Elso Barghoorn and Stanley Allen Tyler’s photographs of a number of microorganisms ushered in a new era of Precambrian paleontology, and the fossil discoveries were the first in a series crucial to understanding the origin and early development of life on Earth.
On April 30, 1954, the American journal Science published a brief article by Elso Barghoorn and Stanley Allen Tyler titled “Occurrence of Structurally Preserved Plants in Pre-Cambrian Rocks of the Canadian Shield.” The article described five morphologically distinct types of microorganisms, including slender, unbranched filaments and spherical colonies made up of filaments, which were judged to resemble living bluegreen algae, and branched filaments and spherical bodies, which were compared with living aquatic fungi.
These fossils had first come to the attention of Tyler, a geologist working with banded iron deposits of Precambrian age on the shores of Lake Superior. A puzzling circumstance was the association of coal with the iron ore deposits, since the iron ore was known to be approximately 2 billion years old, and no convincing evidence of co-occurring life-forms that could have produced coal was known at the time. Examination of the coal revealed what appeared to be microscopic plants. Tyler showed them to William Schrock
Barghoorn agreed that the material appeared to be microbial. As a biologist, he recognized that convincing proof of life-forms so early in geologic time would be immensely significant. He suggested returning to the site and conducting a systematic search for life-forms in the coal-bearing rocks. Following the coal seams into Canada, they collected samples of black gunflint chert, which was cut into thin sections with a diamond saw for microscopic observation. The choice of rock type was deliberate.
The formation contained visible structures known as stromatolites, of a type earlier workers had tentatively identified as beings remains of algal reefs. Moreover, chert is a sedimentary rock that has been infiltrated by fine-grained quartz; its stability, impermeability, and fine grain led to excellent preservation of organic remains. For example, the Devonian Rhynie chert
When examined under the microscope, the thin sections revealed spheres and filaments. Both clearly were hollow and bounded by a sturdy wall of organic material; there was doubt that these were microorganisms. In the 1954 publication, some specimens were identified as blue-green algae, with a bacterial level of cellular organization, and others as relatively more advanced aquatic fungi. Nevertheless, subsequent detailed investigations have cast doubt on the identification of fungi in this or any other assemblage more than 1.5 billion years old. In papers published in 1965 and 1971, Barghoorn attributed a bacterial or blue-green algal origin to all of the gunflint organisms.
Tyler and Barghoorn were not the first to report microorganisms from Precambrian rocks of the Canadian shield; John W. Gruner
Tyler and Barghoorn continued to collaborate on their investigations of gunflint organisms. After an interval of ten years, they published in Science a more detailed paper on their findings titled “Microorganisms from the Gunflint Chert.”
The impact of Tyler and Barghoorn’s paper is appreciated best in the context of the historical development of Precambrian paleobiology. Since the beginning of systematic paleontological investigation in the early nineteenth century, it has been recognized that the fossil record of readily visible plants and animals begins abruptly with the Cambrian period, originally defined by its stratigraphic position and characteristic fossils and now known to have begun approximately 580 million years ago. Cambrian sedimentary rocks contain abundant fossils of corals, sponges, and shellfish. The abrupt appearance of diverse, relatively advanced animals and the absence of fossils in Precambrian sediments were remarked upon by Charles Darwin, who could find no explanation for this phenomenon.
Charles Doolittle Walcott, an American geologist who has been called the founder of Precambrian geology, made a systematic search for fossils in Precambrian rocks at the beginning of the twentieth century. He found a colonial alga, a few primitive shells, and worm trails in late Precambrian formations in the Grand Canyon but confirmed the lack of macrofossils in most Precambrian sedimentary rocks.
In the years since Tyler and Barghoorn’s paper, no direct fossil evidence for chemical evolution has emerged. The oldest sedimentary rocks that have not been subject to extensive deformation that would obliterate or distort organic traces—the 3.2-billion-year-old Fig Tree group in South Africa and the 3.5-billion-year-old Warrawoona group in Australia—contain rods and spheroids of presumed bacterial or blue-green algal origin, and stromatolites 3 billion years old have been found in Africa. This implies that the structure, mechanisms of heredity, and mechanisms of photosynthesis in prokaryotic cells evolved more than 3 billion years ago.
