Origin of lower Metazoa

The lower Metazoa comprises a diverse assemblage of animal phyla traditionally considered primitive by most biologists in the nineteenth and twentieth centuries. Despite claims about their primitive appearance (e.g., simple anatomy, small size), the lower Metazoa collectively displays some of the greatest morphological and developmental diversity within the Animalia. There is also mounting evidence to suggest that the primitiveness of many lower metazoans is a secondary phenomenon, i.e., the lack of complexity is interpreted as a loss of derived features of more complex ancestors. Moreover, many animals once thought to have simple anatomy are now regarded as morphologically complex by way of advanced techniques to explore their organ systems (electron microscopy, immunofluorescence, and molecular probes). Still, debate exists over the evolutionary relationships of many of the lower Metazoa, and a proper examination of metazoan origins is necessary to discern the basis of these arguments.

The evolutionary origin of the Metazoa has long been a source of major controversy, and several theories have been proposed to explain the phylogenetic jump from unicellular organism to multicellular animal. Ernst Haeckel (1874) was the first biologist to speculate that animals share a common ancestry with unicellular protists, and his subsequent theories on metazoan origins revolve around the perceived similarities between unicellular protists and the process of animal embryology. In Haeckel's 1866 penultimate work, in which he formulated his fundamental biogenetic law, he posits that embryonic development (ontogeny) recapitulates evolution (phylogeny), or rather, that the development of individuals (from zygote to larva) is a stepwise progression through adult ancestral forms. While the idea of recapitulation (one-to-one correspondence between ontogeny and phylogeny) has in the ensuing period been rejected, developmental characters still play an important role in reconstructing metazoan evolution. Two of Haeckel's theories on the origin of the Metazoa have relied almost exclusively on developmental characters and the biogenetic law: the cellularization theory and the colonial theory.

In Haeckel's cellularization theory, a ciliate-like ancestor became multicellular through a process of cellularization, i.e., cytokinesis that results in the synchronous formation of many cells. This organism, variously regarded as planula larva-like (phylum Cnidaria) or even an acoel flatworm (Platy-helminthes, Acoela), served as a model of the metazoan ancestor and was subsequently championed in numerous theories on metazoan evolution (e.g., planula or planuloid-acoeloid theory of von Graff in the 1880s and Hyman in the 1950s; the syncytial or ciliate-acoel theory of Hadzi in 1953 and Steinbock in 1963). The implications of these theories are threefold: first, the cnidarian planula larva or adult acoel flatworm represents the most primitive metazoan; second, that acoel flatworms are the link between Cnidaria and the rest of the animal kingdom (Bilateria); and third, that the acoelomate body organization is primitive within the Bilateria. Recent findings using ultrastructural and molecular sequence data have essentially disproved these theories.

Haeckel's second theory, and one that has received more widespread acceptance, is the colonial theory (1874), which states that the most primitive metazoans were derived from a colonial amoeba-like organism (synamoebium) that later became ciliated, similar in appearance to modern-day Volvox (phylum Chlorophyta). While protists such as Volvox are colonial flagellates, and not amoeba-like (phylum Rhizopoda) or ciliated (phylum Ciliophora), they nonetheless have a characteristic hollow, spherical appearance analogous to the blastula stage of metazoan embryos. Haeckel coined this metazoan precursor a "blastaea." Following his biogenetic law, Haeckel suggested the blastaea evolved into a "gastraea," a two-layered sac with two germ layers, corresponding to the gastrula stage of animal embryogeny. This theory is important for two reasons: first, it introduced comparative embryology into phylogenetic discussions that would later have prominence in the creation of Protostomia and Deuterosto-mia; and second, it presupposed the ancestral metazoan to have had primary radial symmetry, as is the case for some primitive metazoans (e.g., Cnidaria). Adding further support to this theory was the discovery of choanoflagellates (phylum Choanoflagellata), a group of protists that possesses a collar complex (a forwardly directed flagellum surrounded by an inverted cone-shaped collar of 30-40 retractile microvilli). The collar complex is interpreted as a synapomorphy uniting Choanoflagellata and Metazoa (=Animalia) because a homologous structure is present in basal metazoans (e.g., the choanocyte layer of sponges). In 1880, the British protozo-

Humans

Apes

Mammalia

Protostomes

Lophotrochozoans Ecdysozoans

Petromyzontida

Myxinoidea

Crustacea

Arthropoda Echinodermata

Annelida

The first phylogenetic "tree" traced the origin of life to Moneran. Interrelationships were mapped in a branching structure.

