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Evolution and the history of life on Earth​



Heritability​

major evolutionary events
major evolutionary events
The geologic time scale from 650 million years ago to the present, showing major evolutionary events.
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.



Convergence​

parallel evolution of marsupial and placental mammals
parallel evolution of marsupial and placental mammals
Parallel evolution of marsupial mammals in Australia and placental mammals on other continents.
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.



Spontaneous generation​

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?



Geologic record​

geologic time
geologic time
The stratigraphic chart of geologic time.
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.”


stromatolites
stromatolites
Living stromatolites in Hamelin Pool of Shark Bay, Western Australia.
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 C12, 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 C12 preferentially to C13, were present in the early Archean Eon.


Spriggina fossil
Spriggina fossil
Spriggina fossil from the Ediacaran Period, found in the Ediacara Hills of Australia.
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).









The origin of life​


Hypotheses of origins​

Perhaps the most fundamental and at the same time the least understood biological problem is the origin of life. It is central to many scientific and philosophical problems and to any consideration of extraterrestrial life. Most of the hypotheses of the origin of life will fall into one of four categories:
  1. The origin of life is a result of a supernatural event—that is, one irretrievably beyond the descriptive powers of physics, chemistry, and other science.
  2. Life, particularly simple forms, spontaneously and readily arises from nonliving matter in short periods of time, today as in the past.
  3. Life is coeternal with matter and has no beginning; life arrived on Earth at the time of Earth’s origin or shortly thereafter.
  4. Life arose on the early Earth by a series of progressive chemical reactions. Such reactions may have been likely or may have required one or more highly improbable chemical events.
Michelangelo: The Creation of Adam
Michelangelo: The Creation of Adam
The Creation of Adam, detail of the ceiling fresco by Michelangelo, 1508–12; in the Sistine Chapel, Vatican City.
Hypothesis 1, the traditional contention of theology and some philosophy, is in its most general form not inconsistent with contemporary scientific knowledge, although scientific knowledge is inconsistent with a literal interpretation of the biblical accounts given in chapters 1 and 2 of Genesis and in other religious writings. Hypothesis 2 (not of course inconsistent with 1) was the prevailing opinion for centuries. A typical 17th-century view follows:


[May one] doubt whether, in cheese and timber, worms are generated, or, if beetles and wasps, in cow’s dung, or if butterflies, locusts, shellfish, snails, eels, and suchlike be procreated of putrefied matter, which is apt to receive the form of that creature to which it is by the formative power disposed. To question this is to question reason, sense, and experience. If he doubts of this, let him go to Egypt, and there he will find the fields swarming with mice begot of the mud of the Nylus [Nile], to the great calamity of the inhabitants.
(Alexander Ross, Arcana Microcosmi, 1652.)
Click to expand...
It was not until the Renaissance, with its burgeoning interest in anatomy, that such spontaneous generation of animals from putrefying matter was deemed impossible. During the mid-17th century the British physiologist William Harvey, in the course of his studies on the reproduction and development of the king’s deer, discovered that every animal comes from an egg. An Italian biologist, Francesco Redi, established in the latter part of the 17th century that the maggots in meat came from flies’ eggs, deposited on the meat. In the 18th century an Italian priest, Lazzaro Spallanzani, showed that fertilization of eggs by sperm was necessary for the reproduction of mammals. Yet the idea of spontaneous generation died hard. Even though it was clear that large animals developed from fertile eggs, there was still hope that smaller beings, microorganisms, spontaneously generated from debris. Many felt it was obvious that the ubiquitous microscopic creatures generated continually from inorganic matter.


Maggots were prevented from developing on meat by covering it with a flyproof screen. Yet grape juice could not be kept from fermenting by putting over it any netting whatever. Spontaneous generation was the subject of a great controversy between the famous French bacteriologists Louis Pasteur and Félix-Archimède Pouchet in the 1850s. Pasteur triumphantly showed that even the most minute creatures came from “germs” that floated downward in the air, but that they could be impeded from access to foodstuffs by suitable filtration. Pouchet argued, defensibly, that life must somehow arise from nonliving matter; if not, how had life come about in the first place?