It has been recognized for some time that living organisms have transformed the face of the earth in the Phanerozoic; the importance of their role in geological processes in the Precambrian was evident only when a usable fossil record became available. The fossil record provides information about the types and in some cases the abundance of microorganisms that were present at various stages in geologic time; the inorganic geologic record provides evidence for atmospheric and climatic changes, and comparison of the biochemistry of living forms provides clues as to probable conditions at the time various metabolic processes evolved.
The fossil record has been helpful in developing hypotheses about the role of oxygen in the early biosphere. The iron formations that were the subject of Tyler’s investigation are the remains of a massive global geochemical event that took place approximately 2 billion years ago and that is the ultimate source of most of the world’s high-grade iron ore. Fossil assemblages corresponding in age to these “red beds” contain numerous simple microbial forms whose size and level of organization correspond well to modern blue-green algae.
Like more advanced algae and higher plants, blue-green algae produce oxygen as a by-product of their manufacture of starch and other carbohydrates from carbon dioxide and water. According to hypothesis, the beginning of the red beds in the geologic column marks the point in time when oxygen-producing photosynthesis evolved. This released free oxygen into water, which held quantities of soluble iron. Over a period of millions of years, oxygen released by blue-green algae oxidized iron in ocean water; the precipitated iron formed the red beds. When soluble iron was depleted, the oxygen content of the atmosphere began to rise.
Indirect fossil evidence for rising oxygen levels exists in the form of specialized nitrogen-fixing cells in blue-green algae and the appearance of eukaryotes, whose metabolism depends on free oxygen. Free oxygen made possible creation of the ozone layer, which was crucial to colonization of the land by plants and animals.
Large-scale geologic changes made by living organisms and the global interaction of the living and nonliving components of the planet are of more than academic interest. Human industrial processes that alter the composition of the earth’s atmosphere may have precedent in the remote geologic past, and an understanding of the dynamics of the system throughout geologic time is crucial to preventing disastrous mistakes in the present.
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citation-type="booksimple" xlink:type="simple">Barghoorn, Elso. “The Oldest Fossils.” Scientific American 224 (May, 1971): 12, 30-42. Includes electron micrographs of microorganisms from the 3-billion-year-old Fig Tree formation in South Africa and excellent color photographs of organisms from the gunflint chert. -
citation-type="booksimple" xlink:type="simple">Day, William. Genesis on Planet Earth. 2d ed. New Haven, Conn.: Yale University Press, 1984. A clear, well-presented account of theory about the origin and nature of life on Earth. Focuses on the chemistry of life processes and possible evolutionary pathways. Best for readers with some familiarity with scientific terminology and organic chemistry. -
citation-type="booksimple" xlink:type="simple">Gould, Stephen Jay. The Lying Stones of Marrakesh: Penultimate Reflections in Natural History. New York: Harmony Books, 2000. By a writer of popular works on the science of life, this book is a good starting point for those interested in natural history and paleontology. Discusses fossils as well. -
citation-type="booksimple" xlink:type="simple">Knoll, Andrew H. The Early Evolution of Life on Earth: The First Three Billion Years of Evolution on Earth. Princeton, N.J.: Princeton University Press, 2003. A well-written work on microscopic fossils of the Precambrian and Cambrian periods, geared to general readers with introductory knowledge of biology as well as to specialists in the field of paleontology. Includes color illustrations and black-and-white photographs. -
citation-type="booksimple" xlink:type="simple">Margulis, Lynn, Clifford Matthews, and Aaron Haselton, eds. Environmental Evolution: Effects of the Origin and Evolution of Life on Planet Earth. 2d ed. Cambridge, Mass.: MIT Press, 2000. A unique study of the effects of life and evolution on Earth’s surface environment. Includes an essay by Elso Barghoorn, coauthored with Paul K. Strother. -
citation-type="booksimple" xlink:type="simple">Schopf, J. William. “The Evolution of the Earliest Cells.” Scientific American 239 (September, 1978): 16, 110-112. Written for well-informed general readers. Illustrated with color micrographs of a variety of Precambrian microorganisms that give a good impression of the nature of the fossil evidence. -
citation-type="booksimple" xlink:type="simple">_______, ed. The Earth’s Earliest Biosphere. Princeton, N.J.: Princeton University Press, 1983. A collection of papers from a symposium on early life. The chapter by Preston Cloud, “Early Biologic History: The Emergence of a Paradigm,” divides Precambrian studies into four eras. Provides a clear account of research prior to 1954 and places the significance of Tyler and Barghoorn’s research in a larger context.
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