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External WebsitesAstronomer and science writer Carl Sagan (1934-96) was a highly popular and influential figure in the United States and controversial in scientific, political, and religious circles for his views on extraterrestrial.
Carl Sagan , Lynn MargulisDistinguished University Professor, University of Massachusetts, Amherst. Coauthor of Five Kingdoms: An Illustrated Guide to the Phyla of Life on Earth; What Is Life?; and others.
Lynn Margulis • All Fact-checked by The Editors of Encyclopaedia BritannicaEncyclopaedia Britannica's editors oversee subject areas in which they have extensive knowledge, whether from years of experience gained by working on that content or via study for an advanced degree. They write new content and verify and edit content received from contributors.
The Editors of Encyclopaedia Britannica Last Updated: Sep 3, 2024 • Article History Table of ContentsAsk the Chatbot a Question
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The evidence is overwhelming that all life on Earth has evolved from common ancestors in an unbroken chain since its origin. Darwin’s principle of evolution is summarized by the following facts. All life tends to increase: more organisms are conceived, born, hatched, germinated from seed, sprouted from spores, or produced by cell division (or other means) than can possibly survive. Each organism so produced varies, however little, in some measurable way from its relatives. In any given environment at any given time, those variants best suited to that environment will tend to leave more offspring than the others. Offspring resemble their ancestors. Variant organisms will leave offspring like themselves. Therefore, organisms will diverge from their ancestors with time. The term natural selection is shorthand for saying that all organisms do not survive to leave offspring with the same probability. Those alive today have been selected relative to similar ones that never survived or procreated. All organisms on Earth today are equally evolved since all share the same ancient original ancestors who faced myriad threats to their survival. All have persisted since roughly 3.7 billion to 3.5 billion years ago during the Archean Eon (4 billion to 2.5 billion years ago), products of the great evolutionary process with its identical molecular biological bases. Because the environment of Earth is so varied, the particular details of any organism’s evolutionary history differ from those of another species in spite of chemical similarities.
Everywhere the environment of Earth is heterogeneous. Mountains, oceans, and deserts suffer extremes of temperature, humidity, and water availability. All ecosystems contain diverse microenvironments: oxygen-depleted oceanic oozes, sulfide- or ammonia-rich soils, mineral outcrops with a high radioactivity content, or boiling organic-rich springs, for example. Besides these physical factors, the environment of any organism involves the other organisms in its surroundings. For each environmental condition, there is a corresponding ecological niche. The variety of ecological niches populated on Earth is quite remarkable. Even wet cracks in granite are replete with “rock eating” bacteria. Ecological niches in the history of life have been filled independently several times. For example, quite analogous to the ordinary placental mammalian wolf was the marsupial wolf, the thylacine (extinct since 1936) that lived in Australia; the two predatory mammals have striking similarities in physical appearance and behaviour. The same streamlined shape for high-speed marine motion evolved independently at least four times: in Stenopterygius and other Mesozoic reptiles; in tuna, which are fish; and in dolphins and seals, which are mammals. Convergent evolution in hydrodynamic form arises from the fact that only a narrow range of solutions to the problem of high-speed marine motion by large animals exists. The eye, a light receptor that makes an image, has evolved independently more than two dozen times not only in animals on Earth but in protists such as the dinomastigote Erythropsodinium. Apparently eyelike structures best solve the problem of visual recording. Where physics or chemistry establishes one most efficient solution to a given ecological problem, evolution in distinct lineages will often tend toward similar, nearly identical solutions. This phenomenon is known as convergent evolution.
Life ultimately is a material process that arose from a nonliving material system spontaneously—and at least once in the remote past. How life originated is discussed below. Yet no evidence for spontaneous generation now can be cited. Spontaneous generation, also called abiogenesis, the hypothetical process by which living organisms develop from nonliving matter, must be rejected. According to this theory, pieces of cheese and bread wrapped in rags and left in a dark corner were thought to produce mice, because after several weeks mice appeared in the rags. Many believed in spontaneous generation because it explained such occurrences as maggots swarming on decaying meat.
By the 18th century it had become obvious that plants and animals could not be produced by nonliving material. The origin of microorganisms such as yeast and bacteria, however, was not fully determined until French chemist Louis Pasteur proved in the 19th century that microorganisms reproduce, that all organisms come from preexisting organisms, and that all cells come from preexisting cells. Then what evidence is there for the earliest life on Earth?
