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In 1950, when Ledyard Stebbins' Variation and Evolution in Plants first appeared, the known history of life—the familiar progression from spore-producing to seed-producing to flowering plants, from marine invertebrates to fish, amphibians, then reptiles, birds, and mammals—extended only to the beginning of the Cambrian Period of the Phanerozoic Eon, roughly 550 million years ago. Now, after a half-century of discoveries, life's history looks strikingly different—an immense early fossil record, unknown and assumed unknowable, has been uncovered to reveal an evolutionary progression dominated by microbes that stretches seven times farther into the geologic past than previously was known. This essay is an abbreviated history of how and by whom the known antiquity of life has been steadily extended, and of lessons learned in this still ongoing hunt for life's beginnings.

PIONEERING PATHFINDERS
Darwin's Dilemma

Like so many aspects of natural science, the beginnings of the search for life's earliest history date from the mid-1800s and the writings of Charles Darwin (1809–1882), who in On the Origin of Species first focused attention on the missing Precambrian fossil record and the problem it posed to his theory of evolution: “There is another . . . difficulty, which is much more serious. I allude to the manner in which species belonging to several of the main divisions of the animal kingdom suddenly appear in the lowest known [Cambrian-age] fossiliferous rocks . . . If the theory be true, it is indisputable that before the lowest Cambrian stratum was deposited, long periods elapsed . . . and that during these vast periods, the world swarmed with living 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. The case at present must remain inexplicable; and may be truly urged as a valid argument against the views here entertained” (Darwin, 1859, Chapter X).

Darwin's dilemma begged for solution. And although this problem was to remain unsolved—the case “inexplicable”—for more than 100 years, the intervening century was not without bold pronouncements, dashed dreams, and more than little acid acrimony.

J. W. Dawson and the “Dawn Animal of Canada”

Among the first to take up the challenge of Darwin's theory and its most vexing problem, the missing early fossil record, was John William



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