The announced discovery of amino acids—the building blocks of life—in rocks up to 3.1 billion years old influenced thinking about evolution and the fundamental nature of biological systems.

On November 16, 1967, J. William Schopf and Elso Barghoorn of Harvard University and Keith Kvenvolden of the U.S. Geological Survey presented a paper to the National Academy of Sciences summarizing their search for traces of amino acids in the oldest known sedimentary rocks. These findings were published in the Proceedings of the National Academy of Sciences in 1968. This team of scientists, consisting of two paleontologists and one organic geochemist, had analyzed organic material leached from pulverized black cherts from three Precambrian formations: the 1-billion-year-old Australian Bitter Springs formation Bitter Springs formation , the 2-billion-year-old Canadian Gunflint chert Gunflint chert , and the 3-billion-year-old Fig Tree chert Fig Tree chert from South Africa. The latter was the oldest undeformed Precambrian sedimentary rock known at the time. Amino acids
Paleontology
Fossils
Life, origins of
[kw]Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks (Nov. 16, 1967)
[kw]Amino Acids in 3-Billion-Year-Old Rocks, Barghoorn and Colleagues Find (Nov. 16, 1967)[Amino Acids in Three Billion Year Old Rocks, Barghoorn and Colleagues Find]
Amino acids
Paleontology
Fossils
Life, origins of
[g]North America;Nov. 16, 1967: Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks[09490]
[g]United States;Nov. 16, 1967: Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks[09490]
[c]Biology;Nov. 16, 1967: Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks[09490]
[c]Chemistry;Nov. 16, 1967: Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks[09490]
[c]Archaeology;Nov. 16, 1967: Barghoorn and Colleagues Find Amino Acids in 3-Billion-Year-Old Rocks[09490]
Barghoorn, Elso
Schopf, J. William
Kvenvolden, Keith
Miller, Stanley L.

The Gunflint locality had already yielded abundant evidence of early life in the form of many examples of structurally preserved microorganisms; it was the subject of a classic 1954 paper in the journal Science by Barghoorn and Stanley Allen Tyler Tyler, Stanley Allen , which contained the first indisputable reports of early Proterozoic microfossils. Well-preserved microorganisms had been reported in the Bitter Springs formation by Barghoorn and Schopf in 1965. The fossil evidence for life in the Fig Tree chert was not as compelling, but Schopf and Barghoorn were in the process of examining this material and had reported bacterial microfossils visible with electron microscopy in it in Science in 1966.

In a sense, chemical fossils are potentially more informative than fossils of whole organisms in reconstructing the evolutionary history of life, especially in its earliest stages. All life on Earth today shares a unique carbon-based chemistry. The presence of deoxyribonucleic acid Deoxyribonucleic acid;fossils (DNA) and amino acids is virtually a definition of the presence of life, and these chemicals or their breakdown products are one of the things for which scientists routinely search in meteorites and lunar samples.

If the presence of DNA or amino acids could be unequivocally demonstrated in the earth’s earliest sedimentary rocks, it would be profoundly significant from several perspectives. First, it would provide corroboration that the ambiguous microfossils found in the Fig Tree chert and other extremely ancient (greater than 3 billion years old) Archaean formations were microbial cells and not aggregations of organic matter formed by some nonbiological process. Second, it would be evidence that these earliest cells had a chemical composition and metabolism similar at least in its broad outlines to present-day microorganisms. Third, it might provide direct fossil evidence for stages in evolution prior to the emergence of discrete microbial cells.

The theory that a period of chemical evolution preceded the emergence of the earliest true life-forms was first coherently articulated by Soviet chemist and biologist Aleksandr Ivanovich Oparin Oparin, Aleksandr Ivanovich in the 1920’s and 1930’s. In the early 1950’s, Harold C. Urey Urey, Harold C. and Stanley L. Miller expanded this theory for the early evolution of life, postulating a “soup” of organic chemicals that existed in water bodies on primeval Earth. Heat, electrical discharge, and cosmic rays acted on this “soup” to produce a broad spectrum of organic compounds, including those that characterize living systems. On a global scale, over millions and millions of years of geologic time, chemical reactions led to increasingly complex molecules, eventually producing a proto-DNA molecule, which was able to make copies of itself and to direct the synthesis of other complex compounds. The ability to synthesize a membrane to enclose the replicating genetic material was a crucial step in the evolution of simple cells.

