All developed organisms have evolved from common ancestors. Evidences suggest that there has been continuous improvement in existing species from simple to complex and from imperfect to more perfect. It can be supported by providing evidences from different disciplines of biology such as morphology, anatomy, embryology, study of vestigial organ, biogeography, etc. Embryology is the developmental study of an egg into an adult. Von Baer, in 1828 suggested the theory of divergence. According to this theory, during embryonic development, general characteristics appear before specific characteristics. During embryonic development, an animal progressively departs from other animals. Ernst Haeckel, in 1866 proposed the theory of recapitulation or biogenetic law. The theory states ‘Ontogeny Recapitulates Phylogeny’, i.e., an organism tends to repeat the evolutionary history of its ancestors during its development. Ontogeny is the history of development of an organism. Phylogeny is the history of evolution of an organism from its ancestors. The early developmental stages of all multicellular animals are similar and their life from a single celled zygote which develops into morula, blastula and gastrula. In the gastrula, three germinal layers; ectoderm, mesoderm and the endoderm are formed, which develop the same types of parts in all the animals. Different animals start showing divergence in their developmental stages. Animals that show greater similarity during their embryonic development show closer evolutionary relationships. This supports a common ancestry of all animals. The embryos of certain animals develop temporary and non-functional structures, which disappear before hatching or before birth. In the development of various organs like the kidneys, the gonoducts, the gonads, the heart, the aortic arches, the brain and the ear, all the vertebrates follow the same basic plan and show a common ancestry.
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