Spontaneous generation is the idea that organisms could form miraculously from
But Pasteur's results posed a new riddle for evolutionary biologists: If life could only arise from life, how did living organisms initially appear on the planet?
The answer came in the 1920's when A.I. Oparin of Russia and J.B.S. Haldane of England independently presented compelling arguments that the origin of life could be explained, not as the result of rapid spontaneous generation of whole organisms in a few weeks, but from a long and gradual process of chemical evolution.
Much progress has been made since the 1920's, but, as with most complex scientific questions, many uncertainties remain and many new avenues of inquiry have been uncovered.
Today, research into the origin of life is interdisciplinary with workers trying to answer four main questions:
This is what we know:
Our solar system formed about 4.6 billion years ago (abbreviated b.y.a.) when a swirling cloud of
gas and dust began to contract.
This bombardment, measured from radioactive dating of the moon's craters, and by comparing lunar, martian, and mercurian cratering records, gradually declined in intensity until it reached present-day levels about 3.5 billion years ago.
The effect of the bombardment on the origin of life was significant. Bombardment affected:
Let's look at each of these effects in more detail:
About 3.9 bya, the earth had solidified but the violent bombardment would have prevented the continuous existence of life on the planet before 3.8 billion years ago. Large impacts would have produced globally lethal conditions by boiling large volumes of ocean water effectively sterilizing the surface of the planet with steam.
This has been called the impact frustration of the origin of life.
The interesting thing is that the oldest fossils from rocks in Greenland are about 3.8 billion years old. This means that life
was existed on the planet almost as soon as it was physically possible for it to do so.
As the planet cooled, an atmosphere formed. When scientists initially tried to work out what was
in the first atmosphere they reasoned that, because most of the matter in the solar system is
hydrogen the early atmosphere of Earth must have been rich in hydrogen. Therefore, they
concluded other elements (such as carbon, oxygen, nitrogen, and sulfur) would be bound to
hydrogen in their reduced forms as
BUT if such an atmosphere did exist it would have been blasted away by meteor impacts during the bombardment phase.
The more probable source of the atmosphere present 3.8bya would have been gases released from the cooling rocks such as we now get from volcanoes and other vents through the crust.
Based on an analysis of gases vented by modern volcanoes, it seems likely that this early atmosphere consisted mostly of water vapor (H2O) and carbon dioxide (CO2) and nitrogen gas (N2).
Note that this atmosphere is very different from the one we have today.
2. Where did the oxygen come from? -Photosynthesis
3. Nitrogen on early Earth would have been at approximately its same percentage as Earth's present atmosphere.
4. Trace amounts of hydrogen (H), methane (CH4), hydrogen sulfide(H2S), hydrogen
cyanide(HCN), and formaldehyde(CH2O) would have been present.
As the Earth cooled, water vapor condensed as rain and formed oceans and seas.
In addition to an atmosphere very different from the one we know, lightning, volcanic activity (crust thiner), and ultraviolet radiation (no ozone) were much more intense when the Earth was young.
As these organic compounds accumulated they formed an "organic soup" in which additional reactions could take place.
The energy needed to drive the formation of these organic molecules was derived from the sun's radiation, electrical discharges in the form of lightening, and heat from the cooling earth.
Important point - Laboratory simulations cannot establish that the kind of chemical evolution that has been
described here actually created life on primordial earth, but only that some of the key steps could
Proteins are made up of subunits, called amino acids so we must first talk about how to make amino acids.
There are 20 different types of amino acids used in living organisms, but all have the same central
structure consisting of a carboxyl group (COO-), an amino group (NH3), and a carbon with a
hydrogen and a variable side chain. For example:
First he created a "comic strip version" of primitive Earth.
In a flask he put:
For example, Adding an amino group (in other words an ammonia) to acetic acid and you have an
amino acid glycine:
1. Amino acids are asymmetrical molecules that exist in right and left-handed forms that are mirror images of each other: the L-isomer and the D-isomer. When amino acids are made in the laboratory a mixture of L- and D-amino acids is found. However, the amino acids present in living organisms are all L-isomers. Why? How?
Perhaps organic polymers were synthesized and accumulated on rock or clay surfaces. If the conditions are hot and dry enough, the water molecule can be lost and amino acids can join.
Using this process, Sidney Fox and his research group at the University of Miami have formed substances like proteins, called protenoids, by drying warm mixtures of amino acids. They suggest that volcanic activity could have generated high temperature to form proteins on the early Earth, even if only temporarily and locally. It is possible to imagine waves or rain splashing dilute solutions of chemicals onto fresh lava or other hot rocks on the early earth and then rinsing proteinoids back into the water.
Problem with this idea: High temperatures pose a problem, because organic molecules tend to break down as they are heated.
An alternative idea suggests that clay, even cool clay, may have been used. Clay has some interesting properties:
1. It has a slight charge that can attract and hold other molecules.
2. Clays may contained small amounts of metal atoms, such as copper, iron, or zinc. These metal atoms function as catalysts facilitating the dehydration reactions that link amino acids together.