The map shows three of the better-known sites where amino acids, the building blocks of life, were found in ancient rocks.

In theory, sensitive chemical analysis could detect stages in the precellular evolutionary sequence, although the evidence that Barghoorn and colleagues had in hand suggested that the earliest material available to them contained primitive cells. Nevertheless, the chemistry of these early microorganisms, especially those of the 3-billion-year-old Fig Tree formation, was of profound potential significance to students of evolution.

The types and quantities of amino acids present in the samples were determined by pulverizing carefully cleaned samples of hard, virtually impermeable chert and leaching any organic material present with various solvents. The nature of the organic material was determined by gas chromatography. Twenty amino acids were identified in all samples; a twenty-first occurred only in the Bitter Springs formation. Concentrations were extremely low and decreased with increasing geologic age. Most of the organic matter in the samples consisted not of amino acids or other unaltered biological compounds, but of kerogen, an amorphous mixture of organic hydrocarbons. Kerogen is a precursor of petroleum.

Barghoorn and coauthors noted that the distribution of concentrations of various amino acids in all three samples corresponded to the distribution of amino acids from living organisms but differed from results obtained from nonbiological synthesis. The absence of amino acids in controls all but eliminated the possibility of field or laboratory contamination of the samples. Because the amino acids occurred with microfossils in formations high in organic matter, the scientists concluded that they had a biological origin contemporaneous with the microfossils, and that this provided both further confirmation of the existence of life as early as 3 to 3.1 billion years ago and evidence that these fundamental chemical building blocks of cells had remained essentially unchanged from the dawn of organized cellular life to the present.

The discovery of amino acids in ancient Precambrian sediments attracted attention throughout the scientific community, because it confirmed predictions about the nature of early evolutionary events that had been postulated on theoretical grounds and by comparisons of living organisms. The validity of the discovery, however, was questioned almost immediately.

An important piece of information that the authors acknowledged to be missing from the 1968 paper was the optical configuration of the amino acids that had been discovered. Amino acids, and many other complex biological molecules, exist in two forms, a dextro-, or right-handed, form, and a levo-, or left-handed, form, which are mirror images of each other. Although identical in chemical composition and in most physical and chemical properties, they are not identical in living systems. Owing, presumably, to some chance event in early evolutionary history, living systems are composed of left-handed organic molecules. Therefore, one would expect that a sample of amino acids of biological origin would consist primarily of levo-forms, whereas one of nonbiological origin would consist of equal parts levo- and dextroforms. After long exposure to elevated temperature and pressure, however, biologically produced amino acids undergo racemization; that is, they also become mixtures of the two forms.

In 1969, Kvenvolden and colleagues published a short paper in Nature on the optical configuration of amino acids in the Fig Tree chert, in which they reported that they had determined that levoforms predominated. Thus, the case for their biological origin was strengthened, but the authors also raised the question of whether the amino acids were indeed contemporaneous with the formation. If so, it indicated that the quality of chemical preservation in these 3-billion-year-old rocks was better than that in much younger formations that contained well-preserved organic micro- and macrofossils. The care that had been taken in processing, the consistency of results in multiple samples from the same site, and the absence of amino acids in controls argued compellingly against contamination by the workers themselves, but there remained the possibility that the chert itself was not completely impermeable to dissolved chemicals and that the amino acids were the result of more recent biological activity.

In a 1983 review titled “Precambrian Organic Geochemistry,” “Precambrian Organic Geochemistry” (Hayes)[Precambrian Organic Geochemistry] J. M. Hayes Hayes, J. M. and coauthors expressed doubt that any of the complex soluble chemical fossils reported from the Precambrian are as old as the rocks that contain them, citing Kvenvolden’s paper on optical configuration and a calculation that predicts that amino acids racemize within a million years at 25 degrees Celsius. They conclude that “the status of chemical fossils in the Precambrian can, thus, be easily summarized: the low concentrations, evident mobilities, and recognized instabilities of many ’Precambrian’ chemical fossils makes their interpretation problematic and leads to the conclusion that these materials do not provide compelling evidence regarding any details of early life.”

