Study of Origin of Metazoa and various theories associated with the Origin
Keywords:
Metazoa, Protozoa, OriginAbstract
On a rough estimate, life began on Earth about 3.7 billion years ago. First cell to appear was a prokaryote cell without a definite nucleus. Later, the eukaryote cell developed having a well-organized nucleus. The earlier unicellular or nonmulticellular organisms were neither plant (Protophyta) nor animal (Protozoa) in nature. Ernst Haeckel (1866) called them protista which may be defined as organisms, usually of small size, consisting of a nuclear apparatus (no definite nucleus, one nucleus or many nuclei) in a cytoplasmic body which is not divided into cells. With an increase in complexity of life, multicellularity developed in plants and animals. As mentioned earlier, the term Metaphyta is used for multicellular plants and Metazoa for multicellular animals.
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