Origin of Metazoa
Origin of Metazoa
Unraveling the origin of multicellular animals (metazoans) has presented many problems for zoologists. Three prominent hypotheses for the origin of metazoans from unicellular ancestors are (1) that metazoans arose from a syncytial (multinucleate) ciliated form in which cell boundaries later evolved, (2) that they arose from a colonial flagellated form in which cells gradually became more specialized and interdependent, and (3) that metazoans are polyphyletic, having been derived independently from more than one group of unicellular organisms.
Proponents of the syncytical ciliate hypothesis believe that metazoans arose from an ancestor shared with the single-celled ciliates. The common ancestor of metazoans acquired multiple nuclei within a single cell membrane and later became compartmentalized into the multicellular condition. It is assumed that the body form of the ancestor resembled that of modern ciliates and thus tended toward bilateral symmetry. Therefore the earliest metazoans would have been bilateral and similar to some present flatworms. There are several objections to this hypothesis. It ignores embryology of the flatworms in which nothing similar to cellularization occurs; it does not explain the presence of flagellated sperm in metazoans; and, perhaps more important, it implies that the radial symmetry of cnidarians is derived from a primary bilateral symmetry, for which there is no evidence.
The colonial flagellate hypothesis— first proposed by Haeckel in 1874—is the classical scheme, which, with various revisions, still has many followers. According to this hypothesis, metazoans descended from ancestors characterized by a hollow, spherical, colony of flagellated cells. Individual cells within the colony became differentiated for specific functional roles (reproductive cells, nerve cells, somatic cells, and so on), thus subordinating cellular independence to welfare of the colony as a whole. The colonial ancestral form was at first radially symmetrical, similar perhaps to the free-swimming planula larvae of the cnidarians (jellyfishes and others). This larva is radially symmetrical and has no mouth. Cnidarians, with their radial symmetry, could have evolved from this form.
Bilateral symmetry could have evolved later when some of these planula-like ancestors became adapted for a creeping form of locomotion on the ocean floor. Dorsal and ventral surfaces would have differentiated, a ventral mouth would have appeared, and a start would have been made toward cephalization (a concentration of neurons and sensory structures at the anterior). These adaptations for creeping locomotion would have led to primitive bilateral symmetry, resembling that of flatworms.
Some zoologists prefer the idea that metazoans had a polyphyletic origin and suggest that the sponges, cnidarians, ctenophores, and remaining eumetazoans evolved independently. Thus no single scheme might account for them all.
We now have phylogenetic evidence based on small-subunit ribosomal RNA sequences and on similarities in complex biochemical pathways.* This evidence generally supports the colonial flagellate hypothesis that metazoans represent a monophyletic assemblage including choanoflagellates (“collared” flagellates such as Codosiga). The sister group of metazoans appears to be fungi. The molecular evidence excludes the syncytial ciliate hypothesis because metazoa are apparently closer to eukaryotic algae and higher plants than they are to ciliates.
Unraveling the origin of multicellular animals (metazoans) has presented many problems for zoologists. Three prominent hypotheses for the origin of metazoans from unicellular ancestors are (1) that metazoans arose from a syncytial (multinucleate) ciliated form in which cell boundaries later evolved, (2) that they arose from a colonial flagellated form in which cells gradually became more specialized and interdependent, and (3) that metazoans are polyphyletic, having been derived independently from more than one group of unicellular organisms.
Proponents of the syncytical ciliate hypothesis believe that metazoans arose from an ancestor shared with the single-celled ciliates. The common ancestor of metazoans acquired multiple nuclei within a single cell membrane and later became compartmentalized into the multicellular condition. It is assumed that the body form of the ancestor resembled that of modern ciliates and thus tended toward bilateral symmetry. Therefore the earliest metazoans would have been bilateral and similar to some present flatworms. There are several objections to this hypothesis. It ignores embryology of the flatworms in which nothing similar to cellularization occurs; it does not explain the presence of flagellated sperm in metazoans; and, perhaps more important, it implies that the radial symmetry of cnidarians is derived from a primary bilateral symmetry, for which there is no evidence.
The colonial flagellate hypothesis— first proposed by Haeckel in 1874—is the classical scheme, which, with various revisions, still has many followers. According to this hypothesis, metazoans descended from ancestors characterized by a hollow, spherical, colony of flagellated cells. Individual cells within the colony became differentiated for specific functional roles (reproductive cells, nerve cells, somatic cells, and so on), thus subordinating cellular independence to welfare of the colony as a whole. The colonial ancestral form was at first radially symmetrical, similar perhaps to the free-swimming planula larvae of the cnidarians (jellyfishes and others). This larva is radially symmetrical and has no mouth. Cnidarians, with their radial symmetry, could have evolved from this form.
Bilateral symmetry could have evolved later when some of these planula-like ancestors became adapted for a creeping form of locomotion on the ocean floor. Dorsal and ventral surfaces would have differentiated, a ventral mouth would have appeared, and a start would have been made toward cephalization (a concentration of neurons and sensory structures at the anterior). These adaptations for creeping locomotion would have led to primitive bilateral symmetry, resembling that of flatworms.
Some zoologists prefer the idea that metazoans had a polyphyletic origin and suggest that the sponges, cnidarians, ctenophores, and remaining eumetazoans evolved independently. Thus no single scheme might account for them all.
We now have phylogenetic evidence based on small-subunit ribosomal RNA sequences and on similarities in complex biochemical pathways.* This evidence generally supports the colonial flagellate hypothesis that metazoans represent a monophyletic assemblage including choanoflagellates (“collared” flagellates such as Codosiga). The sister group of metazoans appears to be fungi. The molecular evidence excludes the syncytial ciliate hypothesis because metazoa are apparently closer to eukaryotic algae and higher plants than they are to ciliates.
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