The geologic record is the history of Earth as recorded in the rocks that make up its crust. Rocks have been forming and wearing away since Earth first started to form, creating sediment that accumulates in layers of rock called strata. The way these strata are arranged and what fossils are in them give scientists clues about what Earth was like billions of years ago.
The concept of what is called geologic time is somewhat difficult to fully grasp because it deals in such enormous blocks of time. When people first began to seriously study Earth around the seventeenth century, their first estimate of Earth's age was in the thousands of years. One famous example is that of the Irish clergyman James Ussher (1581–1656), who used the Bible to calculate that Earth was created in 4004 b.c. A century later, estimates by others had only raised that number to about 75,000 years, and it was not until the Scottish geologist (a person specializing in the study of Earth) James Hutton (1726–1797) made his famous statement that the Earth contains "no vestige of a beginning—no prospect of an end," did the notion of millions and perhaps billions of years begin to be considered. Today, with advanced tools, scientists are able to say with some certainty that Earth is about 4,600,000,000 years old.
READING THE GEOLOGIC RECORD
By examining the progress of geology, humans touch upon the major breakthroughs that allowed scientists to be able to "read" Earth's geologic record. This geologic history of the planet's evolution (changes occurring over time) and developmental changes is recorded in its rocks. One of the earliest breakthroughs was the eighteenth-century realization that there was something to be learned by the obvious relationships of one type of rock to another. Called the law of superposition, this idea stated that in an undisturbed section of sedimentary rocks (formed in layers or "strata" by weathering and erosion), each layer is older than the one above it and younger than the one below it. Although this seems fairly simple and obvious today, it was a major breakthrough in being able to start to date the age of Earth.
A related idea that also proved very helpful was the principle of faunal succession. This states that the fossils found in rocks also succeed one another in a definite order, and that a time period can be recognized by the type of fossils contained in its rocks. This principle applies throughout the world, so that geologists can identify rocks of the same age even if they are found in widely separated locations. The importance of fossils cannot be overemphasized, since without them, science would lose its primary tool for subdividing geologic time periods into smaller and smaller chunks.
ERAS, PERIODS, AND EPOCHS
Geologists have divided the geologic record into periods that can be organized or charted onto a timescale. The major divisions of geologic time are known as eras which are described by some as "chapters" in Earth's history. Each era is naturally different from another, especially in terms of the nature of life it contained. The eras are then divided into periods. These have nothing to do with the passage of a certain amount of time, meaning that they are not of equal length. Instead, they are based upon the nature of the rocks and fossils found there. Some may be longer than others. The main subdivisions of periods are called epochs. These eras, periods, and epochs usually were named after places on Earth (mostly in Western Europe) where the rocks of those times were first discovered.
In terms of the geologic record, life on Earth is first seen about 3,500,000,000 years ago. This means that for about 1,000,000,000 years from the time Earth first formed, there was no life on the planet. The oldest primitive fossils found were simple prokaryotic organisms (bacteria whose cells did not have a nucleus or any other structures). The first eukaryotic organisms (whose cells contain a nucleus that is surrounded by a membrane) appeared about 1,800,000,000 years ago in what is called Precambrian era.
The first multicellular organisms did not appear on Earth until somewhere between 700,000,000 and 1,000,000,000 years ago. Then, during the Cambrian period of the Paleozoic era, an explosion of multicellular life in the sea took place. Continuing explosions occurred, going from marine invertebrates (animals without a backbone) to the beginnings of actual fishes. Around 435,000,000 years ago, the first land plants evolved, followed by great swamp trees, amphibians, and primitive reptiles. The dinosaurs came on the scene during the Triassic period (about 225,000,000 years ago), and by about 180,000,000 years ago, the Jurassic period saw the first birds and mammals.
Called the father of modern geology (the study of the Earth), Scottish geologist Charles Lyell (1797–1875) offered proof that Earth's surface was the result of natural forces operating very slowly over millions of years. His work laid the foundations not only for modern geology but for the study of evolutionary biology as well.
Charles Lyell was born to a well-to-do family in Kinnordy in eastern Scotland; his family moved to Southampton, England, when he was two years old. His father was an amateur naturalist who kept a well-stocked library that the young Lyell often used. As a youngster, Lyell was more interested in observing nature and collecting butterflies and insects than in school. He entered Oxford University at nineteen and his interest in geology increased, although he prepared for a law career. At twenty-two he graduated and moved to London to study law, although he spent every free moment on geological trips and excursions. By the time he finished law school and was admitted to the bar, he had conducted several geological tours in England and the continent and met with some of the best minds in geology.
By the age of twenty-eight, Lyell still had his father's financial support and was doing much more geology than law. By now, he was doing serious geological research and writing and beginning to formulate his own ideas. After making a long and difficult geological trip through France and Italy, Lyell returned to begin his Principles of Geology, which would become one of the most influential textbooks ever written. The state of geology at this time was such that it was still dominated by individuals who believed that Earth was not very old, perhaps only several thousand years, and that the obvious changes that had taken place were the result of sudden, catastrophic occurrences. Lyell's readings, and mostly his field trips, had convinced him of just the opposite. He agreed with the earlier ideas of his countryman, the Scottish physician James Hutton (1726–1797), who had long before put forth his idea of "uniformitarianism." This theory (written before Lyell was born) said that natural geological forces, like earthquakes, volcanoes, and erosion acted upon the Earth over an extremely long period of time, and that geological change was very slow-acting and sometimes barely noticeable. Lyell's Principles of Geology was published in three volumes from 1830 to 1833. It was so well-written and so well-documented, that despite the fact that it contained few original ideas, it communicated and explained the basic principles of the new geology so well and so effectively that it became a steady best-seller. It went through twelve editions in Lyell's lifetime.
One of those who closely read Lyell's books and who was deeply influenced by them was the English naturalist Charles Darwin (1809–1882). Darwin and Lyell had become good friends, and when Darwin left to take his famous trip in 1831 on the H.M.S. Beagle, he had the first volume of Lyell's book with him. Lyell's ideas of timelessness and gradual change proved to be highly influential when Darwin began to formulate his own ideas about biological evolution (physical changes that occur over generations). In fact, Darwin is said to have once stated, "I always feel as if my books came half out of Lyell's brain." Darwin admitted that he drew heavily on Lyell's book, both for its excellent writing style and its real content. As a result, both men would shape the thinking of their disciplines with their ideas that Earth and life itself was much more ancient than thought, and that existing species appeared to have evolved from previous ones now extinct. Another important geological concept championed by Lyell concerned what is called the geological record. That is, that older rocks are generally buried beneath younger ones, and that careful excavation and study of the geological layers and fossils found there contain the evolutionary history of Earth itself.
Sometime during the Paleocene epoch of the Cenozoic era (about 65,000,000 years ago), the Age of Mammals began. Around 2,000,000 years ago, Homo habilis appeared and was the first human species to be given the genus name Homo, meaning "man."
Finally, because of the geologic record, scientists also know that a major phenomenon like continental drift (the movement of the plates on Earth's crust) is responsible for the current position of the continents. Scientists also know that the record contains evidence of several extinctions. None of these are more dramatic and puzzling than the disappearance of the dinosaurs about 100,000,000 years ago during the Cretaceous period. Scientists have described the geologic record as a history book to be read to learn about Earth's past. However, the geologic record is only useful if one knows how to read its signs and interpret them.