Past time on Earth, as inferred from the rock record, is divided into four immense periods of time called eons. These are the Hadean (4.6 billion to 4 billion years ago), the Archean (4 billion to 2.5 billion years ago), the Proterozoic (2.5 billion to 541 million years ago), and the Phanerozoic (541 million years ago to the present). For the Hadean Eon, the only record comes from meteorites and lunar rocks. No rocks of Hadean age survive on Earth. In the figure , eons are divided into eras, periods, and epochs. Such entries in the geologic time scale are often called “geologic time intervals.”
Among the oldest known fossils are those found in the Fig Tree Chert from the Transvaal, dated over three billion years ago. These organisms have been identified as bacteria, including oxygenic photosynthetic bacteria (cyanobacteria)—i.e., prokaryotes rather than eukaryotes. Even prokaryotes, however, are exceedingly complicated organisms that grow and reproduce efficiently. Structures of communities of microorganisms, layered rocks called stromatolites, are found from more than three billion years ago. Since Earth is about 4.6 billion years old, these finds suggest that the origin of life must have occurred within a few hundred million years of that time.
Chemical analyses on organic matter extracted from the oldest sediments show what sorts of organic molecules are preserved in the rock record. Porphyrins have been identified in the oldest sediments, as have the isoprenoid derivatives pristane and phytane, breakdown products of cell lipids. Indications that these organic molecules dating from 3.1 billion to 2 billion years ago are of biological origin include the fact that their long-chain hydrocarbons show a preference for a straight-chain geometry. Chemical and physical processes alone tend to produce a much larger proportion of branched-chain and cyclic hydrocarbon molecular geometries than those found in ancient sediments. Nonbiological processes tend to form equal amounts of long-chain carbon compounds with odd and even numbers of carbon atoms. But products of undoubted biological origin, including the oldest sediments, show a distinct preference for odd numbers of carbon atoms per molecule. Another chemical sign of life is an enrichment in the carbon isotope C 12 , which is difficult to account for by nonbiological processes and which has been documented in some of the oldest sediments. This evidence suggests that bacterial photosynthesis or methanogenesis, processes that concentrate C 12 preferentially to C 13 , were present in the early Archean Eon.
The Proterozoic Eon, once thought to be devoid of fossil evidence for life, is now known to be populated by overwhelming numbers of various kinds of bacteria and protist fossils—including acritarchs (spherical, robust unidentified fossils) and the entire range of Ediacaran fauna. The Ediacarans—large, enigmatic, and in some cases animal-like extinct life-forms—are probably related to extant protists. Almost 100 species are known from some 30 locations worldwide, primarily sandstone formations. Most Ediacarans, presumed to have languished in sandy seaside locales, probably depended on their internal microbial symbionts (photo- or chemoautotrophs) for nourishment. No evidence that they were animals exists. In addition to the Ediacarans, acritarchs, and other abundant microfossils, clear evidence for pre-Phanerozoic, or Precambrian, life includes the massive banded-iron formations (BIFs). Most BIFs date from 2.5 billion to 1.8 billion years ago. They are taken as indirect evidence for oxygen-producing, metal-depositing microscopic Proterozoic life. Investigations that use the electron microprobe (an instrument for visualizing structure and chemical composition simultaneously) and other micropaleontological techniques unfamiliar to classical geology have been employed to put together a much more complete picture of pre-Phanerozoic life.
The earliest fossils are all of aquatic forms. Not until about two billion years ago are cyanobacterial filaments seen that colonized wet soil. By the dawn of the Phanerozoic Eon, life had insinuated itself between the Sun and Earth, both on land and in the waters of the world. For example, the major groups of marine animals such as mollusks and arthropods appeared for the first time about 541 million years ago at the base of the Cambrian Period of the Phanerozoic Eon. Plants and fungi appeared together in the exceptionally well-preserved Rhynie Chert of Scotland, dated about 408 million–360 million years ago in the Devonian Period. Solar energy was diverted to life’s own uses. The biota contrived more and more ways of exploiting more and more environments. Many lineages became extinct. Others persisted and changed. The biosphere’s height and depth increased, as did, by implication, the density of living matter. The proliferation and extinctions of a growing array of life-forms left indelible marks in the sedimentary rocks of the biosphere (see evolution: The concept of natural selection).
