Theory of Asynchronous Evolution

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Evolutionary Theories of Asymmetrization of Organisms, Brain and Body (IV)

Right and left-handedness are not the norm and pathology. They both are normal, adaptive phenotypes for stable and variable environment, regulating behavioral plasticity of the society.

The main distinction from all the existing theories is that handedness is considered in relation to the dominant hemisphere (as cis- and trans-handedness), and not to the side of the body (left and right handedness).

People with right hemisphere dominance are rare (less than 1% of the population), therefore they will not be considered. People with left hemisphere dominance can be further subdivided according to the position of their dominant hand, which can be located on the opposite to the dominant hemisphere side of the body (discordant or trans-position), or on the same side (concordant or cis-position):

Left Hemisphere Dominance with Right Hand Dominance
(trans-position, ~90% of the population)

Right-handedness improves the blood supply to the subordinate hemisphere; this decreases lateral dimorphism, evens the hands, and increases symmetry.

 

 

 

 

Legend: CS, OS - conservative, operative subsystems brain;  cs, os - conservative, operative subsystems hands;
l, r - left, right; H - hemispheres of the brain; h - hands; CD - configuration of dominance;
≡ - corpus callosum;
x
- contra lateral nerve connections; | | - ipsilateral nerve connections; carotid arteries:         - subordinate,
         -
dominant,            - super dominant;  four levels of dominance:  ■ > ■  >   > ;

Left Hemisphere Dominance with Left Hand Dominance
(cis-position, ~9% of the population)

Hyperoxy of the Left Hemisphere of Lefthanders

Left-handedness (a higher functional load on left organs) improves the blood supply to the dominant left hemisphere.  This favors innovations in the right hand of the left-handed individual, i.e., creates or increases lateral dimorphism and asymmetry.

How new functions can appear in left-handed individuals?

All begins, probably, when the organism has a need to do something new, say, to draw. The lefthander, of course, tries to do it with the left hand. This improves the blood supply to the dominant left hemisphere (a brown arrow) increasing its dominance even more. Contact between the left hand and the left hemisphere is carried out through new, ipsilateral nerve connection. So, the left hand through ipsi-communication induces, as though forces the left hemisphere to solve a problem of new function development. As soon as the problem is solved and a control centre of a function is created, “instruction” gets to the right hand through the traditional contra lateral nerve connection (blue arrow). In the next generation, these innovations may appear in right-handed individuals as a new norm.

A "vanguard" subsystem in most cases is the left cerebral hemisphere and right organs.

Right organs acquire new functions, while left organs keep the old ones. Abundant experimental data on asymmetry in cetaceans confirm this theoretical suggestion. At dolphins usually the left ovary ovulates. Sperm whale is breathing with its left nostril only.

Population Level

Even more generalized concepts of left–right-handedness refer to a phenomenon as a whole or to many individuals. They should be applied to a population level, to characterize the asymmetry in quantity, variation and modality of subpopulations of left– and right–handers. They define evolutionary plasticity of a society at a level of behavior and mentality. These are analogues of, accordingly, the sex ratio, variation of sexes and sexual dimorphism. Differentiation into right- and left-handed individuals ensures the optimal conditions for the evolution of behavioral and mental traits.

[ETS: Differentiation into two sexes ensures the population-environment informational contact that is optimal for effective evolution]

The left-handed individuals constitute the operative subsystem of lateral differentiation and are analogous to males.  Conversely, right-handed individuals constitute the conservative subsystem (an original, basic, or regular state) and are analogous to females.  Therefore, at the population level, evolutionary innovations must originally appear in left-handed individuals and are then transferred to right-handed ones.

In the course of evolution, two behavioral (mental) patterns have developed for the two contrasting types of environment, optimal (stable) and extreme (changing).  These are the conservative pattern or preservation mentality, which is analogous to the female sex and is associated with right-handedness, and the operative pattern or exploration (reforming) mentality, which is analogous to the male sex and is associated with left-handedness.  A mechanism must exist for the environment-dependent regulation of the ratio between left- and right-handed individuals (L/R) in a population, which ensures the mental and behavioral evolutionary plasticity of the population.

[ETS: The sex ratio (M/F) determines evolutionary plasticity of the population.  Special regulatory mechanisms change this ratio depending on environmental conditions].

In humans, the L/R ratio is determined during the first trimester of embryonic development.  In the embryogenesis dominant hemisphere realizes dominant paired organs: in optimum environment—in trans-position, in the extreme one—in cis-position (on the same side of the body) and creates the new form of asymmetry for the organism and the population.  Under optimal conditions, a normal blood supply to the left hemisphere of an embryo results in regular (standard) right-handedness through contra lateral connections.  Under extreme conditions, any ecological or psychological stress in a pregnant woman induces embryonic hypoxia, suppresses the left hemisphere, and results in the dominance of the right hemisphere and, hence, left-handedness.  This inversion of dominance may occur in any paired organs (feet, eyes, kidneys, etc.). The more organs that are inverted, the more pronounced is the left-handedness of the entire organism.

Sex hormones determine the development of trans- and cis-phenotypes.