Deuterostomes

Reptilia

Amphibia

Deuterostomes

Reptilia

Ctenophores

Porifera

Deuterostomes Humans

Ctenophores

Porifera

Planaria

Amoeba

Moneran

Polychaete, a segmented worm, is one of the most primitive of annelids.

Phylogeny of lower metazoans, lesser deuterostomes, and protostomes. (Illustration by Christina St. Clair)

ologist Saville-Kent went so far as to consider the sponges (phylum Porifera) as colonial protists derived directly from choanoflagellates. The existence of colonial choanoflagellates such as Proterospongia (Greek: protero = earlier or former) has added fuel to this theory, and since this time, it has remained conventional wisdom that Choanoflagellata is the most likely sister-group of Metazoa.

The early radiation of the Metazoa still remains a major puzzle in animal evolution. The earliest known metazoan fossils were once thought to extend only as far back as the Cambrian Period, 540 million years ago (mya). The Cambrian marks a time when most of the major animal phyla first appear in the fossil record. The period of time over which much of this diversification appeared, approximately 30 million years, is relatively rapid compared to the known age of meta-zoan life (low estimate: 600 million years based on paleon tology; high estimate: 1,100 based on molecular clock). Consequently, it is often referred to as the Cambrian explosion. However, intriguing evidence suggests that there was a cryptic Precambrian metazoan history. Some of the most recent finds (Xiao et al., 1998) include the smallest known fossils, in the form of embryos and early larval stages, dating back to the Neoproterozoic (570 mya). These fossils clearly indicate the existence of a fauna prior to the Cambrian explosion, and perhaps allude to the presence of small, planktonic or interstitial organisms that did not readily fossilize. While molecular and fossil evidence verifies Precambrian existence, most of the larger animal phyla clearly diversified during the Cambrian. Some of the earliest molecular studies (Field et al., 1988; Christen et al., 1991) also provided evidence to suggest that the Metazoa was diphyletic, and that two subkingdoms, Radiata and Bilateria, arose independently from flagellated protozoa in the Precambrian. Both terms allude to the ani

Fossil Evidence Biology Term

Artist's rendition of Precambrian life forms based on fossils from the Ediacara Hills of South Australia. Prominent in the diorama are jellyfish (Edi-acaria flindersi), Mawsonites spriggi, Kimberella quadrata, sea pens (Charniodiscus arboreus), pink paddles (Charniodiscus oppositus), flat worms (Dickensonia costata), and algae. (Photo by ┬ęChase Studios, Inc/Photo Researchers, Inc. Reproduced by permission.)

Artist's rendition of Precambrian life forms based on fossils from the Ediacara Hills of South Australia. Prominent in the diorama are jellyfish (Edi-acaria flindersi), Mawsonites spriggi, Kimberella quadrata, sea pens (Charniodiscus arboreus), pink paddles (Charniodiscus oppositus), flat worms (Dickensonia costata), and algae. (Photo by ┬ęChase Studios, Inc/Photo Researchers, Inc. Reproduced by permission.)

mals' primary axis of symmetry, and though the terms remain active in the literature, there is conclusive evidence to support monophyly of the Metazoa; synapomorphies include multicellularity, cell junctions, collagen, gametic meiosis with haploid egg and sperm, sperm with acrosome, and fundamentally radial cleavage. These ground-pattern characteristics are still found in basal and derived metazoans, despite up to one billion years of animal evolution. As of recent times, most phyla are considered monophyletic, i.e., a clade consisting of an ancestral species and all its descendants and all sharing a characteristic combination of specific anatomical features (Nielsen, 2003).

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