Fossils of the earliest life-forms

Identification and study of the earliest fossils have expanded the time frame during which life is known to have existed on the Earth, revealing information that has important implications for the origins of life and, thus, for human evolutionary ancestry.

Discovery of Precambrian Fossils

The search for the earliest fossils has focused on the remains of microscopic, single-celled organisms. The study of these microfossils has yielded a better appreciation of the antiquity of life. Fossils visible to the naked eye have been found in rocks dating back 600 million years. This record corresponds to the Cambrian diversification, sometimes called the Cambrian “explosion”—a proliferation of hard-shelled animals that left their imprint in the geological ledger and the preceding Ediacaran, in which soft-bodied organisms are preserved. The Cambrian period heralded the arrival of complex organisms that had skeletons and that were composed of many cells adapted to specialized functions. Prior to 1950, little was known about the Precambrian history of life, principally because there were no fossils to provide the necessary information. Did organisms suddenly appear after the Earth endured four billion years of a lifeless existence? Scientific intuition suggested that simple, unicellular creatures must have preceded the arrival of macroscopic organisms, yet no evidence of this development had been found.

In 1954, Stanley Tyler and Elso Barghoorn published a report of bacteria-like fossils found in the Gunflint iron formation of Canada, dated nearly two billion years before the present. Through the Gunflint research and related studies on early life, Barghoorn emerged as a leading figure in the growing discipline of paleobotany—the study of ancient plants—until his death in 1984. In the words of his colleagues, “Elso will undoubtedly be most clearly remembered as the man who pushed the history of life back sixfold.”

The Gunflint fossils have been extensively studied in the years since their discovery. The geological formations that house them are known as stromatolites, a term that refers to a layered structure formed by successive generations of sticky microbial mats that trap sediment and gradually build a dome-shaped structure up to several meters high. Such mat communities are known to be associated with photosynthetic cyanobacteria and certain other microorganisms that occur widely on the modern Earth. The singular advantage of stromatolites from a paleontological viewpoint is that, after the organic material decomposes, the sedimentary structure provides a permanent record of the morphology of the organisms that formed it. The Gunflint rocks are composed of chert, a form of quartz that is especially resistant to compression and thus helps to preserve the fossils. The stromatolites that were studied by Barghoorn and Tyler revealed the presence of six distinct organisms, indicating a highly diversified biological community from two billion years ago.

Later, a millimeter-wide fossil of a new species of alga was discovered near Gold Point, Nevada. The discovery of this Precambrian-era soft-tissue preservation of this species was among the only of its kind in North America and was one of only a few in the world. Scientists named the species Elainabella deepspringensis. Another twenty-first-century discovery in North America provided trace evidence of a phyllocarid crustacean (an ancestor of crabs and lobsters) trapped in a tidal pool from a time before animals left the water for land This discovery, however, was dated to the middle Cambrian period.

Precambrian Paleobiology Research Group

The documentation of the Gunflint fossils from the Proterozoic era (about 540 million to 2.5 billion years ago) spurred the search for more ancient remains of life. The African continent, considered by many scientists to be the cradle of human evolution, has also provided a rich source of microbial fossils. In the 1960s, Barghoorn and his student J. William Schopf studied a set of 3.1-billion-year-old rocks known as the Fig Tree Cherts from the region in South Africa and Eswatini (formerly Swaziland) called the Barberton Greenstone Belt, largely located in the Makhonjwa Mountains. Microscopic analysis revealed two unknown microbes, to which Barghoorn and Schopf assigned characteristic Latin names in the naturalist's tradition: Eobacterium isolatum for a rod-shaped relic and Archaeosphaeroides barbertonensis for a spheroidal fossil. The latter resembles modern algae-like cyanobacteria and, like the Gunflint study, suggests the operation of photosynthesis at an early stage in biological evolution. More significantly, the Fig Tree Cherts established that life has existed on Earth for the majority of its history. Hence the origin of life had to be placed in the Archean eon, the period from about 2.5 to 4 billion years before the present.

