EVOLUTION OF METAZOANS

EVOLUTION OF METAZOANS

All multicellular animals are called metazoans and single celled animals, Protozoa Metazoa (Meta= later; zoa = animals) after Metazoa (Meta= later; zoa = animals) after the Protozoans (Proto = first; zoa = animals); thus metazoans believed to have originated from Protozoans.

Protozoans belong to the Kingdom Protista or Proticsta and include single celled animals such as : a) Amoeba (Phylum Sarcodina), b) Flagellates (Phylum Mastigophora), c) Ciliates (Phylum Ciliophora) d) Sporozoans (Phylum Sporozoa or Apicomplexa). These four groups of protozoans are based on their locomotory structures that they use.

1. Flagellates (Phylum Mastigophora) Flagellates have a locomotory structure called flagellum which is a long, threadlike appendage or a whip-like extension structure. It propels the animal forward either by pulling the animal along or by pushing it forward An example of a flagellate is Euglena.

Euglena

2. Cilliates (Phylum Ciliophora) Ciliates are covered with tiny hair-like projections called cilia also used for movement. There are two types of cilia: motile cilia, which constantly beat in a single direction, and non-motile cilia, which typically serve as sensory organelles Animals with these structures can move relatively fast Example of ciliates is Paramecium.

Paramecium

3. Amoeba (Phylum Sarcodina) Amoebas use a temporary extension or projection of the cytoplasm called pseudopodia (False feet) which form in any part of the animal when moving or engulfing food Animals with these locomotory structures are called amoeboids. An example of amoeba is Amoeba Proteus

Amoeba

4. Sporozoans (Phylum Apicomplexa or Sporozoa) Apicomplexa or sporozoans - large group of protists, characterized by the presence of a unique organelle called an apical complex parasites that complete part of their life cycle inside of cells of a host Examples of sporozoans are Plasmodium (Malaria), Gregarines and Coccidia (coccidiosis)

Evolution of Multicellularity What possible processes might have led to the production of multicellular animals from these single cell animals?

The first organisms that existed were unicellular. However, multicelularity is thought to have evolved many times independently - plants, animals and fungi had independent evolution paths,

1. Aggregation hypothesis Aggregation of single cells into multicellular mass could produce a multicellular organism. Furthermore, aggregation of cells into sheets or ridges often occurs during embryogenesis in invertebrates. Theory of aggregation not accepted as the origin of multicellularity, because all animals begin life as single cells that divide rather than as an association of independent cells. (rejected)

2. Symbiotic hypothesis This hypothesis suggests that the first multicellular organisms occurred from symbiosis (cooperation) of different species of single celled organisms, each with different roles. Over time these organisms would become so dependent on each other they would not be able to survive independently, eventually leading to their genomes being incorporated However, the problem with this theory is that it is still not known how each organism's DNA could be incorporated into one single genome to constitute them as a single species. (rejected)

3. Colonial hypothesis This is the concept that animals arose from colonial flagellated protists that form a colony. An organism often cited to support this hypothesis is the sponges Phylum Porifera, which are a collection of different types of cells performing different functions. The type of colony often cited as the forerunner of the animal kingdom is Volvox. This theory asserts that cooperation (symbiosis) of many organisms of the same species led to a multicellular organism. This is the hypothesis which is highly embraced by many biologists. (accepted)

4. Syncytial hyphothesis (Cellularisation hypothesis) This is the concept that animals arose from a multinucleated ciliated protists. This concept stresses the possibility of a ciliate organism whose nucleus divided repeatedly to give a multinucleated protozoan. The organism ended up with a large mass of cytoplasm with many nuclei scattered This was followed by compartmentalisation of individual nuclei by the development of septa. It is assumed that the partitioning of the nuclei was caused by a mutation. This mass now clearly cellular, could become even larger, more complex and more perfectly integrated.