The great naturalhistory museums of the world, in Paris, London, New York, Chicago, and Cleveland, boast of magnificenthalls of ancientlife. Dioramasface the corridor,beginning with the oldest layers of rock in which huge numbers of invertebrate animal The "Sudden skeletons have been found. Simple trilobites, segmented worms, brachiopods, and solitary corals (fig. 1) inhabited ancient seas some 580 million years E I *nt ago during Cambriantimes (fig. 2). By the Ordovician Period, beginning about 480 million years ago, new groups of animals appeared:cephalopods, graptolites, Animal Fossils colonial bryozoans, and primitive fishes made their debut, as reconstructedby, for example, fossiliferous limestones underlying what is now Cincinnati. Later, Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/76/40171/4447635.pdf by guest on 29 September 2021 About 600 Million by the Devonian Period, which began 400 millionyears ago, huge reef-likecomplexes extended for kilometers across shallow seas. These calcareous mounds were Years Ago: Why? formed by coralsand cemented by "stromatoporoids," tiny, rectangular skeletalized animals. Most paleon- tologists believe stromatoporoids were colonial sponges with calcium carbonate spicules. The museumgoer delights in undersea worlds which are Sharon Kaveski now entirely extinct. What was the scenario before the Cambrian?The Lynn Margulis rocks underlying those from Cambriantimes in many parts of the world are folded and metamorphosed by high temperaturesand pressures. Yet, some sedimen- tary rocks as old as 3,500 million years, far more an- cient than those of the Cambrian Period, have also been found which seem to be devoid of animal fossils. Why? The "sudden appearance" of invertebrate fossil skeletons in different sediments from many parts of the world about 600 million years ago (at the base of the Cambrian)has been referred to, for example, by Princeton paleontologist A. Fischer and many others as the "most vexing" of all paleontological problems. What happened between 3,500 and 600 million years ago? Why do the many older rocks lack clear evidence of animalremains? Does the sudden appearanceof in- vertebratessupport a contention that all animals were Sharon Kaveski received her B.A. degree in biology from Boston University,where she concentratedin evolutionarybiology. She has created simultaneously? Did it take most of Earth also studied marinescience at the Universityof Miami-CoralGables, history for life itself to emerge from non-life? Did Florida,and participatedin an intensive shipboard oceanography shocking catastrophes in the form of worldwide programat OccidentalCollege in Los Angeles. Lynn Margulisis a professor of biology at the Boston University Biological Science volcanoes and earthquakespreclude life from gaining Center, Boston, MA 02215, where she has taught since 1967. She a foothold among the chaos? received her A.B. degree from the University of Chicago, her M.S. Many attempts have been made to explain the degree fromthe Universityof Wisconsin,and her Ph.D. degree from the University of California-Berkeley.Her research interests have Precambrian boundary problem, the comparative taken her to Mexico, Ghana, Brazil, and Spain. She worked on paucity of Precambrian animal fossils, and their elementary school science curriculumreform for the U.S., Colom- remarkableabundance in the PhanerozoicAeon, in all bia, and Africa.She has receivedmany awardsincluding the Boston UniversityFaculty Publication Merit Award (1967)and the Sherman marine fossiliferous strata since the base of the Cam- FairchildFellowship at the CalifomiaInstitute of Technology(1977). brian. Perhaps the most satisfactoryexplanation is the Dr. Margulisis a fellow of AAAS, and a Guggenheim Foundation one we present here: there is no "Precambrianbound- Fellow. She has publishedextensively (see ABT43(9):482).Her most recent book is EarlyLife, Science Books International,Boston, MA ary problem" at all. It depends on how you look at (1982). evolution. 76 THE AMERICAN BIOLOGY TEACHER, VOLUME 45, NO. 2, FEBRUARY 1983 9K ~9 , t-g ~ ~ ~ ~ 4~ FIGURE 1. Cambrian scene, Cambrian fauna. The beginning of the Phanerozoic Aeon: a marinelandscape of the Cambrian Periodnearly 600 million years ago. (Draw- ing by Laszlo Meszoly, reprinted from M7 ~ B Margulis,Early Life, 1982, with permission of ScienceBooks International, Boston MA.) Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/76/40171/4447635.pdf by guest on 29 September 2021 At Ediacara remarkable animal fossils have been The Boundary and the EdiacaranFauna found in rocks beneath those harboring Cambrian When we examine the nature of the famous Precam- trilobitesand brachiopods. As imprints in sandstones brian/Cambrian boundary, we find that nearly or casts of bodies, these animals have been identified everywhere that it has been studied there lies an un- as part of a rich marinefauna by MartinGlaessner and conformity,or missing sequence of rocklayers between his colleagues of the University of Adelaide. Six ex- the barrenPrecambrian and the overlying fossiliferous tinct genera of jellyfish as well as soft corals related Cambrian-for example in Wales (the ancientname for to today's sea pens have been identified. Segmented which was Cambria),or in the Grand Canyon of nor- annelid worms with strong head shields and odd, thern Arizona (fig. 3). This unconformity really does bilaterally symmetrical, worm-like fossils are also not occur everywhere in the world; there are places among the collectionof crawling, free-floating,sessile, where the entire unbroken sequence of Precam- and swimming animals (fig. 5). These animals, it is brian/Cambrian rock surfaces. Unfortunately for thought, were preserved as molds or casts in sand- western geologists, most of the continuous rock se- stone when they became stranded in mud flats in quences occur in remote, inaccessible places such as shallow waters, and were subsequentlyburied by shift- the Alden River formation in Siberia (fig. 4; see ing sands that washed over the flats (Glaessner 1961). Rosanov 1975.) However, there is one readily accessi- The key factorin explaining this great and beautiful ble site where the relation between Precambrianand collectionof Precambriananimals is the realizationthat Cambrianrocks can be seen in the field. This site is all the animals were soft-bodied. They lacked the hard the Ediacarahills of southwest Australia,near Sydney. calcareous skeletons of the later Cambrian animals. 0 Millions of , years ago P P c~~~~~~~~ j 30 Periods 2 E Uu i7n n0 u C u - I- C Eras Paleozoic Mesozoic enozoic Pre-Phanerozoic Eras Aphebian Riphsean Vendian _ Aeons J Hadean Archaen Proterozoic Paarzi Milliotns of 400f0 3900 3000 2600 2)000 1000 580) years ago Origin Oldest Oldest strotmatolitesand Abundant Oldest OldEst Oldest plant of Earth Earth rocks moneran microfossils stromatolites protoctist animal and fungus fossils fossils fosils FIGURE2. A chronologyof Earthhistory showing the majortime-rock divisions. (Reprintedfrom Margulis,Early Life, 1982, with permission of Science Books International,Boston MA.) SUDDEN EXPLOSION 77 FIGURE 3. Cambrian boundary: United States, Grand Canyon. The boundary between prePhanerozoic rocks in the Monument Creek Xi7- =area of the Grand Canyon of Arizona. The discontinuity (unconfor- ._ :; ~ ~mity) here is profound. Because no obvious signs of life are found here below the arrow, and fossils are abundant above in the Cam- brian strata, it was thought that this boundary represented some major change in the history of the Earth. (Photograph by Bradford Downloaded from http://online.ucpress.edu/abt/article-pdf/45/2/76/40171/4447635.pdf by guest on 29 September 2021 4U ~~~~~~~~~~Washburn,Boston Museum of Science, reprinted from Margulis, Early Life, 1982, with permission of Science Books International, Boston MA.) Since the early '60s, the Ediacaranfauna has been iden- bacteriain the process of reproducingby division. An- tified at 14 other locations worldwide, including Great cient microfossils seen in rocks dated to be about the Britain,Canada, Sweden, and the Soviet Union. Thus, same age, 3,400 million years old, have also been the idea that there were no animals before the Cam- reported from the Warrawoona rock formation at brian has now been entirely dispelled. During the late North Pole, Western Australia (one of the warmest Precambrian,a period of time which is better called places on Earth). the prePhanerozoic (see fig. 2), there were perfectly Another amazing and diverse assemblage of well-developed metazoanforms belonging to recogniz- microfossils has been found in the 2 billion-year-old able phyla. But, since they bore no hard parts, relative Gunflint Iron formation of Northern Minnesota and to theirCambrian successors, they were preservedonly Southern Ontario (fig. 6.) Still another remarkably very poorly. well-preserved collection of fossils comes from Cen- tralAustralia, near Alice Springs. This fossil find, from And Before the Ediacaran Fauna? the BitterSprings formation,is about 900 million years Did other forms of life precede the Ediacaranfauna, old. These and many other findings confirm that life or did all of evolution begin with these soft-bodied, was already well established at least 3.4 billion years late prePhanerozoic metazoans? ago, a mere billion years after the formation of the Amazingly enough, there has been a revolution in Earth itself as a solid planet replete with its satellite, our thinking about early life in the last 30 years. The