Colloquium Solution to Darwin’s dilemma: Discovery of the missing Precambrian record of life J. William Schopf* Department of Earth and Space Sciences, Institute of Geophysics and Planetary Physics (Center for the Study of Evolution and the Origin of Life), and Molecular Biology Institute, University of California, Los Angeles, CA 90095-1567 In 1859, in On the Origin of Species, Darwin broached what he Darwin’s dilemma begged for solution. And although this regarded to be the most vexing problem facing his theory of problem was to remain unsolved—the case ‘‘inexplicable’’—for evolution—the lack of a rich fossil record predating the rise of more than 100 years, the intervening century was not without shelly invertebrates that marks the beginning of the Cambrian bold pronouncements, dashed dreams, and more than little acid Period of geologic time (Ϸ550 million years ago), an ‘‘inexplicable’’ acrimony. absence that could be ‘‘truly urged as a valid argument’’ against his all embracing synthesis. For more than 100 years, the ‘‘missing J. W. Dawson and the ‘‘Dawn Animal of Canada.’’ Among the first to Precambrian history of life’’ stood out as one of the greatest take up the challenge of Darwin’s theory and its most vexing unsolved mysteries in natural science. But in recent decades, problem, the missing early fossil record, was John William understanding of life’s history has changed markedly as the doc- Dawson (1820–1899), Principal of McGill University and a giant umented fossil record has been extended seven-fold to some 3,500 in the history of North American geology. Schooled chiefly in million years ago, an age more than three-quarters that of the Edinburgh, Scotland, the son of strict Scottish Presbyterians, planet itself. This long-sought solution to Darwin’s dilemma was Dawson was a staunch Calvinist and devout antievolutionist (3). set in motion by a small vanguard of workers who blazed the trail in the 1950s and 1960s, just as their course was charted by a few In 1858, a year before publication of Darwin’s opus, specimens pioneering pathfinders of the previous century, a history of bold of distinctively green- and white-layered limestone collected pronouncements, dashed dreams, search, and final discovery. along the Ottawa River to the west of Montreal were brought to the attention of William E. Logan, Director of the Geological Survey of Canada. Because the samples were known to be n 1950, when Ledyard Stebbins’ Variation and Evolution in ancient (from ‘‘Laurentian’’ strata, now dated at about 1,100 Plants (1) first appeared, the known history of life—the I million years) and exhibited layering that Logan supposed too familiar progression from spore-producing to seed-producing to regular to be purely inorganic (Fig. 1), he displayed them as flowering plants, from marine invertebrates to fish, amphibians, then reptiles, birds, and mammals—extended only to the begin- possible ‘‘pre-Cambrian fossils’’ at various scientific conferences, ning of the Cambrian Period of the Phanerozoic Eon, roughly where they elicited spirited discussion but gained little accep- 550 million years ago. Now, after a half-century of discoveries, tance as remnants of early life. life’s history looks strikingly different—an immense early fossil In 1864, however, Logan brought specimens to Dawson who record, unknown and assumed unknowable, has been uncovered not only confirmed their biologic origin but identified them as to reveal an evolutionary progression dominated by microbes fossilized shells of giant foraminiferans, huge oversized versions that stretches seven times farther into the geologic past than of tiny calcareous protozoal tests. So convinced was Dawson of COLLOQUIUM previously was known. This essay is an abbreviated history of their biologic origin that a year later, in 1865, he formally named how and by whom the known antiquity of life has been steadily the putative fossils Eozoon canadense, the ‘‘dawn animal of extended, and of lessons learned in this still ongoing hunt for Canada.’’ Dawson’s interpretation was questioned almost im- life’s beginnings. mediately (5), the opening shot of a fractious debate that raged on until 1894 when specimens of Eozoon were found near Mt. Pioneering Pathfinders Vesuvius and shown to be geologically young ejected blocks of Darwin’s Dilemma. Like so many aspects of natural science, the limestone, their ‘‘fossil-like’’ appearance the result of inorganic beginnings of the search for life’s earliest history date from the alteration and veining by the green metamorphic mineral ser- mid-1800s and the writings of Charles Darwin (1809–1882), who pentine (6). in On the Origin of Species first focused attention on the missing Yet despite the overwhelming evidence, Dawson continued to Precambrian fossil record and the problem it posed to his theory press his case for the rest of his life, spurred by his deeply held of evolution: ‘‘There is another . difficulty, which is much more