METAZOA EVOLUTION by Arcano Nelson Gomes

METAZOA EVOLUTION

CLADOGRAMA COM OS NÚMEROS DOS CARACTERES

This phylogeny was made in 1990, when I was teaching INVERTEBRATES. 

 

Still too good not changed anything. Many things are not widely accepted, because there's always those who fail to understand the classics.

 

I see the work of Hyman still majestic. It's where I picked up all the best, great ideas are all there.But I have the sense to get, I have chosen what I believe is the best. I searched for a unique vision consistent without getting into endless controversies.

 

Introduction

The classification of animals had its beginning probably when man created language, but a more detailed classification appeared only in Greek civilization. Aristotle was the first to excel by constituting a meaningful classification that takes into account, not only the external morphology, but also the internal structure of organisms.

In the eighteenth century Linnaeus laid the basis of modern classification creating the binomial system for designation of species, besides gathering species and species groups into larger units.

With the emergence of the theory of evolution the classifications have undergone a significant change.

Lamarck was the first to develop an evolutionary theory that suggested a slow and gradual transformation of living beings. Darwin - Wallace described satisfactorily a mechanism that came to explain the process of these transformations: the "Theory of Natural Selection."

The theory of evolution proposes that all living organisms descended from a common ancestor and some organisms are more closely related to each other. This idea led scholars to propose classifications that to the extent possible, reflects this degree of kinship. These classifications were called "Natural classifications" because reflected the natural order of evolution.

In the last decades we live such changes in the way to face the classification of living beings. The Phylogenetic Systematics proposed by Hennig brought a more harmonic view with respect to the classification procedure. Hennig proposed these ideas in the mid twentieth century but they were slow to gain respectability that they have today, they encompass all previous achievements proposing a more objective methodology.

By recognizing that phylogenetic systematics is the more objective way to organize and to present a classification, I propose here a phylogeny for the Metazoa, which will seek to provide a uniform and consistent view on existing conflicting ideas.

Using basically the henniggian ideas I seek mainly an evolutionary understanding of the basic plan of each phylum always concerned with a coherent vision in relation to its evolution.

Two important points need preliminary remark:

1.The first relates to the criterion of homology which no doubt is the center of the whole phylogenetic analysis. For this I propose the following procedure: One structure similar to another must be "a priori" considered as homologous. Of course the trustworthiness will be greater or less according to the accumulation of evidences in favor of a determined posture.

2.The second is related to the determination of convergence. To solve this problem I believe that parsimony was and remains the best way to identify them. If three subgroups (monophyletic) of a main monophyletic group, can be assembled two by two by means of autapomorphic characters (3 characters, each joining two groups), then two of these characters should be considered as necessarily convergence. More characters would be needed to better understand these relationships. The number of evidences will point the best hypothesis. This is the basic approach.

It should be a special call that comes to bring perhaps, “the main contribution of this work to understand the phylogenetic relationships of Metazoa”. Maybe for the sake of conservatism or even carelessness, wrong posture was perpetuated in the analysis of the characters of Metazoa. There is an absurd error in two of the most important characteristics that define the major groups of Metazoa. When we divide two groups as Protostomia and Deuterostomia, we are assuming that the exclusive common ancestor of these two groups was not Protostomia nor Deuterostomia. Otherwise we could not use these features as synapomorphies. Since we do not define which was this plesiomorphic character, we can assume that this character is NOTHING. So we have two synapomorphic characters that have come from NOTHING. This same thinking also applies to Spiralia and Radialia. So the ancestor of the first group was quoted or Protostomia or Deuterostomia. As the second group was either Spiralia or Radialia. My analysis showed that, as a matter of parsimony, the Deuterostomia necessarily derived from Protostomia, this second group is parafiletic and that the condition related to the origin of the mouth from the blastopore is the ancestral condition for all Metazoa. The cleavage radial should be, by parsimony, derived from the spiral cleavage that occurs at the base of Bilateria. Another case very similar is the ancestor of the Coelenterata and Bilateria. The former have radial symmetry and the other, bilateral symmetry. In this case, it is considered that the ancestor was asymmetric (Placozoa), so no symmetry. But since this ancestor had a hollow body, should be elongated or rounded, since he had only one way and not two. Polypoid like shape is the simplest and elongated should be preferred because Bilateria are elongated. Moreover, if it is elongated in both groups and in both cases are simpler, this is parsimony. So consider here the form polypoid elongated shape as nearest ancestor of Bilateria, by parsimony.