Since 1968, several sedimentary formations older than the Fig Tree chert have been identified, including the Onverwacht formation Onverwacht formation , which underlies it, the 3.5-billion-year-old Warrawoona group Warrawoona group in Australia, and the 3.8-billion-year-old Isua formation Isua formation in Greenland. The Warrawoona group contains stromatolites, simple filamentous microfossils that Schopf accepts as being the oldest plausible microbial microfossils known, and extensively dehydrogenated kerogen with a carbon isotope ratio suggesting a biotic origin. Information on the chemical composition of the microorganisms in this formation would be tremendously useful to scientists studying evolution. Had photosynthesis already evolved at this early date, or did these organisms rely on a chemical energy source? How did the basic building blocks of life 3.5 billion years ago compare to those today? Unfortunately, the state of the rocks is such that any complex chemicals contained in them are almost certainly of recent origin, so a fossil answer to these questions seems unlikely.

The investigation of chemical fossils failed to fulfill its promise as a tool for the study of the earliest phases in the evolution of life on Earth, and scientists studying evolution continue to rely on comparative biochemistry of living forms, mathematical models incorporating known physical parameters, and laboratory experiments in their efforts to unravel the story of the emergence of the unique chemical processes that characterize life on Earth. Amino acids
Paleontology
Fossils
Life, origins of

• Brooks, J., and G. Shaw. Origin and Development of Living Systems. New York: Academic Press, 1973. A comprehensive textbook for students of biology and paleontology that covers the origins of the solar system and Earth, the common patterns of life’s biochemistry and metabolism, theories of chemical evolution prior to the emergence of the first true organisms, the Precambrian fossil record, including chemical fossils, and the relevance of organic chemicals in meteorites.
• Day, William. Genesis on Planet Earth. 2d ed. New Haven, Conn.: Yale University Press, 1984. A clear, well-presented account of current theory about the origin and nature of life on Earth. The discussions of the basic building blocks of life are useful background for understanding what scientists would have liked to have found in Precambrian chemical fossils. Presupposes familiarity with scientific terminology and organic chemistry.
• Kvenvolden, K. A., E. Peterson, and G. E. Pollock. “Optical Configuration of Amino Acids in Precambrian Fig Tree Chert.” Nature 221 (1969): 141-143. Casts serious doubt on the Precambrian origin of the amino acids reported by Barghoorn, Schopf, and Kvenvolden a year earlier.
• Schopf, J. William. Cradle of Life: The Discovery of Earth’s Earliest Fossils. Princeton, N.J.: Princeton University Press, 1999. A good introduction to the fossil record and its significance to evolution. A well-illustrated work, with a glossary, bibliography, and indexes.
• _______, ed. The Earth’s Earliest Biosphere. Princeton, N.J.: Princeton University Press, 1983. A collection of papers from a symposium on early life. Chapter 4, “Prebiotic Synthesis and the Origin of Life,” describes current notions about the physical environment of the early Earth, from its initial formation to the first fossil evidence for life 3.5 billion years ago. Chapter 5 summarizes the literature casting doubt on Precambrian records of complex organic compounds.
• _______. Life’s Origin: The Beginnings of Biological Evolution. Berkeley: University of California Press, 2002. A journey for general readers that looks at the beginnings of life on Earth, with discussion also of the chemistry of amino acids and its role as a building block of life. Includes a useful glossary, bibliographies, and an index.
• Schopf, J. William, Keith A. Kvenvolden, and Elso S. Barghoorn. “Amino Acids in Precambrian Sediments: An Essay.” Proceedings of the National Academy of Sciences 69 (1968): 639-646. A straightforward technical report that describes the research methods in detail, including the issue of modern contamination.

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