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Origin of the animals

 

The first protistas animals appeared about 1,400 million ago years being aerobic autótrofos organisms able to make the photosynthesis. Of the initial protista animal some appear numerous groups appearing that lose the plástidos ones and have cellular membrane undresses giving rise to the heterótrofos protistas. From it is considered here that the animals arise. But these changes are not guaranteed by the fossil presence that could clarify it, due to their lack. The Kingdom animal appears more or less makes 700 million years, hypotheses that try to explain how the animals arise.


THEORIES ON THE ORIGIN OF THE ANIMALS.


Colonial theory (HAECKEL, 1874)

The origin animal that would be in some protista could form colonies by union of different individuals, or being united individual independent or by cells product of the own cellular division. The colonies are spherical and hollow, a species of celoblástula, situation that is known like blastaea, now a invaginación by the one of the poles, forming gastraea would take place and finally from this the Planula would already form. This author compares these stages with the stages of the human development. The first rejections appear when it is observed that gástrula in some animals forms by ingresión and not by invaginación.

Butschli (1883)

It indicates that the formation of the blastaea and the gastraea come from an individual bulk or one constituted by two layers. To the organism reference to an animal flattened with two differentiated layers denominated PLAKULA doing affluent (dorsal and ventral). It would be an organism that would live at heart and that would take the food being placed raises and bending over opening a small internal space of the two layers, like a small cavity that surrounds to the particle. Once thus, one occurs to the digestion extra-physical of the particle and the absorption of the nutrients. From the discovery of the Placozoos, this theory has had much force. The problems arise when anlizar the animals of the Volvox sort. Autótrofo is a protista and not heterótrofo like which we needed for our study. In addition, the cells of Volvox are not so independent since there are cytoplasm bridges between the same ones.

Symbiotic theory

For this theory the first animals arise by the union from different protistas from different species establishing among them symbiosis relations. The lichens and the model lean in that they present/display. The problems appear when trying to demonstrate since they become independent, to carry out the reproduction.

Sincitial theory (HADZI, 1953)

Based on the nuclei that present/display the protistas, most external they would be different from the rest by means of membrnas, forming of this form cells in sincitio. This protista would be ciliado feeding by means of the creation on furrows or channels. We would find similarities according to the saying with a the Edge Flatworms (Acelos order) are individual small of form extended with ventral opening buco with external wall. According to the indicated thing, these animals would be flattened

Types of animals


Colonial theory

The primitive one would be the group of the Cnidarios, with radial symmetry

Sincitial theory

The first animal comes from an extended protista and nucleado forming individual resemblances to the flatworms and bilateral symmetry, this symmetry is not as old as the radial one. The Cnidarios it is resembled to them with radial symmetry indicating:

The sponges present/display rare time radial symmetry.
The Ctenóforos does not have radial symmetry either (presence of tentacles)
In the Cnidarios, safe in some Hidrozoos, radial symmetry does not exist either

Habitat

The first animals would be pelágicos and with habitat in the water (colonial hypothesis); the sincitial would indicate that the life form would be bentonica and habitat at heart of the ocean

Embryonic development and number of layers

The colonial theory would indicate that oldest they are the radiated ones (with two leaves). The sincitial is based on the flatworms (with three leaves). In Cnidarios and Cnetóforos there are specialized cells very which contradicts the theory of being the most primitive individuals, since the specialization is synonymous of evolution, supporting this therefore to a later origin in these animals.

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