lions of years — a long time for anyone 

 who has ever set a dynamite charge, but 

 awfully quick relative to a history of life 

 measured in billions (remember that one 

 thousand millions make a billion). But 

 how many millions? 



Paleontologists have always hedged on 

 this crucial question because we had no 

 precise dates for the inception of the Cam- 

 brian period. The Cambrian ended some 

 505 to 510 million years ago, but we had 

 no good fix on the beginning until last 

 September, when several of my colleagues 

 in the Cambridge mafia (Harvard plus 

 MIT) joined with Russian geologists in fi- 

 nally nailing the early Cambrian, based on 

 data "so beautiful you could cry," to quote 

 my grandmother, who would have under- 

 stood (S. A. Bowring, J. P. Grotzinger, 

 C. E. Isachsen, A. H. Knoll, S. M. 

 Pelechaty, and P. Kolosov, "Calibrating 

 Rates of Early Cambrian Evolution," Sci- 

 ence, Septembers, 1993, pp. 1293-98). 



Previous estimates for the Cambrian's 

 beginning ranged from nearly 600 to 530 

 milfion years ago (I have been using 590 

 in my introductory course for years, but 

 must change the date this time around). 

 The older dates (favored by most) permit- 

 ted quite a good stretch for the Cambrian 

 explosion, perhaps 30 million years or so 

 (still a moment among billions, but at least 

 a relaxed moment). My colleagues have 

 now pinpointed the explosion by caUbrat- 

 ing the radioactive decay of uranium to 

 lead within zircon crystals obtained from 

 volcanic rocks interbedded with Siberian 

 sediments containing earliest Cambrian 

 fossils. 



The earliest Cambrian, like Caesar's 

 Gaul, is divided into three parts called, 

 from oldest to youngest, Manakayan, 

 Tommotian, and Atdabanian. (The names 

 are all derived from Russian localities 

 where early Cambrian rocks are particu- 

 larly well exposed.) The Manakayan con- 

 tains many fossihzed bits and pieces of 

 cousins and precursors, but not the re- 

 mains of major modem phyla. The Man- 

 akayan therefore predates the Cambrian 

 explosion. By the end of the Atdabanian, 

 virtually all modem phyla had made their 

 appearance. The Cambrian explosion 

 therefore spans the Tommotian and Atda- 

 banian stages. 



My colleagues have dated the base of 

 the Manakayan at 544 milfion years ago 

 (with potential error of only a few hundred 

 thousand years) and have determined that 

 this initial stage lasted some 14 million 

 years. The Tommotian began about 530 

 million years ago and — get this, for now 



the intellectual impact occurs — the subse- 

 quent Atdabanian stage ended only 5 to 6 

 (at the very most, 10) million years later 

 Thus, the entire Cambrian explosion, pre- 

 viously allowed 30 or even 40 million 

 years, must now fit into 5 to 10 (and al- 

 most surely nearer the lower limit), from 

 the base of the Tommotian to the end of 

 the Atdabanian. In other words, fast is 

 much, much faster than we ever thought. 



This story rocked the airwaves (insofar 

 as any scientific tale merits the cliche). 

 The New York Times awarded front-page 

 billing in its weekly science section; Na- 

 tional Public Radio featured my col- 

 leagues on its weekly science talk show. 

 The primary theme was intense surprise: 

 evolution means slow; how could so much 

 happen so fast? Was the entire conceptual 

 world of evolutionary theory about to be 



undermined? I was absolutely delighted 

 by my colleagues' result, but I was not sur- 

 prised. I have believed for many years that 

 fast was at least this fast. (1 had regarded 

 the old limits of 30 to 40 million years 

 merely as an upper bound, and had as- 

 sumed that the Cambrian explosion only 

 occupied a small segment at the beginning 

 of this full interval.) Why such a differ- 

 ence between public perception and my 

 personal reaction? 



2. bisects and flowers. Nothing displays 

 human hubris more than the old textbook 

 designation of recent geological times as 

 the "age of man." First of all, if we must 

 use an eponymous designation, we live 

 today, and have always lived, in the "age 

 of bacteria." Second, if we insist on multi- 

 cellular parochialism, modem times must 

 surely be called the "age of insects." 



c-i,, ■ / 





y' n 





"We're out of electricity." 



17 