Pasteur’s experimental results were definitive: life does not spontaneously appear from nonliving matter. American historian James Strick reviewed the controversies of the late 19th century between evolutionists who supported the idea of “life from non-life” and their responses to Pasteur’s religious view that only the Deity can make life. The microbiological certainty that life always comes from preexisting life in the form of cells inhibited many post-Pasteur scientists from discussions of the origin of life at all. Many were, and still are, reluctant to offend religious sentiment by probing this provocative subject. But the legitimate issues of life’s origin and its relation to religious and scientific thought raised by Strick and other authors, such as the Australian Reg Morrison, persist today and will continue to engender debate.


Toward the end of the 19th century, hypothesis 3 gained currency. Swedish chemist Svante A. Arrhenius suggested that life on Earth arose from “panspermia,” microscopic spores that wafted through space from planet to planet or solar system to solar system by radiation pressure. This idea, of course, avoids rather than solves the problem of the origin of life. It seems extremely unlikely that any live organism could be transported to Earth over interplanetary or, worse yet, interstellar distances without being killed by the combined effects of cold, desiccation in a vacuum, and radiation.


Although English naturalist Charles Darwin did not commit himself on the origin of life, others subscribed to hypothesis 4 more resolutely. The famous British biologist T.H. Huxley in his book Protoplasm: The Physical Basis of Life (1869) and the British physicist John Tyndall in his “Belfast Address” of 1874 both asserted that life could be generated from inorganic chemicals. However, they had extremely vague ideas about how this might be accomplished. The very phrase “organic molecule” implied, especially then, a class of chemicals uniquely of biological origin. Despite the fact that urea and other organic (carbon-hydrogen) molecules had been routinely produced from inorganic chemicals since 1828, the term organic meant “from life” to many scientists and still does. In the following discussion the word organic implies no necessary biological origin. The origin-of-life problem largely reduces to determination of an organic, nonbiological source of certain processes such as the identity maintained by metabolism, growth, and reproduction (i.e., autopoiesis).


Darwin’s attitude was: “It is mere rubbish thinking at present of the origin of life; one might as well think of the origin of matter.” The two problems are in fact curiously connected. Indeed, modern astrophysicists do think about the origin of matter. The evidence is convincing that thermonuclear reactions, either in stellar interiors or in supernova explosions, generate all the chemical elements of the periodic table more massive than hydrogen and helium. Supernova explosions and stellar winds then distribute the elements into the interstellar medium, from which subsequent generations of stars and planets form. These thermonuclear processes are frequent and well-documented. Some thermonuclear reactions are more probable than others. These facts lead to the idea that a certain cosmic distribution of the major elements occurs throughout the universe. Some atoms of biological interest, their relative numerical abundances in the universe as a whole, on Earth, and in living organisms are listed in the table. Even though elemental composition varies from star to star, from place to place on Earth, and from organism to organism, these comparisons are instructive: the composition of life is intermediate between the average composition of the universe and the average composition of Earth. Ninety-nine percent of the mass both of the universe and of life is made of six atoms: hydrogen (H), helium (He), carbon (C), nitrogen (N), oxygen (O), and neon (Ne). Might not life on Earth have arisen when Earth’s chemical composition was closer to the average cosmic composition and before subsequent events changed Earth’s gross chemical composition?



atom universe life (terrestrial vegetation) Earth (crust)
Relative abundances of the elements
(percent)
*0 percent here stands for any quantity less than 10–6 percent.
hydrogen 87 16 3
helium 12 0* 0
carbon 0.03 21 0.1
nitrogen 0.008 3 0.0001
oxygen 0.06 59 49
neon 0.02 0 0
sodium 0.0001 0.01 0.7
magnesium 0.0003 0.04 8
aluminum 0.0002 0.001 2
silicon 0.003 0.1 14
sulfur 0.002 0.02 0.7
phosphorus 0.00003 0.03 0.07
potassium 0.000007 0.1 0.1
argon 0.0004 0 0
calcium 0.0001 0.1 2
iron 0.002 0.005 18

