Variation and Evolution in Plants and Microorganisms Toward a New Synthesis 50 Years After Stebbins (2000) / Chapter Skim
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2 Solution to Darwin's Dilemma: Discovery of the Missing Precambrian Record of Life
2 Solution to Darwin's Dilemma: Discovery of the Missing Precambrian Record of Life
Pages 6-20
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From page 6... ...
But in recent decades, understanding of life's history has changed markedly as the documented fossil record has been extended seven-fold to some 3,500 million years ago, an age more than three-quarters that of the planet itself. This long-sought solution to Darwin's dilemma was set in motion by a small vanguard of workers who blazed the trail in the 1950s and 1 960s, just as their course was charted by a few pioneering pathfinders of the previous century, a history of bold pronouncements, dashed dreams, search, and final discovery.
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From page 7... ...
[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.
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From page 8... ...
In 1864, however, Logan brought specimens to Dawson who not only confirmed their biologic origin but identified them as fossilized shells of giant foraminiferans, huge oversized versions of tiny calcareous protozoal tests. So convinced was Dawson of their biologic origin that a year later, in 1865, he formally named the putative fossils Eozoon canadense, the "dawn animal of Canada." Dawson's interpretation was questioned almost immediately (King and Rowney, 1866)
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From page 9... ...
. In 1878, as a 28-year-old apprentice to lames Hall, Chief Geologist of the state of New York and acknowledged dean of American paleontology, Walcott was first introduced to stromatolites wavy layered moundshaped rock masses laid down by ancient communities of mat-building microbes Cambrian-age structures near the town of Saratoga in eastern New York State.
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From page 10... ...
Over the next several field seasons, he and his comrades charted the geology of sizable segments of Arizona, Utah, and Nevada, including unexplored parts of the Grand Canyon, where in 1883 he first reported discovery of Precambrian specimens of Cryptozoon (Walcott, 1883~. Other finds soon followed, with the most startling in 1899small, millimeter-sized black coaly discs that Walcott named Chuaria and interpreted to be "the remains of .
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From page 11... ...
His studies in the Canadian Rockies, from 1907 to 1925, were even more rewarding, resulting in discovery of an amazingly well-preserved assemblage of Cambrian algae and marine invertebrates the famous Burgess Shale Fauna that to this day remains among the finest and most complete samples of Cambrian life known to science (Walcott, 1911; Gould, 1989~. Walcott's contributions are legendary he was the first discoverer in Precambrian rocks of Cryptozoon stromatolites, of cellularly preserved algal plankton (Chuaria)
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From page 12... ...
, Tyler published a short note announcing the discovery (Tyler and Barghoorn, 1954) , a rather sketchy report that on the basis of study of petrographic thin sections documents that the fossils are indigenous to the deposit but fails to note either the exact provenance of the find or that the fossils are present within, and were actually the microbial builders of, large Cryptozoonlike stromatolites (an association that, once recognized, would prove key to the development of the field)
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From page 13... ...
, both attracted by questions posed by the abrupt appearance and explosive evolution of shelly invertebrate animals that marks the start of the Phanerozoic Eon. A feisty leader in the development of Precambrian paleobiology, Cloud was full of energy, ideas, opinions, and good hard work.
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From page 14... ...
Breakthrough to the Present My own involvement dates from 1960, when as a sophomore in college I became enamored with the problem of the missing Precambrian fossil record, an interest that was to become firmly rooted during the following few years, when I was the first of Barghoorn's graduate students to focus on early life. I have recently recounted in some detail my recollections of those heady days (Schopf, 1999)
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From page 15... ...
Gunflint chert of southern Canada.
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From page 16... ...
, publication of which viewed in light of the earlier articles on the Gunflint organisms not only served to dispell lingering doubts about whether Precambrian fossils might be some sort of fluke, but seemed to show that the early fossil record was surprisingly richer and easier to unearth than anyone had dared imagine. Indeed, it now appears that the only truly odd thing about the Gunflint and Bitter Springs fossils is that similar finds had not been made even earlier.
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From page 17... ...
Bitter Springs chert of central Australia. Because the petrified microbes are three-dimensional and sinuous, composite photos have been used to show the specimens A-G, I, K, and L
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From page 18... ...
Precambrian paleobiology is thriving the vast majority of all scientists who have ever investigated the early fossil record are alive and working today; new discoveries are being made at an ever quickening clip progress set in motion by the few bold scientists who blazed this trail in the 1950s and 1960s, just as their course was charted by the Dawsons, Walcotts, and Sewards, the pioneering pathfinders of the field. And the collective legacy of all who have played a role dates to Darwin and the dilemma of the missing Precambrian fossil record he first posed.
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From page 19... ...
(1965) Microorganisms from the Late Precambrian of central Australia.
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From page 20... ...
(1966) The late Precambrian fossils from Ediacara, South Australia.
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