In an effort to codify and extend the evidence of early life, Schopf initiated an organized project in the later 1970s known as the Precambrian Paleobiology Research Group (PPRG). With support from the National Aeronautics and Space Administration (NASA), Schopf and a team of twenty-three scientists from every relevant field compiled an exhaustive study of ancient fossils with the goal of characterizing the earliest biosphere of the Earth. A major site of the PPRG effort was a set of fossils from the Pilbara province of northwest Australia known as the North Pole chert, a name given by early prospectors to this remote location. Following the discovery of stromatolites in 1977 by an Australian graduate student named John Dunlop, Schopf's group conducted extensive fieldwork and laboratory research to establish the age and authenticity of these microfossils. Dating techniques used by the Geological Survey of Western Australia confirmed the age of this formation, called the Warrawoona sequence (or Warrawoona group), at 3.5 billion years. The PPRG scientists identified a variety of round and wormlike structures that provide strong evidence for the existence of life only one billion years after the Earth formed. Isotopic analysis also supports a biological origin for these structures, some of which correspond in shape to cyanobacteria and other modern organisms but also others that have no modern counterpart. Most researchers agree that Australia's North Pole stromatolites, along with the prokaryote fossils of South Africa's Barberton region, are among the oldest evidence of life yet discovered.

Classification

The classification of the organisms that created the Archean stromatolites has generated considerable debate. If the fossils in the structures represent the ancestors of modern cyanobacteria, as suggested by their general size and shape, then photosynthesis had presumably become an important metabolic pathway at this stage in evolution. Geologists have argued, however, that the photosynthetic production of oxygen must have been a later development since the geological record shows little evidence of free oxygen during the Archean eon. Stromatolites can also be formed by bacteria that are not photosynthesizers, so the existence of such formations in the Archean does not require the existence of photosynthesis. Schopf, who led the study of the North Pole fossils, believes that the biological production of oxygen did not emerge until one billion years after these stromatolites were formed.

Other paleobotanists, such as Andrew Knoll (like Schopf, a student of Elso Barghoorn), proposed that photosynthesis had already developed 3.5 billion years ago but that such organisms were so limited by the small continental shelf and by the harsh ultraviolet radiation on the early Earth that they did not significantly alter the atmosphere. An alternative way to reconcile the postulated early photosynthesis with the geological record is to consider that the biologically produced oxygen might have disappeared by combining with organic molecules in the ocean. The similarity between carbon isotope ratios in stromatolites and modern cyanobacteria also suggests an Archean beginning for photosynthesis. Nevertheless, the limited evidence prevents any definitive conclusions regarding the metabolism of the organisms that made up the North Pole communities.

Rocks that are older than the Warrawoona sequence have been discovered by geologists, but the existence of fossils in these formations is more problematic. Much attention has centered on a set of rocks from Greenland known as the Isua deposits, dated 3.8 billion years before the present. Unlike the North Pole fossils, these rocks have undergone extensive deformation as a result of the extreme temperatures and pressures to which they have been subjected throughout geologic time. While fossils may once have existed, their presence is difficult to establish because of the lack of pristine evidence. The longer a rock has been on the Earth, the higher the probability that it will have been deformed in the manner of the Isua deposits. Thus, attempts to push the frontier of life beyond the 3.5-billion-year mark set by the Australian discoveries will undoubtedly encounter this dilemma, though the search for more ancient fossils will continue.

Study of Cellular Fossils

The two primary techniques that are employed to characterize cellular fossils are microscopic examination and chemical analysis. The former method is important for studying the shape of the organism and thus relating it to contemporary cells with a similar form. Thin sections of the rock are examined first under a light microscope using reflected and transmitted white light. In addition, impressions of the fossils are examined by electron microscopy, which reveals even finer details. Since the organisms themselves have long since decayed, such an inspection of the sedimentary imprint provides only the outlines of the cellular structure, from which inferences must be drawn regarding its relationship to modern biology. Special care must be taken to ensure that the suspected fossil is not merely a bubble or other artifact and that a true biological structure is not a result of contamination by a modern organism.