photo of Jupiter taken by Voyager 1
photo of Jupiter taken by Voyager 1
Photograph of Jupiter taken by Voyager 1 on February 1, 1979, at a range of 32.7 million km (20.3 million miles). Prominent are the planet's pastel-shaded cloud bands and Great Red Spot (lower centre).
The Jovian planets (Jupiter, Saturn, Uranus, and Neptune) are much closer to cosmic composition than is Earth. They are largely gaseous, with atmospheres composed principally of hydrogen and helium. Methane, ammonia, neon, and water have been detected in smaller quantities. This circumstance very strongly suggests that the massive Jovian planets formed from material of typical cosmic composition. Because they are so far from the Sun, their upper atmospheres are very cold. Atoms in the upper atmospheres of the massive, cold Jovian planets cannot now escape from their gravitational fields, and escape was probably difficult even during planetary formation.
Earth and the other planets of the inner solar system, however, are much less massive, and most have hotter upper atmospheres. Hydrogen and helium escape from Earth today; it may well have been possible for much heavier gases to have escaped during Earth’s formation. Very early in Earth’s history, there was a much larger abundance of hydrogen, which has subsequently been lost to space. Most likely the atoms carbon, nitrogen, and oxygen were present on the early Earth, not in the forms of CO2 (carbon dioxide), N2, and O2 as they are today but rather as their fully saturated hydrides: methane, ammonia, and water. The presence of large quantities of reduced (hydrogen-rich) minerals, such as uraninite and pyrite, that were exposed to the ancient atmosphere in sediments formed over two billion years ago implies that atmospheric conditions then were considerably less oxidizing than they are today.
In the 1920s British geneticist J.B.S. Haldane and Russian biochemist Aleksandr Oparin recognized that the nonbiological production of organic molecules in the present oxygen-rich atmosphere of Earth is highly unlikely but that, if Earth once had more hydrogen-rich conditions, the abiogenic production of organic molecules would have been much more likely. If large quantities of organic matter were somehow synthesized on early Earth, they would not necessarily have left much of a trace today. In the present atmosphere—with 21 percent of oxygen produced by cyanobacterial, algal, and plant photosynthesis—organic molecules would tend, over geological time, to be broken down and oxidized to carbon dioxide, nitrogen, and water. As Darwin recognized, the earliest organisms would have tended to consume any organic matter spontaneously produced prior to the origin of life.
The first experimental simulation of early Earth conditions was carried out in 1953 by a graduate student, Stanley L. Miller, under the guidance of his professor at the University of Chicago, chemist Harold C. Urey. A mixture of methane, ammonia, water vapour, and hydrogen was circulated through a liquid solution and continuously sparked by a corona discharge mounted higher in the apparatus. The discharge was thought to represent lightning flashes. After several days of exposure to sparking, the solution changed colour. Several amino and hydroxy acids, familiar chemicals in contemporary Earth life, were produced by this simple procedure. The experiment is simple enough that the amino acids can readily be detected by paper chromatography by high school students. Ultraviolet light or heat was substituted as an energy source in subsequent experiments. The initial abundances of gases were altered. In many other experiments like this, amino acids were formed in large quantities. On the early Earth much more energy was available in ultraviolet light than from lightning discharges. At long ultraviolet wavelengths, methane, ammonia, water, and hydrogen are all transparent, and much of the solar ultraviolet energy lies in this region of the spectrum. The gas hydrogen sulfide was suggested to be a likely compound relevant to ultraviolet absorption in Earth’s early atmosphere. Amino acids were also produced by long-wavelength ultraviolet irradiation of a mixture of methane, ammonia, water, and hydrogen sulfide. At least some of these amino acid syntheses involved hydrogen cyanide and aldehydes (e.g., formaldehyde) as gaseous intermediates formed from the initial gases. That amino acids, particularly biologically abundant amino acids, are made readily under simulated early Earth conditions is quite remarkable. If oxygen is permitted in these kinds of experiments, no amino acids are formed. This has led to a consensus that hydrogen-rich (or at least oxygen-poor) conditions were necessary for natural organic syntheses prior to the appearance of life.
Under alkaline conditions, and in the presence of inorganic catalysts, formaldehyde spontaneously reacts to form a variety of sugars. The five-carbon sugars fundamental to the formation of nucleic acids, as well as six-carbon sugars such as glucose and fructose, are easily produced. These are common metabolites and structural building blocks in life today. Furthermore, the nucleotide bases and even the biological pigments called porphyrins have been produced in the laboratory under simulated early Earth conditions. Both the details of the experimental synthetic pathways and the question of stability of the small organic molecules produced are vigorously debated. Nevertheless, most, if not all, of the essential building blocks of proteins (amino acids), carbohydrates (sugars), and nucleic acids (nucleotide bases)—that is, the monomers—can be readily produced under conditions thought to have prevailed on Earth in the Archean Eon. The search for the first steps in the origin of life has been transformed from a religious/philosophical exercise to an experimental science.








