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What does the dreaded "E" word mean, anyway
Natural History, Feb, 2000 by Stephen Jay Gould
Two studies published within the past month led me to this topic, because each discovery confirms the biological, variational, and Darwinian "take" on evolution while also, and quite explicitly, refuting a previous, transformational interpretation--rooted in our culturally established prejudices for the more comforting, astronomical view--that had blocked our understanding and skewed our thoughts about an important episode in life's history:
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1. Vertebrates "all the way down." In one of the most crucial and enigmatic episodes in the history of life--and a challenge to the older, more congenial idea that life has progressed in a basically stately, linear manner through the ages--nearly all animal phyla made their first appearance in the fossil record at essentially the same time, an interval of some 5 million years (about 525 million to 530 million years ago) called the Cambrian explosion. (Geological firecrackers have long fuses when measured by the inappropriate scale of human time.) Only one major phylum with prominent and fossilizable hard parts did not appear in this incident or during the Cambrian period at all--the Bryozoa, a group of colonial marine organisms unknown to most nonspecialists today (although still relatively common in shallow oceanic waters) but prominent in the early fossil record of animal life.
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.)
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