - ipsilateral nerve connections.

All abovementioned evolutionary innovations are closely related to lateralization, however their purpose is unclear. Moreover some scientists think that intra hemisphere connections can only level their changes, therefore different hemispheres should require less connections than similar (Pribram, 1975).

Evolutionary role of corpus callosum, contra and ipsilateral nerve connections

Contra lateral nerve connections. One can assume that increasing brain blood flow can improve regulation. , then contra lateral nerve connections form negative feedback thus providing symmetry of hands and other organs. So, evolutionary role of contra connections is to preserve lateral symmetry of bilaterally symmetric or certain level of asymmetry of triaxially asymmetric organisms.

Corpus callosum creates sequential connection between hemispheres and their asymmetry.

Evolutionary role of ipsilateral connections is to create positive relations that change the equilibrium state. They provide adaptation to new conditions.

The picture closely resembles that of the refrigerator: negative feedback preserves the required temperature, while positive connection—changes it.

The evolution of Asymmetry (I)              ◄ Brain Asymmetry (II)                         Asymmetry of paired organs, hands (IV) ►
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