Planet Earth was formed roughly 4.6 billion years ago. For human beings—used to measuring time in terms of days, weeks, and months—such an enormous span of time can be a difficult concept to grasp. Geologists, scientists who study the Earth and the processes that continue to shape it, have broken up this vast expanse of “deep time” into major divisions based on what they have learned from the study of ancient rocks and fossils. The first of these divisions—from approximately 4.6 billion until 542 million years ago—is known as the Precambrian, meaning everything that happened before the Cambrian period. (Today some people prefer to call this period the Cryptozoic, which means “hidden life.”) Almost all of planet Earth’s history is Precambrian. Until recently, however, it has remained the most unknown, the strangest, and most perplexing period in all geologic history—what some have referred to as the “Dark Ages” of Earth’s existence.
For centuries, the Earth yielded no fossil record to help humans envision Precambrian time. Discoveries of rich caches of fossils from the Cambrian period enabled scientists to assemble a vivid picture of the creatures that inhabited the planet during that time, but the Precambrian Earth remained largely unimaginable. What was known as the “missing fossil record” of the Precambrian period stood for more than a century as one of the great unsolved mysteries of the natural sciences. This mystery perplexed Charles Darwin and many other scientists who followed in his footsteps. Humans had not yet developed the various methods to accurately determine the age of rocks formed during this interval of geologic time. They had not yet identified and interpreted the remains of the microscopic bacteria that formed in the earliest oceans. The story scientists were beginning to piece together was full of gaps and inconsistencies. It offered glimpses of an alien Earth with an unstable, roiling surface, rocked by volcanic events and cosmic collisions, alternated between extremes of ice and fire, and an atmosphere that would poison most life as we know it today. How did such a hellish place give rise—over the course of an almost unimaginable span of years—to all the familiar features of our planet: oceans, mountains, and valleys, and an oxygen-rich atmosphere that sustains the flowering of plant and animal life in all its countless forms?
Scientists who study the Precambrian period grapple with some of the most profound questions that human beings have ever asked: How old is the Earth? Where did the Moon come from? What made the oceans and the mountains and valleys? How and when did life begin? People of different cultures and religious faiths have shared creation stories to help explain these ancient mysteries. Paleogeologists and paleobiologists—scientists who study the ancient Earth and the life-forms that arose on it—have devised scientific tools to begin answering the same questions.
What has enabled scientists to speak with authority about what occurred on this planet billions of years ago? Geologists have developed systems of dating that enable them to study the geologic processes taking place on the planet today and make educated guesses about its past. James Hutton (1726–1797), considered by many to be the founder of modern geology, laid the groundwork for this earth science in 1785 when he presented his scientific papers at the Royal Society of Edinburgh in Scotland. Hutton’s bold thesis had to do with the concept of geologic cycles—the recognition that processes such as erosion, deposition, sedimentation, and upthrusting are cyclical and must have been repeated many times over the long course of Earth’s history, with “no vestige of a beginning, no prospect of an end.” Entire mountain ranges rise up and are eroded away, over and over again. Hutton reasoned that since each geologic cycle takes many millennia to complete, the Earth must be far older than anyone had previously believed. The term “uniformitarianism” was introduced to describe this process by Cambridge scholar William Whewell in 1832.
The Scottish geologist Charles Lyell elaborated on Hutton’s theory with his own theory of gradualism. When Charles Darwin embarked aboard the HMS Beagle on his legendary voyages, he brought along Charles Lyell’s book Principles of Geology (1830). In this volume, Lyell sets forth the argument (radical at that time) that present-day geological processes can explain the history of the Earth. Many people believed that the biblical story of the flood, or some such cataclysmic event, accounted for Earth’s geological features. Lyell instead argued that these features were produced gradually over millennia by geologic processes still occurring today, which have operated uniformly throughout history. (The motto of uniformitarian science might be summed up as “The present is the key to the past.”)
Another benchmark in geochronology, the dating of events in the Earth’s history, stemmed from William Smith’s work with faunal sequence. During preparations for the digging of a coal canal in southwestern England in 1793, Smith observed that layers, or strata, of sedimentary rock contained fossils in a definite sequence, and that the same sequence could be found in rocks elsewhere. The discovery that fossil plants and animals succeed one another in time in a predictable manner, now known as the law of faunal succession, became one of the keys to unlocking the secrets of deep time. Another key was provided by French zoologist Georges Cuvier’s 1812 hypothesis that fossils do record geologic events. These advances in geochronology were based on the phenomenon of stratification, the naturally occurring sequence of rocks in layers with the youngest rocks on the top and the oldest on the bottom. Stratigraphy, a branch of geology concerned with studying the stratification of rocks, enables scientists to determine the age of rocks and fossils relative to one another.
