N EWS F OCUS 65 But only if it’s built, of course. DOE’s heavy-ion laboratory is among the experi- probably won’t put the r-process problem to 64 Decker says that no decision on the project ments that it deemed deserving of attention. rest; its broader mission means that it 63 is imminent, assigning it a status that makes This unnamed laboratory, which would be wouldn’t be able to study quite as many ele- 62 U.S. supporters uncomfortable. Last week, built at the GSI heavy-ion research center in ments as RIA would. 61 Germany’s Science Council set priorities for Darmstadt, Germany, has a great deal of “We’re poised and ready to go,” he adds. 60 major science projects, and a $675 million overlap with RIA, although Gelbke says it “All we need is a decision.” –CHARLES SEIFE 59 58 EVOLUTION 57 than the signatures of those below it. A sim- 56 ilar isotopic shift marks the boundary be- 55 tween the and the 54 A Trigger for the elsewhere in the world. And radiometric dat- 53 ing of volcanic ash layers shows that the 52 ? shift took less than a million years and oc- 51 curred by 542.0 ± 0.5 million years ago, 50 Sediments in Oman provide evidence that an extinction 542 million years within the documented age range of the 49 ago set the stage for a proliferation of wild and wonderful life forms boundary’s carbon isotopic shift elsewhere. 48 If Cloudina and Namacalathus were not 47 Before the Cambrian period began 542 mil- down in Oman, and, to the Omanis’ good alone in disappearing, the resulting extinction 46 lion years ago, life was microscopic, vegeta- fortune, oil eventually filled the spaces (blue at the dawn of the Cambrian could have set 45 tive, or just so odd that it now seems other- in figure) between the carbonate clots. off an evolutionary explosion, Grotzinger ar- 44 worldly. Then, in a geologic moment, an Enter the age of fuel. Drilling for oil gues. Such an explosion might occur, he says, 43 evolutionary explosion littered the fossil has penetrated all six thrombolite layers. In a “if you cleared the playing field [through an 42 record with the recognizable remains of ev- drill core provided by Petroleum Develop- extinction] and started over again.” Such a 41 ery basic form of that we know to- ment Oman, Grotzinger found two of the late dramatic event could open up new possibili- 40 day. What caused this change is controver- Precambrian’s emblematic inhabitants ties for life in the same way the extinction of 39 sial. Some think of the lead-in to this explo- —cone-shaped Cloudina and gobletlike the dinosaurs opened the way for mammals. 38 sion as a “slow fuse” of gradually accumu- Namacalathus—throughout the first three But rather than envisioning an asteroid im-

37 lating genetic traits that finally produced thrombolite layers. Apparently, these carbon- pact, Grotzinger sees geochemical signs in the on October 14, 2008 36 large, complex ; others believe a ate-shelled animals of uncertain affinities Oman cores—similar to those others have 35 “trigger” of some sort set off the mechanism thrived attached to or lying on top of Oman’s seen elsewhere—that oxygen-deficient, car- 34 that suddenly produced Cambrian animals. bon dioxide–rich waters welled 33 At last month’s annual meeting of the up into the shallow sea at the 32 Geological Society of America in Denver, same time. That could have been 31 Colorado, sedimentologist John Grotzinger enough to wipe out any marine 30 of the Massachusetts Institute of Technology species not adept at taking up 29 and his colleagues reported the latest evi- oxygen or fending off the toxic

28 dence of a trigger for the Cambrian explo- carbon dioxide (Science, 1 De- www.sciencemag.org 27 sion: an extinction 542.0 million years ago, cember 1995, p. 1441). 26 possibly brought on when the deep sea dis- No one, including Grotzinger, 25 gorged noxious waters. “We do have evi- thinks the case for a global 24 dence for point-blank extinction,” says Precambrian-Cambrian extinc- 23 Grotzinger. But even in the data-sparse tion is closed. “It would be con- 22 realm of early life, this one page from the sistent with an extinction,” says 21 fossil record of Oman isn’t enough to prove paleontologist Sören Jensen of 20 that a near-knockout punch to primitive life the University of California, Downloaded from 19 set off the Cambrian explosion. Despite the Loser. Did the extinction of Cloudina (cross-sectioned as Riverside, “but you wouldn’t 18 rarity of sediment and preserved oval) and others trigger the Cambrian explosion? want to say it proves it.” First, the 17 from that time, more records must be found. evidence so far involves only two 16 Forming the Oman record entailed some carbonate reefs. But in the next three reef de- species at a single place. Second, no extinc- 15 geologic happenstance; finding it required posits, Cloudina and Namacalathus were tion this abrupt has been proposed before, 14 some lingering crude oil. Late in Precambrian gone, even though the abundance of throm- notes paleontologist Douglas Erwin of the 13 times, what is now Oman on the far eastern bolite would suggest that the living condi- National Museum of Natural History in 12 tip of the Arabian Peninsula held a deep tions were pretty much as they had been dur- Washington, D.C., but “the Omani data pro- 11 basin filled with water from the adjacent ing earlier intervals. To Grotzinger, that’s vides further support of there having been a 10 ocean, a sea much like the Mediterranean strong evidence that Cloudina and Namaca- biological crisis then. It’s certainly an increas- 9 today. In the basin’s shallower waters, great lathus were gone from the world, not just ingly reasonable idea.” Major extinctions 8 reefs formed as microorganisms helped pre- missing from this little corner of it. mark the other important turning points of 7 cipitate millimeter-scale clots of carbonate, By the best measures of time in this dis- evolution that have occurred since the Cam- 6 known as thrombolites. But the water level tant era, the apparent extinction comes right brian explosion, such as the end of the “old 5 would sometimes fall enough to cut off the at the jump from the Precambrian to the life” of the Paleozoic era 250 million years 4 sea’s shallow connection to the open ocean, Cambrian. Grotzinger and his colleagues ago. Proving that a trigger set off the most 3 the seawater would evaporate, and salt in- found that the carbon isotopic signature of fundamental evolutionary event since life’s 2 stead of carbonate would be deposited. Six the first thrombolite layer that lacks the Pre- origin will take some more digging or per-

1 CREDIT: J. GROTZINGER/MIT pairs of salt and carbonate layers were laid cambrian creatures is significantly lighter haps drilling. –RICHARD A. KERR

www.sciencemag.org SCIENCE VOL 298 22 NOVEMBER 2002 1547