The structure of Priapulida - a new ecological approach

Until then, metazoans were merely interstitial forms, small organisms with a body constituted by few number of cells, locomotion based in ciliary movements and feed on particles. In this new approach arises a different type of reproduction, larger bodies with great production of EGGS, planktonic larvae trochophore and tornaria types. The biggest Coelenterates, as anemones and jellyfish appeared only later. The Platyhelminthes parasites groups emerged only after the Coelomata. Nematode parasites with larger size did not existed before the onset of Coelomata. In these groups of larger body animals with large gonads, emerged two major branches: the Metameria on one side and the worms with gut in a "U" shape with the mouth close to the anus and tentacles around the mouth, on the other hand. The gonads gave rise to the coelom. This coelom enabled a new type of locomotion. All of these animals with larger body become host by Nematoda and Platyhelminthes and prey by large coelenterates.

Priapulids are cylindrical worm-like animals, ranging from 0.2-0.3 to 39 centimetres in length 

 

Sediment Eaters and Filter Feeders -  by far the major forms of life in the oceans.

The descendants of one of the two branches of celomated became immediately filter feeders, Sipuncula type worms originated lofoforates and Deuterostomes, and the condition of Deuterostomia is derived from that of Protostomia, there are not two large independent groups. The transformation of the intestine into a "U" shape, took the condition of Deuterostomia. It was an adaptation to sedentary life. Filter Feeders of particles arose independently in many phyla: Mollusca [bivalves], Arthropoda [barnacles type of crustacean] and polychaetes.

As expected the Coelomata divided into Filter Feeders and Sediment Eaters - The Sediments Eaters had a DOUBLE adaptation:

The First: living in tubes, tunnels within the sediment, the coelom is partitioned in a metamerism, which enabled a rapid shift in the tunnel. Worm Locomotion.

The Second adaptation: the use of celomatic compartments (gonads) to produce many EGGS which were released into the surrounding environment. The external fertilization providing a really great offspring. The planktonic filtering water larvae is present in both branches of Coelomata. The internal fertilization has advantages in critical situations under extreme or specific conditions.

 

Discussion

Between the phylogenies of the animal kingdom more widespread, we can mention those proposed by Hyman (1940, 1951), Marcus (1958), Hanson (1958, 1977) and Hadzi (1963).

They all put the flagellate protozoa in the base as having originated the other groups. Therefore it is accepted unanimously that the Metazoa multicellular stem from a unicellular organism.

From there occur two basic disagreements. The first is related to the inclusion or not of Porifera with other multicellular phyla. The second is related to the origin of Metazoa from the ciliated protozoa (Syncytial Theory) or from a colony of flagellates (Colonial Theory).

In this analysis we shall not take into account the problem of Porifera that would be treated as Parazoa originated independently of the Metazoa from unicellular organisms, Huxley (1875) and Sollas (1884).

The Syncytial theory of Hadzi (1953) and Hanson (1958.1977) suggests an origin for the Metazoa from ciliated protozoa multinucleate who suffered compartmentalization and became multicellular. The main evidence in favor of this theory is the fact that Acoela be considered basal metazoans. These organisms were associated with the Platyhelminthes or more recently placed in the phylum apart Acoelomorpha, would be detached from other metazoan at the beginning of its diversification keeping so much of the overall structure of the common ancestor. These characteristics are: A solid body structure, a syncytial organization and a size comparable to the ciliated protozoa. Currently the Acoela are not being placed at the base of the Platyhelminthes, this position being occupied by Catenulida, Ehlers (1985) and Smith III (1985). However, the syncytial structure and the absence of a gastric cavity in Acoela were considered by these authors as autapomorphic characters of this group do not represent the ancestral condition of the Platyhelminthes or Metazoa thus providing no support Syncytial Theory.

The Colonial Theory was first proposed by Haeckl (1874) and subsequently modified by Metschnikoff (1887 ) and Hyman (1940). In essence the theory proposes the origin of Metazoa from colonial flagellate protozoa. Presents as evidence embryo formation of the Metazoa, all are sourced from a egg cell (unicellular condition) which for successive divisions resulting in a grouping cells similar to each other (undifferentiated). This unicellular condition would represent the starting point of a process of embryonic induction, according to the "law of recapitulation" of Haeckl, ie multicellular organisms have an order in their development and the various steps to be performed can only occur in a certain sequence. The formation of a structure induces the formation of a second and so on. Thus, organisms in their ontogeny preserve these important steps of induction because they represent an indispensable link, without which the whole process would be unfeasible.

Hyman (op. cit.) proposed a hypothetical ancestor planuloid for the Metazoa, based on the development of Coelenterata (planula) and the Platyhelminthes. Recent Studies in Tricoplax adhaerens (Placozoa), Grell (1971.1972), support this idea. Tricoplax presents a planuloid structure and no doubt is related to the Metazoa with which it shares apomorphic characters (see list of characters). Therefore, we can consider the body as the most basal groups of the Metazoa and use it as outgroups in phylogenetic analysis that follows.