Encyclopædia Britannica, Inc. Source: International Commission on Stratigraphy (ICS)
In the 20th century, scientists devised methods of dating rocks that were based on chemicals rather than fossil sequences. Antoine-Henri Becquerel’s discovery of radioactivity in 1896 proved key to the development of radiometric dating techniques that revolutionized the science of geochronology. Some elements, such as uranium, undergo radioactive decay, a process that happens at a uniform and predictable rate. In 1905, John William Strutt was the first person to successfully apply a radiometric technique to the study of earth materials when he succeeded in determining the age of a radium-containing rock by analyzing its helium content.
Radioactive decay is a property of certain naturally occurring elements, and radioactive isotopes can also be created under laboratory conditions. Each radioactive isotope has a fixed rate of decay, called its half-life, during which radioactive “parent” atoms transform into “daughters,” or atoms of a different chemical element. Naturally occurring uranium isotopes transform into lead, and rubidium isotopes become strontium as they decay. By determining the ratio of parent to daughter atoms, one can accurately calculate the age of a rock containing those elements.
Today the geologist’s toolkit includes many radiometric techniques that use isotopes to determine the age of ancient rocks and fossils. There are two main methods of observing radioactive decay. The first of these detects and counts radioactive atoms through the radiation they emit. Carbon-14 dating is an example of this first method. This technique depends upon the fact that the radioactive isotope carbon-14 present in the atmosphere and contained in all organic matter will begin to decay once an organism has expired. By comparing the carbon-14 present in a dead object to the carbon-14 in living matter, one can determine when an organism died. The second method uses a device called a mass spectrometer to count the atoms of each isotope, sifting them one atom at a time according to weight. The potassium-argon dating method is an example of this second method. Unlike stratigraphic methods of dating that supply relative ages, radiometric methods enable scientists to determine the approximate age of an object. Both absolute and relative dating techniques are important and complementary tools in geochronology, however, since not every rock can be dated radiometrically.
Geologic processes can serve as absolute chronometers, as well. Measurements based on weathering processes (as in obsidian dating), tree-ring and coral growth, and variations in the Earth’s magnetic field—where magnetic minerals in rocks provide the record of ancient change—have enabled geologists to precisely calculate the ages of rocks. The nature of the most ancient rocks suggests there are even older rocks that have yet to be found.
All these dating methods, along with recent fossil discoveries, have helped geologists formulate a clearer picture of the earliest period in Earth’s history. Precambrian time is currently divided into three eons: the Hadean Eon, the Archean Eon, and the Proterozoic Eon. Each has a unique character and represents a major stage in the planet’s life. The Hadean (4.6 to about 4 billion years ago), which takes its name from Hades, the ancient Greek underworld, predates the formation of any rocks, so what people know about this time is largely based on computer models and guesswork. During the Hadean Eon, many scientists believe that the solar system formed out of gases and dust, the sun began to shine, and Earth took shape in the midst of meteor showers and other galactic debris.
The Archean Eon dates from approximately 4 billion to 2.5 billion years ago and marks the beginning of life and geological processes on Earth. During the Archean Eon, Earth’s atmosphere was a brew of nitrogen, methane, and carbon dioxide. A crust formed on the surface of the cooling planet, and structures called stromatolites, consisting mainly of blue-green algae and other microorganisms, developed in the oceans. The oldest rocks on Earth date from this time, and all the planet’s continents have Archean cores.
The Proterozoic Eon, the longest chapter in Earth’s history, dates from 2.5 billion to 542 million years ago and includes the beginning of the geological process called plate tectonics. Moving plates on the planet’s surface converged to create mountains and fractured entire continents when they pulled apart, a process that continues today. The oldest known glacial episode, or ice age, occurred in the Proterozoic Eon, during which all or nearly all of Earth’s surface was covered by some amount of ice. During warmer periods, new varieties of bacteria began to harness the power of the sun through the biochemical process of photosynthesis. The oxygen they produced collected in the oceans and then in the atmosphere, sparking an ecological event known as the Great Oxidation Event, creating conditions poisonous to the anaerobic life-forms on Earth. The ancient anaerobes retreated underground, and oxygen-dependent life-forms began to evolve.
Studying the Precambrian is like having a front-row seat at the creation of the Earth. Here, in deep time, one finds the oldest minerals and rocks, the earliest oceans, and the first stirrings of biological life—evidence of which has been found in western Greenland. The Precambrian period also contained the first stages of sexual differentiation and the beginnings of sexual reproduction with the appearance of the first eukaryotes, a huge group of organisms that would eventually include protozoans, fungi, plants, animals, and, finally, human beings.
Contemporary concerns about global warming have spurred interest in paleoclimatology, the study of climatic conditions of past geologic ages. Scientists know that during the long course of Precambrian time, the climatic conditions of Earth changed radically. Evidence of this can be found in the sedimentary record, which documents significant changes in the composition of the atmosphere and oceans over time. The Earth’s deepest secrets still abide in the stones under our feet, but the more people learn about the planet’s Precambrian history, the better prepared we may be to face the challenges of the present. Geological science teaches us that the present is the key to the past, but we may find, before long, that this knowledge is also the key to the future.