Vertebrate brain theory

2.4  The tunicates - ancestors of the vertebrates?

A special form of living beings of replication stage 2 is the spherical form, in which the cells first form a sphere, later a hollow sphere, which is filled with liquid and resembles a blastula. This form occurs in all vertebrates at the beginning of their embryonic development. Thus, in individual development, evolution is virtually repeated on a small scale. The fertilised egg cell corresponds to the stage in which the living being consists of a single cell. This cell begins to divide, whereby the daughter cells that develop do not separate from each other - as in a colony - but form a spherical cell cluster. Afterwards - as in the development of the colony into a multicellular organism - the cells begin to differentiate and divide their work.

One can well imagine the further development in the course of evolution towards higher animals; here too, embryology provides an idea of how the further development of multicellular organisms up to vertebrates could have taken place.

We start our considerations with a multicellular organism, which in its simplest form consists of cells that form a spherically arranged single-cell layer. This corresponds to the appearance of a bubble nucleus, called a blastula, and develops directly from a fertilised egg cell. The inside of the cell sphere is filled with liquid. To the outside and inside, the cell structure is delimited by the associated cell membranes.

If such a cellular hollow sphere consisting of a single-celled layer on one half, like a football from which the air has completely escaped, a hemispherical structure is formed, which consists of two cell layers, between which there is a remaining cavity. One could call them the most elementary hollow animals. The simplest tunicates could have emerged from this structure. The inversion of the blastula is called invagination, the result of the inversion is called gastrula. The transition from the blastula to the gastrula is called gastrulation.

The two layers that make up the gastrula are called cotyledons. The previous outer layer is called ectoderm, the inner layer entoderm. Such a structure can be imagined in the earliest animal life forms. In the course of evolution, further cells formed between the ectoderm and the entoderm, forming the mesoderm, which is called the third cotyledon.

Parallel to the specialization of the existing cells for different tasks, organ formation took place. Cells with the same task were spatially concentrated into organs.

So, the coat-animals (Tunicata), from which the vertebrates derive according to predominant opinion, developed among other things. During the sexual reproduction of tunicates, the fertilized eggs develop into larvae, which have a chorda and a neural tube, among other things. Therefore, the tunicata belong to the chordates, which also include the vertebrates.

Hubert Fechner writes about this in [121] on page 9:

"With the tunicates we encounter a type of building plan, which, after some constructive improvements and functional changes of individual building elements, reaches its highest training levels among the vertebrates.

The essential construction elements that characterize this construction plan are

1.      An elastic axial rod - the chorda - which lies dorsally of the intestine and extends along the longitudinal axis of the body and serves as support and abutment for the locomotor system.

2.      A dilated forearm, the side walls of which are pierced by several slits through which water is transported out of the intestine - a formation known as the gill gut.

3.      A neural tube lying dorsally of the chorda." (end of quote)

 
In [121] on page 10 it says about the tunicates (tunicata):

"The epidermis is named after its ability to separate a more or less powerful cuticle - the tunica - which develops complex structures.  

In [120] on page 838 the jacket is described in more detail:

"Unique in the animal kingdom is that this mantle, which is predominantly formed by the epidermis, contains cellulose fibres ("tunicin") in addition to water and proteins.

So here cellulose is formed by animals, which otherwise occurs predominantly in the plant kingdom. According to the endosymbiont theory, one imagines that a eukaryotic cell takes up a cyanobacterium without digesting it completely. This becomes the chloroplast and gives the resulting new living being the ability for photosynthesis. This could be the origin of the higher (multicellular) plants.

If some cellulose-producing tunicates succeeded in integrating such cyanobacteria into their bodies in early times and using them as chloroplasts, plants (among other things) could have developed from them.

On the other hand, it could have been the other way around. Some Tunicata ancestors may have been sedentary plants that lost the ability to photosynthesize while cellulose formation was maintained.