3. Clay also seems to be able to store energy absorbed from radioactive decay and then discharged this energy at times when the clay changes temperature or degree of dehydration.
One more problem: Proteins are twisted or folded to form a macromolecule with a specific comformation, or three-dimensional shape. This comformation determines the function of the protein. For example, the unique shape of an enzyme permits it to "recognize" and act on the substance it regulates. The shape of the molecule is determined by the order of amino acids in the chain -- how do we get the very precise order of amino acids in early earth's environment?
2. Nucleotides and Nucleic Acids
Each nucleotide is composed of 5-carbon sugar, a nitrogen containing base (and by that I mean either a single ring called a pyrimidine or a double ring called a purine)
and a phosphate group
There are three kinds of nucleotide-based molecules:
Considering the atmosphere of the primitive earth to have contained water vapor, carbon dioxide, carbon monoxide, methane, and ammonia and/or nitrogen, then in the presence of an energy source, such as sunlight or lightning, a number of small molecules like hydrogen cyanide, HCN, are formed. These then undergo spontaneous reaction with other HCN molecules, again in the presence of some energy source and produce adenine and guanine bases (often abbreviated A and G) easily. The other two common bases, cytosine (C) and uracil (U), can also be formed in such experiments, although with more difficulty.
The next step is to combine the nucleotides with other chemicals to make more complex structures like RNA and DNA. For RNA, for example, the adenine or guanine has to be attached to the sugar molecule called ribose and a phosphate group. Ribose and other sugars can be synthesized in the lab from the molecules present in the early earth's atmosphere.
Phosphate is, however, a problem - phosphates appear to have been rare on early Earth.
3. Simple Sugars and Carbohydrates
Most cells use carbohydrates directly or indirectly for energy and as major structural materials. Carbohydrates are monomers or polymers of a sugar. A sugar is composed of carbon, hydrogen, oxygen in a 1:2:1 ratio.
4. Fatty Acids and Lipids
Lipids, which function in the storage of energy and are key components of cell structures such as membranes, are mostly composed of fatty acids.
Fatty acids can be formed simply by adding several carbons to acetic acid.
A fatty acid is a long unbranched hydrocarbon with a carboxyl group (-COOH) at the end:
It is possible that some organic compounds reached the early earth from space - this is called panspermia.
In 1969, a meteor struck near Murchinson, Australia. Analysis of the meteorite fragments revealed the presence of a variety of organic molecules including amino acids, pyrimidines, and molecules resembling fatty acids. Initially, there were even serious proposals that the organic material was biogenic in origin, but consensus was soon reached that abiotic chemical synthesis was the most plausible explanation.
This is significant for two reasons:
1. Heavy bombardment of meteors could have delivered a significant amount of essential compounds to the surface of the Earth.We don't know yet how much organic matter was supplied to the oceans by natural synthesis on Earth and how much by meteor infall. That ratio is not as important as the fact that the right materials were present on the earth for further reactions to act on.
2. The fact that these compounds could appear under abiotic, extraterrestrial conditions makes it seem more likely that similar compounds had been able to form on the primitive Earth.
Cell membranes of all living organisms contain hydrocarbon chains, which constitute an oil-like layer that forms a barrier between the internal and external compartments of cells. Proteins, which provide the enzymatic and transport activities that are primarily functions of membranes, are embedded in this fluid barrier. Therefore, to understand the origin of membrane structure we need to know how lipids and their hydrocarbon moieties provide the essential barrier properties of membranes.
Membrane lipids are typically phospholipid molecules. One of the most common, a constituent of most membranes, is phosphatidylcholine. That just means phosphate is chemically bound to glycerol and choline. The glycerol, in turn, is linked to two fatty acids, each consisting of an acidic carboxyl group (-COOH) attached to a long hydrocarbon chain. other common membrane phospholipids include phosphatidylethanolamine and phosphatidylserine.
Because they are relatively complex, phospholipids probably did not form the earliest membranes. Do molecules simpler than phospholipids assemble into membranes?
Certain molecular structures are polar, with relatively strong electrical charges expressed by their component atoms; others are nonpolar. Polar structures tend to be soluble in water and are usually referred to as hydrophilic. Nonpolar structures are hydrophobic; that is they tend to be soluble in oil and not in water. Some compounds (particularly lipids) that have both hydrophilic and hydrophobic residues on the same molecules are referred to as amohiphilic.
Hydrocarbons, by themselves, are nonpolar molecules. However, if oxygen is added to a long hydrocarbon chain, the molecules become amphiphilic, since oxygen is typically polar. All lipids have oxygen in their molecular structure, usually as carboxyl and phosphate - oxygen containing groups, chemically linked to nonpolar hydrocarbon chains.