Generalized ecological rule: Extreme conditions shift the equilibrium [cis-] <=> [trans-]: for the embryos—to the left (increasing the birth rate of cis-individuals), for the adults—to the right (increasing their mortality), thus “the turnover” of cis-individuals gets increased. This gets done by androgens, which draw the system closer to the environment, and the estrogens which move the system away from it.

[ETS: Extreme conditions shift the equilibrium [♂♂] <=> [♀♀] for the embryos—to the left (rising the birth rate of males), and for the adults—to the right (increasing the mortality of males)].

Trans-individuals have wider reaction norm, narrower phenotypic variation and higher adaptability in the stable environment. For the cis-individuals everything is vice versa.

Terms left– and right–handers should be applied to the whole organism, considering not only hands, but also other paired organs, and more important, to the general manifestations of the lateral asymmetry on the behavioral and psychological attributes of an organism. This psychological profile reflects person’s adaptability in different environments. Right-handers are individuals at which D-hemisphere and D-hand are in trans position (on the different sides of a body). Their behavior is more adaptive in the stable environment. Left-handers have D-hemisphere and D-hand in cis-position (on the same side of the body), and their behavior is more adaptive in the changeable environment.

Psychology of trans- individuals (right–handers) should be similar to women — rational preservation strategy (conservatives, collectivists, conformsts). In politics— centrists, in chess — positional style.

Psychology of cis - individuals (left–handers) should be similar to men — adaptive in changing environment, irrational strategy of change (leaders, reformists, extremists) conservatives. In politics— radicals, in chess — combination style.

Differentiations of a population into left-handed and right-handed groups and an organism into left and right halves yield four types of paired organs (in the given case, hands, namely, right and left hands in each right-handed and left-handed individual. Left organs of right-handers are conservative on both organism and population levels. Similarly, right organs of left-handers are operative on both levels. Left organs of left-handers and right organs of right-handers are positioned somewhere in the middle of these two polar groups. Considering the direction of information flow (operative → conservative) one can come to a conclusion that: The evolutionary changes must emerge in the right organs of left-handed individuals and then be transferred to the right organs of right-handed ones.

The theory provides common grounds for the explanation of various phenomena.  For example, the high prevalence of left-handedness in twins and individuals with low weight at birth may be explained by physiological hypoxia.  The strong correlation between left-handedness and sex (a fivefold excess of men among left-handed, dyslexic, stammering, strabismal, and other individuals whose abnormalities are related to cerebral asymmetry) may be explained by the higher sensitivity of men compared to women.  The high proportions of left-handed individuals and males among prominent people and geniuses, as well as among imbeciles, may be accounted for by the fact that phenotypic variation in left-handed individuals is wider than in right-handed ones, and wider in men than in women.

The advantage provided by left-handedness in tennis, boxing, and fencing is commonly explained by their "inconvenience" for the adversary.  However, in this case, the inconvenience is mutual.  In addition, one can train taking into account the handedness of the adversary.  The true reasons are different.  Firstly, left-handed individuals have a better reaction time, because the right hand is controlled by the slower (rational) left hemisphere, while the left hand, by the quicker (automatic) right hemisphere.  Secondly, high resistance of the right hemisphere to stress is important under the stressful conditions of single combat.  Stress in general switches from the left-hemisphere dominance (the rational behavior) to the right-hemisphere one (instinctive, animal, automatic behavior expressed in the cases of fear, panic, the "crowd effect," etc.).  Therefore, we may expect that left-handedness will be also advantageous in “nonmanual” sports, such as chess, football, racing, jumping, etc.).

Foot

When our ancestors started to walk, the new command center appeared in the left hemisphere. It started to solve problems related to a new position of the body: equilibrium, narrowing aorta, and creating valves in the foot veins. After many generations the center translocated to the right-hemisphere and initial right-footedness became left-footedness. Therefore, in human ontogeny, according to recapitulation law, “child right-footedness” should become “adult left-footedness”. Such a transition was discovered in 1962 (von Bonin). He showed that at children from 6-12 years the right femur is bigger, while after the age 13 — the left one.

ETS—The Evolutionary Theory of Sex

The evolution of Asymmetry (I)                          ◄ Brain Asymmetry (II)                      Mechanisms (III)

   Questions                       Predictions

More about theory of asymmetry:
First publication: Evolutionary Logic of the Functional Asymmetry of the Brain. Geodakyan V. A.  Doklady Biological Sciences, 1992, v. 324, N 1-6, p. 283–287. Translated from Doklady Akademii Nauk, Vol. 324, No. 6, pp. 1327-1331, June, 1992.

Most complete scientific version: Homo sapiens на пути к асимметризации. Антропология на пороге Ш Тысячелетия. Москва 2003, т. 1, с. 170–201. [Homo sapiens on the way to asymmetry. Anthropology on a Threshold of the 3rd Millennium, v.1, Moscow, In Russian]

Available in English: A New Concept on Lefthandedness. Geodakyan V. A. and Geodakyan K. V. Doklady Biological Sciences, Vol 356,1997, pp. 450-454.  Translated from Doklady Academii Nauk Vol 356, No. 6, 1997, pp. 838-842.

 

Copyright 2005-2009 S. Geodakyan. All rights reserved.

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