On the origin of bilateral symmetry PaleoStoria We speculated on the importance of embodiment and what may have been the evolutionary steps that led to the development of this fundamental property of animal life.
Today, I will speak of another great achievement of development, another innovation that significantly changed the lives of animals: the development of bilateral symmetry. As we
seen in the post
Basics Part 1 PaleoStories , most of the bodies with the body can be divided into two main categories, those whose body can be divided into two parts mirror for more than two or more axes, and those on the other hand whose body can be divided into two symmetrical parts (right and left) by a single axis of symmetry. (The situation is a bit 'more complicated, but I'll explain later). The former are such animals radial symmetry (or radial) and are, for example, current cnidarians (including jellyfish) and ctenophores, a bizarre group of marine animals tentacles.
to say so, it seems that the problem is only in the geometric concept symmetry, which in itself may seem irrelevant from the anatomical point of view.
However, if you look at the internal anatomy of animals radial symmetry and symmetry side, you'll immediately notice that the two different forms, there is a gap very meaning of anatomical complexity, two-sided with the animals that possess various anatomical features that the deletions do not have and which are of extreme importance from a functional standpoint. Data in hand, we note that over 99 percent of living species of metazoans are bilateral and that they have a much more complex structure and distribution and a much more diverse ecology than deletions.
then be a two-sided, it's not just!
But if the bilateral symmetry allowed the animals to develop an anatomically far more complex than that radiated, as we have moved from one floor to another body? You can trace the origins of bilateral symmetry, groped to understand when and how did this pass?
The answer is yes, but the question is rather complicated.
The starting point for all phylogenetic analysis (and then groped for something to understand the evolution of different groups) is to recognize what characteristics distinguish one group from another. In this case, therefore, we must consider whether apomorfici that allow us to separate the two-sided by the other animals.
Not all scholars agree in recognizing a set number of synapomorphies for the two-sided, but, in order of importance, we can divide the distinctive features (or rather deemed distinctive) of two-sided in three large groups, in order of consent:
1. Almost all scholars and phylogenetic analysis recognize that all have two-sided two orthogonal axes, an anteroposterior and a dorsoventrale, that just makes these animals "bilaterally symmetrical," a nervous system with a concentration of nerve cells in the central part, which is Then the nerves branch off towards the rear, and triploblastia (during the development of genetic forms a third layer, the mesoderm, deputy to the formation of some special structures, such as the axial skeleton - when present-the muscles, excretory system, and others. In animals not triploblastici we only have two layers, ectoderm and endoderm, whereas in the first two triploblastici addition, in the middle, the mesoderm. I do not want to dwell too much on these biological issues, but I hope to have been clear enough).
2. Other features that are sometimes considered to be distinctive for the two-sided are the presence of an excretory system, a real channel stomach (ie, mouth and anus), a primitive eyes and a set of genes including Hox genes at least 7-8 (I Hox genes are particular types of genes that play an important function to grow the cells and organs in their right place. Which are genes that regulate the development of the anteroposterior body, those that regulate the development of ventral spine, those who "say" in the arts where it grows, or where "should be placed" the eyes, etc.. Hox genes on a specific post will be discussed later)
3. Finally, some would be considered synapomorphies of Bilateria also possess a coelomic cavity, a heart, and a segmented body with appendages. On this third point, however, many scholars agree, and consider these features are not diagnostic for derivatives and Bilateria.
Why have I done here? Why on diagnosis of the distinguishing features of Bilateria run all the theories concerning the origin of bilateral symmetry. Numerous phylogenetic analysis indicate the presence of a common ancestor shared between the bilateral and the cnidarians, then, to understand the origin of the two-sided we groped to reconstruct the features of this common ancestor.
In particular, based on what features we take into account two different scenarios:
- A. The common ancestor of the animal was a simple two-sided, non-segmented and without cavities, similar to the planula cnidarians, and the distinctive features of two-sided would gradually appeared. In this case, therefore, the characteristics of the first two-sided were those of group 1, while those of group 2 and 3 would be appeared later. Note that the planula of cnidarians, which is one of them juveniles, has bilateral symmetry. In the figure we can see a body acelomato fitted with bilateral symmetry, mesoderm, but still simple.
-B. The common ancestor was an animal rather complex two-sided, with the characteristics of two-sided complex, derived from a large larva of a cnidarian cnidarians or an adult. From this common ancestor, they would then develop the other two-sided, some of which (such as two-sided, not fitted to the body cavities, or us, not We segmented) would have then lost or changed some of the points 1,2 and 3.
paleontology is obviously one of the best ways to study the evolution of organisms of the past. Again, a good fossil record is essential in order to provide data to support or detriment of either claim. Unfortunately, the fossil organisms on which we are investigating (we are roughly in the Cambrian, and we are trying to soft-bodied animals such as cnidarians and the first two-sided vermiform, such as flatworms) have to cover my soft and very difficult to find, as they rarely they fossilize.
Therefore, a great help to resolve the issue comes not from paleontology However, the study of this animal, using sophisticated methods of molecular analysis.
The molecular phylogeny is based mainly on the study and comparison of the genetic material of the taxa considered. A major study on the origin of bilateral and was played in excellent Maneiro by Jaume Baguňa Riutort and Martha (2004). Here, in particular, the analysis were conducted by comparing a particular sequence of RNA (18S rDNA). And on the sequencing and comparison of Hox genes, which are considered very important because they are actually (indeed, some of them) that "giving instructions" to the body cells that are arranged in a certain way and promoting the development of an anterior-posterior axis and dorsoventrale.
This analysis showed that within Bilateria, the position is occupied by the most basal clade Acoelomorpha, a group of small invertebrates acelomati, once regarded as flatworms, bearing the characteristics of the group 1 but not the others. This detail, as we shall see, is very useful to hypothesize the origin of the two-sided.
Furthermore, these tests have shown that within Bilateria there is another group, called Eubilateria, which includes three major groups of two-sided, Ecdysozoa, Lophotrochozoa (both joined in the group of protostome) and deuterostome. The matter, as often happens, is wonderfully complicated.
Acoelomorpha represent the most basal clade within Bilateria, to weigh more plausible as the hypothesis of an ancestor of the two-sided rather simple, small, acelomato and not segmented. Acoelomorpha's also a set of Hox genes in between that of cnidarians and that of Eubilateri, to emphasize their key importance in the tree of evolution. Important to note that some cnidarians possess Hox genes similar to those in two-sided deal to organize the body so that an axis has a board anterodorsale dorsoventrale. It may therefore be that, from a planula (which, as we have seen, is already sides) it then developed a mutation in Hox genes, such as to maintain the planula a bilateral structure (for neoteny?) that give rise to a completely two-sided animal in all stages of life.
do not know, but we feel the same imagine what happened: about 615 to 570 million years ago (it seems to indicate the fossil and molecular studies), a body similar to the cnidarian planula for mutations Hox genes or genetic interesting consequence for the formation of new genes, evolved a race of animals from the body with lateral symmetry, not fitted with cavities, they segmentation, but rather simple.
During the Cambrian, perhaps developed several lines of animals of this type, including one that includes the current Acoleomorpha, one group survived.
Then, gradually, other important features were acquired, as a body cavity or a real device stomach, providing new and important means to the animals to grow and radiarsi. Thus, approximately 570 million years ago, the first eubilateri evolved, and, in the wake of this wonderful burst of evolution, to 550 million years ago began to separate the lines of the protostome and deuterostome.
Again, life was never the same.
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Bibliography: J. Baguna and Riutort M., 2004,
The dawn of bilaterian animals: the case of acoelomorph flatworms, BioEssays 26: 1046 to 1057.
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