Where Did We Come From?
It is not known whether metabolism or genetics came first. The main hypothesis which supports genetics first is the RNA world hypothesis, and the one which supports metabolism first is the protein world hypothesis.
Another big problem is how cells develop. All existing forms of life are built out of cells.
Melvin Calvin, recipient of the Nobel Prize in Chemistry, wrote a book on the subject, and so did Alexander Oparin. What links most of the early work on the origin of life is the idea that before life began there must have been a process of chemical change.
Another question which has been discussed by J.D. Bernal and others is the origin of the cell membrane. By concentrating the chemicals in one place, the cell membrane performs a vital function.
Many religions teach that life did not evolve spontaneously, but was deliberately created by a god. Such theories are a part of creationism, which has "old earth" and "young earth" versions. Because of lack of evidence for such views, almost all scientists do not accept them.
Earliest claimed life on Earth
A scientific study from 2002 showed that geological formations of stromatolites 3.45 billion years old contain fossilized cyanobacteria. At the time it was widely agreed that stromatolites were oldest known lifeform on Earth which had left a record of its existence.
Therefore, if life originated on Earth, this happened sometime between 4.4 billion years ago, when water vapor first liquefied, and 3.5 billion years ago. This is the background to the latest discovery discussed above.
Earliest evidence of life comes from the Isua supercrustal belt in Western Greenland and from similar formations in the nearby Akilia Islands. This is because a high level of the lighter isotope of carbon is found there. Living things uptake lighter isotopes because this takes less energy.
Carbon entering into rock formations has a concentration of elemental δ13C of about −5.5. of 12C, biomass has a δ13C of between −20 and −30. These isotopic fingerprints are preserved in the rocks. With this evidence, Mojzis suggested that life existed on the planet already by 3.85 billion years ago.
A few scientists think life might have been carried from planet to planet by the transport of spores. This idea, now known as panspermia, was first put forward by Arrhenius.
There is almost no geological record from before 3.8 billion years ago. The environment that existed in the Hadean era was hostile to life, but how much so is not known. There was a time, between 3.8 and 4.1 billion years ago, which is known as the Late Heavy Bombardment.
It is so named because many lunar craters are thought to have formed then. The situation on other planets, such as Earth, Venus, Mercury and Mars must have been similar. These impacts would likely sterilize the Earth (kill all life), if it existed at that time.
Several people have suggested that the chemicals in the cell give clues as to what the early seas must have been like. In 1926, Macallum noted that the inorganic composition of the cell cytosol dramatically differs from that of modern sea water: "the cell… has endowments transmitted from a past almost as remote as the origin of life on earth"."All cells contain much more potassium, phosphate, and transition metals than modern ... oceans, lakes, or rivers"
."Under the anoxic, CO2-dominated primordial atmosphere, the chemistry of inland basins at geothermal fields would [be like the chemistry inside] modern cells"
If life evolved in the deep ocean, near a hydrothermal vent, it could have originated as early as 4 to 4.2 billion years ago. If, on the other hand, life originated at the surface of the planet, a common opinion is it could only have done so between 3.5 and 4 billion years ago. Lazcano and Miller (1994) suggest that the pace of molecular evolution was dictated by the rate of recirculating water through mid-ocean submarine vents.
Complete recirculation takes 10 million years, so any organic compounds produced by then would be altered or destroyed by temperatures exceeding 300 °C. They estimate that the development of a 100 kilobase genome of a DNA/protein primitive heterotroph into a 7000 gene filamentous cyanobacterium would have required only 7 million years
History of Earth's atmosphere
Originally, the Earth's atmosphere had almost no free oxygen. It gradually changed to what it is today, over a very long time (see Great Oxygenation Event). The process began with cyanobacteria. They were the first organisms to make free oxygen by photosynthesis.
Most organisms today need oxygen for their metabolism; only a few can use other sources for respiration.[ So it is expected that the first proto-organisms were chemoautotrophs, and did not use aerobic respiration. They were anaerobic.
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.
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.
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 , and neon
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?
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 disaccharid 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.
Many separate and rather diverse instances of the origin of living cells may have occurred in the Archean Earth, but obviously only one prevailed. Interactions eventually eliminated all but our lineage. From the common composition, metabolism, chemical behaviour, and other properties of life, it seems clear that every organism on Earth today is a member of the same lineage.
The earliest living systems
Most organic molecules made by living systems inside cells display the same optical activity: when exposed to a beam of plane-polarized light, they rotate the plane of the beam. Amino acids rotate light to the left, whereas sugars, called dextrorotatory, rotate it to the right.
Organic molecules produced artificially lack optical activity because both “left-handed” and “right-handed” molecules are present in equal quantity. Molecules of the same optical activity can be assembled in complementary ways like the stacking of right-handed gloves.
The same monomers can be used to produce longer chain molecules that are three-dimensional mirror images of each other; mixtures of monomers of different handedness cannot. Cumulative symmetry is responsible for optical activity. At the time of the origin of life, organic molecules, corresponding both to left- and right-handed forms, were no doubt formed as they are in laboratory simulation experiments today: both types were produced.
But the first living systems must have employed one type of component, for the same reason that carpenters cannot use random mixtures of screws with left- and right-handed threads in the same project with the same tools. Whether left- or right-handed activity was adopted was probably a matter of chance, but, once a particular asymmetry was established, it maintained itself. Optical activity accordingly is likely to be a feature of life on any planet.
The chances may be equal of finding a given organic molecule or its mirror image in extraterrestrial life-forms if, as Morowitz suspects, the incorporation of nitrogen into the first living system involved glutamine, the simplest of the required amino acid precursors with optical activity.
The first living cells probably resided in a molecular Garden of Eden, where the prebiological origin of food had guaranteed monomers that were available. The cells, the first single-celled organisms, would have increased rapidly. But such an increase was eventually limited by the supply of molecular building blocks. Those organisms with an ability to synthesize scarce monomers, say A, from more abundant ones, say B, would have persisted.
The secondary source of supply, B, would in time also become depleted. Those organisms that could produce B from a third monomer, C, would have preferentially persisted. The American biochemist Norman H. Horowitz has proposed that the multienzyme catalyzed reaction chains of contemporary cells originally evolved in this way.
Life on Earth began more than 3 billion years ago, evolving from the most basic of microbes into a dazzling array of complexity over time. But how did the first organisms on the only known home to life in the universe develop from the primordial soup? One theory involved a "shocking" start. Another idea is utterly chilling. And one theory is out of this world! Inside you'll learn just how mysterious this all is, as we reveal the different scientific theories on the origins of life on Earth
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