Six major steps in animal evolution: are we derived sponge larvae?

Abstract

A review of the old and new literature on animal morphology/embryology and molecular studies has led me to the following scenario for the early evolution of the metazoans. The metazoan ancestor, "choanoblastaea," was a pelagic sphere consisting of choanocytes. The evolution of multicellularity enabled division of labor between cells, and an "advanced choanoblastaea" consisted of choanocytes and nonfeeding cells. Polarity became established, and an adult, sessile stage developed. Choanocytes of the upper side became arranged in a groove with the cilia pumping water along the groove. Cells overarched the groove so that a choanocyte chamber was formed, establishing the body plan of an adult sponge; the pelagic larval stage was retained but became lecithotrophic. The sponges radiated into monophyletic Silicea, Calcarea, and Homoscleromorpha. Homoscleromorph larvae show cell layers resembling true, sealed epithelia. A homoscleromorph-like larva developed an archenteron, and the sealed epithelium made extracellular digestion possible in this isolated space. This larva became sexually mature, and the adult sponge-stage was abandoned in an extreme progenesis. This eumetazoan ancestor, "gastraea," corresponds to Haeckel's gastraea. Trichoplax represents this stage, but with the blastopore spread out so that the endoderm has become the underside of the creeping animal. Another lineage developed a nervous system; this "neurogastraea" is the ancestor of the Neuralia. Cnidarians have retained this organization, whereas the Triploblastica (Ctenophora+Bilateria), have developed the mesoderm. The bilaterians developed bilaterality in a primitive form in the Acoelomorpha and in an advanced form with tubular gut and long Hox cluster in the Eubilateria (Protostomia+Deuterostomia). It is indicated that the major evolutionary steps are the result of suites of existing genes becoming co-opted into new networks that specify new structures. The evolution of the eumetazoan ancestor from a progenetic homoscleromorph larva implies that we, as well as all the other eumetazoans, are derived sponge larvae.

Dynamical patterning modules: a "pattern language" for development and evolution of multicellular form

“…The DPMs, though emerging simultaneously with multicellularity, are nonetheless based on molecules that were present before the split between the architecturally simple poriferans (sponges) (Nielsen, 2008) and placozoans (Miller and Ball, 2005), and the eumetazoans, the evolutionary line that underwent the explosive proliferation of morphologies during the early Cambrian (Nichols et al, 2006). Many of the toolkit genes, therefore, arose first in unicellular organisms, taking on their DPM-associated roles by mobilizing physical processes that only come into play at the "mesoscale" 2 in which multicellular aggregates exist.…”

Developmental gene expression provides clues to relationships between sponge and eumetazoan body plans

“…However, juveniles of demosponges are radially symmetrical with a single osculum and differ from the calcisponge juveniles only by having multiple choanocyte chambers 30 . To circumvent the problem of seemingly incomparable bauplans of adult sponges and eumetazoans, it has been suggested that eumetazoans are derived from sponge larvae 31 . While sponges have an extremely diverse array of larval types, all of them are non-feeding 30 , so evolution of cnidarian-grade organization from sponge larvae would require development of a feeding strategy in addition to paedogenesis.…”

Mucosal immunity: aliment and ailments

“…Other contributions of the microfl oraactivated innate immunity to the homeostasis and functions of the gastrointestinal tract were addressed recently in a series of studies. Collectively, these studies revealed that activation of innate immunity (1) aff ects the composition of the intestinal microfl ora, 11 (2) enhances the integrity and barrier functions of intestinal epithelial cells (IECs) via the induction of tight junction-related proteins, 12 (3) increases the proliferative response and decreases the pace of apoptosis of IECs, as was inferred from studies in wildtype mice treated with TLR agonists 13 or various TLR knockout mice, 14,15 and (4) accelerates the healing process of the injured epithelium. 16 Finally, activation of innate immunity via Nod or TLR results in the secretion of anti-microbial peptides, 17 and triggering of Nod1 supports GALT via the generation of isolated lymphoid follicles (ILFs) in the intestinal mucosa.…”

“…1,2 Subsequently, a decisive evolutionary step took place that led to the formation of a true epithelium and gastrulation in eumetazoans, providing a more effi cient nutritive system. 2 Th is system, which evolved to the primitive gut in higher species, could capture and extract high volumes of nutrients, including various prokaryotes from the primordial aquatic soup. Th is change in the feeding process led to the colonization of the gut tube by certain environmental bacterial species or the primary colonizers.…”