Self-assembly of Lipids into Bilayers - Amphiphilic molecules have a remarkable property: they self-assemble into stable bilayer structures. When lipid such as phosphatidylcholine is dried, for example, the lipid molecules form lamellar structures. (Lamellar means that the structure has layers of molecules.) If water is then added, water molecules penetrate between the lipid layers along hydrophilic planes, causing the lipid to swell. The swelling produces a variety of fairly stable structures such as lipid cylinders, each of which contains thousands of concentric lipid bilayers.
Why do lipid components form a stable bilayer structure in an aqueous environment? For thermodynamic reasons. Hydrocarbon chains can't dissolve in water, and oil do not mix. When lipid molecules are placed in water their hydrocarbon chains tend to stay in contact with one another away from the water. This tendency, which is called the hydrophobic effect, stabilizes the bilayer structure.
If lipid remains in contact with water, stable spherical structures called liposomes form. Liposomes represent a good model for the first types of membranes to appear in the origin-of life saga.
The first cells required some mechanism by which a membrane could encapsulate a system of replicating macromolecules. Although this seems difficult, the drying-wetting procedure offers an easy solution. If membrane-forming lipids are dried in the presence of large molecules, the molecules are sandwiched between alternating lipid bilayers. Upon rehydration, a substantial fraction of the the molecule are encapsulated within vesicular membrane structures. This has the additional advantage of concentrating molecules from dilute solutions. A primary function of amphiphilic compounds is to provide closed microenvironments. If macro molecular catalytic-information system were encapsulated within a vesicular membrane, the components of the system would share the same microenvironment. This would be a step towards true cellular function. Encapsulation would also produce individuals; each cell would be different from its neighbors.
A second role of early membranes was probably related to energy production, because energy-yielding processes are necessary to provide for growth of the catalytic-information system. In contemporary cells, membranes are central to energy production. Chloroplast membranes capture light energy by means of embedded pigment systems. The chemiosmotic synthesis of ATP that takes place on membranes.
That is there are cell membranes - selectively permeable membranes separating metabolic
pathways from the outside environment.
The basic structure of a cell membrane is phospholipid bilayer with proteins dispersed and embedded in it:
There are various simple ways to get lipid bilayers -- the problem is ingetting the proteins embedded into them:
Clues to the origin of life are not easy to find when looking at contemporary organisms. In all contemporary cells, the information about how to build proteins (which, in turn, either catalyze the production of or makeup key structures) is encoded in the double helix of deoxyribonucleic acid (DNA). That information is copied when it is transcribed into a molecule of ribonucleic acid (RNA). Then translated into a particular sequence of amino acids that make up a polypeptide chain. When the chain is folded it becomes a protein. The function of that protein (becoming part of a structure or performing some metabolic catalysis) is determined by the specific type and order of the amino acids in the polypeptide chain. The order in which the amino acids are strung together is crucial because it determines how the chains will fold and twist into the three-dimensional shapes of individual proteins. On the molecular level in contemporary cells, reproduction, then, is the replication of the DNA (since this molecule encodes all of the information of life) when the cell divides.
But here is the problem - when we look at modern organisms, we see that DNA cannot replicate
itself, but must be copied by specific enzymes. These replication proteins, in turn, are controlled
by the transcription and translation of the DNA. If proteins are necessary to reproduce DNA but
to produce proteins we need DNA, which came first in evolution? DNA or proteins?
A. G. Cairns proposed a possible solution in the early 1980s. he suggested that naturally occurring microscopic mineral crystals in clays might have served as the basis for replication until the time when nucleic acids evolved and took over the function of replication. Clay microcrystals consist of flat plates of silicate lattices with regular arrays of ionic sites occupied by various metals. When such a crystal is contained in a droplet of water, the metal ions form irregular patterns of electrostatic potential that can attract particular molecules to the surfaces of the lattice and catalyze chemical reactions. Which reactions are catalyzed depends upon the precise arrangement of the metal ions. Molecules synthesized in this manner could be released back to the water. Because a crystal grows by incorporating silicate and metal ions from the surrounding water, the new materials are similar in composition to the original parts of the crystal that generated them. Thus crystals could, in principle, both replicate information and transfer it to other molecules. What is uncertain is whether they could have done so with sufficient precision to serve as a basis for the evolution of life. According to this theory, the clay lattice first directed the synthesis of primitive enzymes. For a long time, the clay crystals functioned as primitive genetic material, but at some point, by as yet unspecified mechanisms, RNA evolved and took over the role of replicating and transferring information. Once RNA appeared, it was so much better as a genetic material that clay-based life was quickly out competed by RNA-based life and driven to extinction. This scenario is plausible, but critical experiments to test some of its key assumptions have not yet been performed.
RNA + proteins made by inorganic processes cause the RNA to replicate and begin to produce proteins
Finally DNA favored because easier to repair.
But having the molecules of life is not the same as having a life form. However prebiotic chemicals accumulated, polymerized, and eventually reproduced, the leap from an aggregate of molecules that reproduces to even the simplest prokaryotic cell is immense and must have taken many smaller evolutionary steps.