Theory of Asynchronous Evolution

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

“We know far too little about the adaptive importance of directional asymmetry”
R. Palmer (2004)

What is the biological role of left-handedness and it's evolutionary importance ?
What is the fundamental difference between left- and right-handed individuals?
How handedness is related with brain asymmetry and sex?

Features of the symmetry are determined by the isotropy of the environment. Maximal extent of organism symmetry corresponds to a completely isotropic ecological niche.

First organisms on Earth floating in the depths of water unicellular and lower multicellular organisms had the maximum possible spherical symmetry. They appeared approximately 3.5 billon years ago. Anisotropic environment step by step was leading to asymmetry of the organisms.

Asymmetrization along the “top – bottom” axis occurred under the influence of gravity. This led to the appearance of the attached, low-mobile forms (plants and coelenterates) that had radial symmetry.

Asymmetrization along the “front – back” axis occurred due to the interaction with space, when rapid motion was required (to escape from the predator, or to chase a pray). As a result, the main receptors and the brain were moved to the front of the body. Organisms with bilateral symmetry were dominating last 650-800 million years. These are crustaceans, fish as well as the most progressive forms, i.e., mammals, birds, and insects.

V. Beklemishev (1964) distinguished three types of symmetry (spherical, radial, and bilateral) and arranged them in evolutionary array (Fig. 1). Forth type of triaxial asymmetry (aaa) he mistakenly assigned to a primitive organism (amoeba) and placed at the beginning of the array. Organisms of bilateral symmetry were considered the “crown” of evolution.

Amoeba     →      spherical       →         radial           →        bilateral symmetry

Fig. 1. The evolutionary array of organisms’ symmetry types.
        c — symmetry,
а — asymmetry of an organism on
      each of the 3 axis of the three-dimensional space.

What determines asymmetry along the “left – right” axis was unknown.

Basic statements of the new theory:

1.    The evolution of any structure goes from symmetry to asymmetry. The asymmetry of the organisms in the Phylogeny grows on all three axes (I — dorso-ventral (back – belly), II — anterior-posterior (nose – tail), III — and mediolateral (left – right).

2.    Symmetry and asymmetry are features of a shape. Amoeba is shapeless and therefore should be removed from the evolutionary array. Extrapolation of evolutionary logic of the existing array: ccc аcc ааc    have allowed to state a hypothesis about further asymmetrization of modern progressive forms and their natural transition from bilateral symmetry to the last type—triaxial asymmetry (TA) (аас ааа). Growing number of the facts of lateral asymmetry found in modern progressive forms (functional asymmetry of a brain, right handedness in humans, unilateral ovulation and unihemispheric sleep of dolphins) confirms this transition.

3.    Which factor of the environment dictates the development of lateral asymmetry?
According to the idea of asynchronous evolution such factor can be time,
so one side (organ) is more advanced ("vanguard" as, already in the future) and another one is a "rear-guard" (as yet in the past). Triaxial asymmetry type should be the most evolutionary advanced (Fig. 3).

Fig. 3. Adaptive asymmetrization of organisms in anisotropic environment

4.    So, the information flows from the isotropic environment create asymmetry in organisms. How the field acts upon the organism and why it evolves? Let’s take one-dimensional (linear) organism — hydra in the field of gravity and earthworm (or snake) in space.

Phenotype appears from interrelation of genetic and environment information: P = G + E. The source of genetic information is zygote. Since the organism is developed from the zygote, in hydra it should be located at the bottom of the foot, and information flow should move to the top towards tentacles and at earthworm zygote at the end of the tail and genetic information flows to the nose, so tentacles are younger than the leg and nose — younger than the tail. Can development start from the nose? No, because ontogeny repeats phylogeny. Can it start in the middle of linear organism? Yes, if development starts at the border of two environments (ground – atmosphere as in the case of plants, then seed is growing up and down). Information from the environment always go towards (growth) and forms the gradient of potential. Then the new organ (character) should appear at the point of growth. If they are necessary their, like receptors or brain, they stay others drift along the gradient towards the tail. The Evolutionary Theory of Sex allows verifying this hypothesis. Hydra had all three types of reproduction: asexual (using kidneys), hermaphrodite, and dioecious, and earthworm had asexual and hermaphrodite. Theory of Sex predicts that ovaries at hydra should be between kidneys and testes, and at earthworm — closer to the tail than testes. That is so. Hydra has kidneys at the bottom, ovaries higher and testes even higher. Ovaries of earthworm are in the 13th segment and testes — only in the 10th.

5.   Organisms of triaxial asymmetry keep two previous asymmetries. Lateral asymmetry appears on a background of previous ones (a ‘back–belly’ from a jellyfish, and ‘nose-tail’ from the opossum), therefore it should spread from front to back.

6.   Increase in asymmetry can be seen in evolution of many systems:

  • The trend towards asymmetry can be followed in phylogeny of plant organs (flower, leaf, fruits, and seeds). It is known, that zygomorphic (bilateral symmetry) flowers [Gladiolus sp., Orchids, Eyebrights and Violets] are evolutionary more progressive, than actinomorphic (radial symmetry) flowers [Primula, Narcissus, Pyrola], but are less progressive, than triaxial asymmetric ones [Cannaceae and Valerianaceae].

  • The morphology of a leaf during evolution follows the same picture: spherical symmetry of chlorella, radial symmetry of pine needles, bilateral symmetry of Magnolia leafs, and triaxial asymmetry of Begonia or Elms leafs.

  • The same trend can be found in embryogenesis—spherical zygote, radial gastrula, bilateral embryo and triaxial asymmetric child. Vertebrate embryos (and indeed, many invertebrates) exhibit a strikingly conserved left-right asymmetry of the internal organs. Nearly all visceral organs of the thorax and abdomen are left-right asymmetrical in their anatomy, placement, and, in some cases, physiology. Directional left-right asymmetry is conserved throughout chordate evolution, although the details of anatomical asymmetry can vary among species.

One can conclude that asymmetrization is not especially human phenomenon, but general evolutionary phenomenon inherent to all live systems.

Brain Asymmetry (II) ►                    Asymmetry of paired organs, hands (III) ►                      Questions                       Predictions

References

Evolutionary role of asymmetrization of organisms, brain and body (Model and Right-hand rule). Geodakyan V. A. 20th Congress of the I.P. Pavlov Physiological society. Symposium: "Functional interhemisphere asymmetry”. Thesis of presentations. June 4-8, 2007, Moscow. p. 28.

 

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

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