belief that discovery of his ‘‘dawn animal’’ had exposed the serious. I allude to the manner in which species belonging to greatest missing link in the entire fossil record, a gap so several of the main divisions of the animal kingdom suddenly enormous that it served to unmask the myth of evolution’s appear in the lowest known [Cambrian-age] fossiliferous rocks claimed continuity and left Biblical creation as the only answer: . If the theory be true, it is indisputable that before the lowest ‘‘There is no link whatever in geological fact to connect Eozoon Cambrian stratum was deposited, long periods elapsed . and with the Mollusks, Radiates, or Crustaceans of the succeeding that during these vast periods, the world swarmed with living [rock record] . these stand before us as distinct creations. [A] creatures... [But] to the question why we do not find rich fossiliferous deposits belonging to these assumed earliest periods before the Cambrian system, I can give no satisfactory answer. This paper was presented at the National Academy of Sciences colloquium ‘‘Variation and The case at present must remain inexplicable; and may be truly Evolution in Plants and Microorganisms: Toward a New Synthesis 50 Years After Stebbins,’’ urged as a valid argument against the views here entertained’’ held January 27–29, 2000, at the Arnold and Mabel Beckman Center in Irvine, CA. (ref. 2, Chapter X). *E-mail: [email protected]. PNAS ͉ June 20, 2000 ͉ vol. 97 ͉ no. 13 ͉ 6947–6953 Downloaded by guest on September 28, 2021 Fig. 1. Eozoon canadense, the ‘‘dawn animal of Canada,’’ as illustrated in Dawson’s The Dawn of Life (4) (A) and shown by the holotype specimen archived in the U.S. National Museum of Natural History, Washington, DC (B). (Bars ϭ 1 cm.) gap . yawns in our imperfect geological record. Of actual facts [with which to fill this gap], therefore, we have none; and those evolutionists who have regarded the dawn-animal as an evidence in their favour, have been obliged to have recourse to supposition and assumption’’ (ref. 4, p. 227). (In part, Dawson was right. In the fourth and all later editions of The Origin, Darwin cited the great age and primitive protozoal relations of Eozoon as con- sistent with his theory of evolution, just the sort of ‘‘supposition and assumption’’ that Dawson found so distressing.) C. D. Walcott: Founder of Precambrian Paleobiology. Fortunately, Dawson’s debacle would ultimately prove to be little more than a distracting detour on the path to progress, a redirection spurred initially by the prescient contributions of the American paleon- tologist Charles Doolittle Walcott (1850–1927). Like Dawson before him, Walcott was enormously energetic and highly influential (7, 8). He spent most of his adult life in Fig. 2. Cryptozoon reefs near Saratoga, NY. (Photo by E. S. Barghoorn, Washington, DC, where he served as the CEO of powerful November, 1964.) scientific organizations—first, as Director of the U.S. Geological fossils, the first true cellularly preserved Precambrian organisms Survey (1894–1907), then Secretary of the Smithsonian Institu- ever recorded. tion (1907–1927) and President of the National Academy of After the turn of the century, Walcott moved his field work Sciences (1917–1923). Surprisingly, however, Walcott had little northward along the spine of the Rocky Mountains, focusing first formal education. As a youth in northern New York State he in the Lewis Range of northwestern Montana, from which he received but 10 years of schooling, first in public schools and, reported diverse stromatolitelike structures (11) and, later, later, at Utica Academy (from which he did not graduate). He chains of minute cell-like bodies he identified as fossil bacteria never attended college and had no formally earned advanced (12). His studies in the Canadian Rockies, from 1907 to 1925, degrees (a deficiency more than made up for in later life when were even more rewarding, resulting in discovery of an amazingly he was awarded honorary doctorates by a dozen academic well preserved assemblage of Cambrian algae and marine in- institutions). vertebrates—the famous Burgess Shale Fauna that to this day In 1878, as a 28-year-old apprentice to James Hall, Chief remains among the finest and most complete samples of Cam- Geologist of the state of New York and acknowledged dean of brian life known to science (13, 14). American paleontology, Walcott was first introduced to stroma- Walcott’s contributions are legendary—he was the first dis- tolites—wavy layered mound-shaped rock masses laid down by coverer in Precambrian rocks of Cryptozoon stromatolites, of ancient communities of mat-building microbes—Cambrian-age cellularly preserved algal plankton (Chuaria), and of possible structures near the town of Saratoga in eastern New York State. fossil bacteria, all capped by his pioneering investigations of the Named Cryptozoon (meaning ‘‘hidden life’’), these cabbagelike benchmark Burgess Shale fossils.