Taxonomists carve two subkingdoms from the larger animal kingdom: Parazoa and Eumetazoa. The Parazoa (Gr. para, alongside + zoa, animals) are not bilaterally symmetrical and lack tissues and organs while the Eumetazoa (Gr. eu, true + meta, later + zoa, animals) are bilaterally symmetrical and do possess tissues and organs.
Sponges are an ancient but successful animal group whose lineage can be traced back to the beginnings of animal life. The 9,000 or so extant species of sponges and the worm-like mezazoans represent the only parazoa branch on the Tree of Life; all other animal types are eumetazoans.
Sponges Apparently Arose From Choanocyte Ancestors
DNA analysis indicates that the sponge’s immediate evolutionary predecessors are the protistan choanoflagellates, a group biologists long suspected could have been the nearest thing to animals (metazoa) without actually being animals.
These single-celled creatures with whip-like flagella bear an amazing resemblance to the choanocyte cells of present day sponges. In one species, a particular molecule previously found in only multicellular animals was discovered leading scientists to conclude that the choanoflagellates appear to contain the “genetic tool kit” (i.e. genes for inter-cell adhesion, signaling, and differentiation) from which the first animals were made.
Establishing the Family Tree of Sponges
Traditionally sponges were considered to be a monophyletic group. That is, all sponges descended from a common ancestor that was itself a sponge. Furthermore, it was believed that ancestral sponges gave rise to the eumetazoa.
However, when molecular and DNA analysis revealed that choanoflagellates are the nearest evolutionary relatives to all animals, the traditional view of the origin of sponges was shown to be incorrect.
Currently most zoologists believe that choanoflagellates gave rise to sponges and that some sponges gave rise to the eumetazoa.
The Origin of the Various Classes of Sponges
Classes of sponges are distinguished on the basis of the form and chemical composition of their spicules and the complexity of their body form. However, to date, evolutionary relationships between poriferan classes have not been clearly distinguished nor fully understood.
The most widely accepted theory of the origin of sponge classes proposes that choanoflagellate ancestors gave rise to the glass sponges (Class Hexactinellida). Ancestral glass sponges in turn gave rise to demosponges (Class Demospongia) and calcareous sponges (Class Calcarea). Ancestral demosponges in turn gave rise to the eumetazoa.
Most humans know of sponges only as dried remains purchased to scrub cars and floors. However, to hold a sponge in one’s hand is to connect back through over 600 million years of time. These apparently simple animals are worthy of respect as the words of H. V. Wilson so eloquently illustrate:
“If sponges could express themselves, as is the desire of so many of us today, I think a well-speaking sponge might address biologists somewhat in the following fashion: ‘I realize that we are not as widely known as some others and yet I feel that our family memoirs show that we are not an uninteresting race.’ “
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