What does the dreaded "E" word mean, anyway

Natural History, Feb, 2000 by Stephen Jay Gould

One other group, until last month, also had no record within the Cambrian explosion, although late Cambrian representatives (well after the explosion itself) have been known for some time. Whereas popular texts have virtually ignored the Bryozoa, the absence of this other group has been prominently showcased and proclaimed highly significant. No vertebrates had ever been recovered from deposits of the Cambrian explosion, although close relatives within our phylum (the Chordata), if not technically vertebrates, had been collected (the Chordata includes three major subgroups: the tunicates, Amphioxus and its relatives, and the vertebrates proper).

This absence of vertebrates from strata bearing nearly all other fossilizable animal phyla provided a strong ray of hope for people who wished to view our own group as "higher" or more evolved--a predictable direction. If evolution implies linear progression, then later is better--and uniquely later (or almost uniquely, given those pesky bryozoans) can only enhance the distinction. But the November 4, 1999, issue of Nature includes a persuasive article ("Lower Cambrian Vertebrates from South China," by D-G. Shu, H-L. Luo, S. Conway Morris, X-L. Zhang, S-X. Hu, L. Chen, J. Han, M. Zhu, Y. Li, and L-Z. Chen) reporting the discovery of two vertebrate genera within the Lower Cambrian Chengjiang formation of southern China, right within the temporal heart of the Cambrian explosion. (The Burgess Shale of western Canada, the celebrated site for most previous knowledge of early Cambrian animals, postdates the actual explosion by several million years. The recently discovered Chengjiang fauna, with equally exquisite preservation of soft anatomy, has been yielding comparable or even greater treasures for more than a decade. See "On Embryos and Ancestors," Natural History, July-August 1998.)

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.

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.