Production of polymers​

The formation of polymers, long-chain molecules made of repeating units of monomers (the essential building blocks mentioned above), is a far more difficult experimental problem than the formation of monomers. Polymerization reactions tend to be dehydrations. A molecule of water is lost in the formation of a peptide from two amino acids or of a disaccharide sugar from two monomers. Dehydrating agents are used to initiate polymerization. The polymerization of amino acids to form long proteinlike molecules (“proteinoids”) was accomplished through dry heating by American biochemist Sidney Fox and his colleagues. The polyamino acids that he formed are not random molecules unrelated to life. They have distinct catalytic activities. Long polymers of amino acids were also produced from hydrogen cyanide and anhydrous liquid ammonia by American chemist Clifford Matthews in simulations of the early upper atmosphere. Some evidence exists that ultraviolet irradiation induces combinations of nucleotide bases and sugars in the presence of phosphates or cyanides. Some condensing agents such as cyanamide are efficiently made under simulated primitive conditions. Despite the breakdown by water of molecular intermediates, condensing agents may quite effectively induce polymerization, and polymers of amino acids, sugars, and nucleotides have all been made this way.
That adsorption of relevant small carbon compounds on clays or other minerals may have concentrated these intermediates was suggested by the British scientist John Desmond Bernal. Concentration of some kind is required to offset the tendency for water to break down polymers of biological significance. Phosphorus, which with deoxyribose sugar forms the “backbone” of DNA and is integrally involved in cell energy transformation and membrane formation, is preferentially incorporated into prebiological organic molecules. It is hard to explain how such a preference could have happened without the concentration of organic molecules.
The early ocean and lakes themselves may have been a dilute solution of organic molecules. If all the surface carbon on Earth were present as organic molecules, or if many known ultraviolet synthetic reactions that produce organic molecules were permitted to continue for a billion years with their products dissolved in the oceans, a 1 percent solution of organic molecules would result. Haldane suggested that the origin of life occurred in a “hot dilute soup.” Concentration through either evaporation or freezing of pools, adsorption on clay interfaces, or the generation of colloidal enclosures called coacervates may have served to bring the organic molecules in question in contact with each other.
The essential building blocks for life (the monomers) were probably produced in relatively abundant concentrations, given conditions on the early Earth. Although relevant, this is more akin to the origin of food than to the origin of life. If life is defined as a self-maintaining, self-producing, and mutable molecular system that derives energy and supplies from the environment, then food is certainly required for life. Polynucleotides (polymers of RNA and DNA) can be produced in laboratory experiments from nucleotide phosphates in the presence of enzymes of biological origin (polymerases) and a preexisting “primer” nucleic acid molecule. If the primer is absent, polynucleotides are still formed, but they lack specific genetic information. Once such a polynucleotide forms, it can act as a primer for subsequent syntheses.
Even if such a molecular population could replicate polynucleotides, it would not be considered alive. The polynucleotides tend to hydrolyze (break down) in water. In the early 1980s American biochemist Thomas Cech and Canadian American molecular biologist Sidney Altman discovered that certain RNA molecules have catalytic properties. They catalyze their own splicing, which suggests an early role for RNA in life or even in life’s origins. Only the partnership of the two kinds of molecules (proteins and nucleic acids) segregated from the rest of the world by an oily membrane makes the growth process of life on Earth possible. The molecular apparatus ancillary to the operation of the genetic code—the rules that determine the linear order of amino acids in proteins from nucleotide base pairs in nucleic acids (i.e., the activating enzymes, transfer RNAs, messenger RNAs, ribosomes, and so on)—may be the product of a long evolutionary history among natural, thermodynamically favoured, gradient-reducing complex systems. These rules are produced according to instructions contained within the code. American biophysicist Harold J. Morowitz argued cogently that the origin of the genetic system, the code with its elaborate molecular apparatus, occurred inside cells only after the origin of life as a cyclic metabolic system. American theoretical biologist Jeffrey Wicken pointed out that replicating molecules, if they appeared first, would have had no impetus to develop a complex cellular package or associated protein machinery and that life thus probably arose as a metabolic system that was stabilized by the genetic code, which allowed life’s second law-favoured process to continue ad infinitum.
 

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