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What does the dreaded "E" word mean, anyway
Natural History, Feb, 2000 by Stephen Jay Gould
These two creatures--each only an inch or so in length and lacking both jaws and a backbone and in fact possessing no bony skeleton at all--might not strike a casual student as worthy of inclusion within our exalted lineage. But these features, however much they may command our present focus, arose later in the history of vertebrates and do not enter the central and inclusive taxonomic definition of our group. The vertebrate jaw, for example, evolved from hard parts that originally fortified the gill openings and then moved forward to surround the mouth. All early fishes--and two modern survivors of this initial radiation, the lampreys and the hagfishes--lacked jaws.
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The two Chengjiang genera possess all the defining features of vertebrates: the stiff dorsal supporting rod, or notochord (subsequently lost in adults after the vertebral column evolved); the arrangement of flank musculature in a series of zigzag elements from front to back; the set of paired openings piercing the pharynx (operating primarily as respiratory gills in later fishes but used mostly for filter feeding in ancestral vertebrates). In fact, the best reconstruction of branching order on the vertebrate tree places the origin of these two new genera after the inferred ancestors of modern hagfishes but before the presumed forebears of lampreys. If this inference holds, then vertebrates already existed in substantial diversity within the Cambrian explosion. In any case, we now have two distinct and concrete examples of vertebrates "all the way down"--that is, in the very same strata that include the first known fossils of nearly all phyla of modern multicellular animals. We vertebrates do not stand higher and later than our invertebrate cousins, for all "advanced" animal phyla made their first appearance in the fossil record at essentially the same time. The vaunted complexity of vertebrates did not require a special delay to accommodate a slow series of progress-did not require a special delay to accommodate a slow series of progressive steps, predictable from the general principles of evolution.
2. An ultimate parasite, or "how are the mighty fallen." The phyla of complex multicellular animals enjoy a collective designation as Metazoa (literally, "higher animals"). Mobile, single-celled creatures bear the name Protozoa ("first animals"--actually a misnomer, since many of these creatures, in terms of genealogical branching, rank as close to multicellular plants and fungi as to multicellular animals). In a verbal in-between stand the Mesozoa ("middle animals"). Many taxonomic and evolutionary schemes for the organization of life rank the Mesozoa by the literal implication of their name--that is, as a persistently primitive group, intermediate between the single-celled and the multicellular animals and illustrating a necessary transitional step in a progressivist reading of life's history.
But the Mesozoa have always been viewed as enigmatic, primarily because they live as parasites within truly multicellular animals, and parasites often adapt to their protected surroundings by evolving an extremely simplified anatomy, sometimes little more than a glob of absorptive and reproductive tissue cocooned within the body of a host. Thus, the extreme simplicity of parasitic anatomy could represent the evolutionary degeneration of a complex, free-living ancestor rather than the maintenance of a primitive state.
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