Chemical analysis of the fossils is important both to establish their age and to deduce the nature of their previous biochemistry. For both these objectives, measurements of specific isotopes (atomic species with a characteristic mass) play a major role. The decay of radioactive isotopes is the best technique for learning how old a sample is, and a variety of methodologies have been developed for this purpose. Fortunately, nature has provided many different radioisotopes, each with a distinct rate of disintegration. Carbon dating, for example, is usually employed in archaeological research because the half-life (the time required for 50 percent of the element to decay) of carbon-14 is only 5,730 years. For fossils that are billions of years old, a longer-lived isotope is required. Two that are commonly utilized are potassium-40, with a half-life of 1.27 billion years, and rubidium-87, with a half-life of 48.9 billion years. By comparing the ratios of the radioisotopes with their more stable products (argon-40 in the case of potassium and strontium-87 in the case of rubidium), scientists can establish the age of sediments to within an uncertainty of a few percentage points. Carbon dating has improved over time, becoming faster and more reliable. Many modern scientists use a form of radiocarbon dating called accelerator mass spectrometry, also called AMS dating, which is quicker, requires less of a sample, and is more sensitive. However, it is likely that the use of fossil fuels has interfered with dating more recent findings.

The analysis of organic material in fossils can provide clues to the biological function of ancient organisms. Although one cannot usually determine which compounds were originally present in the living microbe, the distribution of carbon, hydrogen, and oxygen isotopes provides a partial record of the metabolic activity of the fossilized cell because organisms selectively incorporate one isotope in preference to another form of the same element. In general, the lighter forms are enriched in living organisms, as enzymatic reactions proceed faster than with the heavier isotopes, which, for carbon chemistry, means that cells contain a higher percentage of carbon-12 than would be expected in the absence of biological activity. The extent of isotopic enrichment in microfossils can provide a marker for the nature of such metabolic activity.

Another direct marker of metabolic productivity consists of rocks deposited during the period when a particular organism flourished. Geologists have focused especially on the presence of minerals that are formed by reactions with molecular oxygen, a by-product of green plant photosynthesis. Analysis of one such mineral, uraninite, indicates that atmospheric oxygen at the end of the Archean eon (about 2.5 billion years ago) was only about 1 percent of the present level. Another possible marker of atmospheric oxygen is the so-called banded iron formations (BIFs), alternating layers composed of the minerals magnetite and hematite. Although the abundance of iron minerals has been correlated with relative amounts of atmospheric oxygen, similar deposits can also be formed by reactions that do not require oxygen, and thus, the interpretation of the banded iron formations is difficult. The key point on which geological studies concur is that free oxygen was nearly absent during the Archean eon when the earliest microbes lived.

Principal Terms

Archean eon: the early Precambrian period, from about 2.5 to 4 billion years ago, roughly corresponding to the earliest known fossils

Cambrian period: the period from about 540 to 485 million years ago, marked by the appearance of hard-shelled organisms

cyanobacteria: previously called blue-green algae; these oxygen-producing, photosynthetic microbes often live in matlike clusters

isotopes: atoms of the same element that differ in mass

microfossil: the characteristic imprint left by a microscopic organism in a geological formation

paleobiology: the study of the most ancient life-forms, typically through the examination of microscopic fossils

photosynthesis: the biological process of using sunlight to form energy-rich compounds, usually with the production of free oxygen

Proterozoic era: the late Precambrian era, from about 600 million to 2.5 billion years ago, before the proliferation of macroscopic life

stromatolite: a structure produced by the trapping or binding of sediment as a result of the growth and metabolic activity of microorganisms

Warrawoona sequence: a geological formation in Western Australia, dated at 3.5 billion years before the present, where “North Pole” stromatolites were discovered

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