The most ancient rocky material of the solar system is estimated to be about 4.6 billion years old. At this distant time, our planet condensed out of the primordial matter and began its transition into the world we know today.

How long did the clock run before life began? Well, one has to pick where you look carefully because most of the rocks on the surface billions of years ago have been weathered away or have been buried and subjected to enormous heat and pressure. This would ruin any fossils. Turns out that where Australia is located is also where 3.5 billion years ago there existed a shallow sea bordered by volcanoes. Whenever a volcano erupted, it put a layer of lava over the sea bottom which could be covered with sediment until the next eruption. This resulted in a paleontologist's paradise--sediment layers with well-preserved fossils.

Microscopic, cell-like fossils resembling bacteria have been found in Australia. They are primitive cells--no nuclei--consisting of filamentous strings of perhaps as many as 40 individual cells. They seem a lot like what used to be called blue-green algae (now known as cyanobacteria). These things have been dated to be between 3.46 and 3.47 billion years old.

How did life appear on earth? Any intelligent person at one time or another encounters this question. A divine origin has been proposed by many religions but the possibility of divine origins falls outside the realm of science because it cannot be tested. The same can be said for notions of life coming here from outer space or from a space probe from another planet billions of years ago. We cannot scientifically test these things.

It turns out that in some ways, today's scientific notion of the origin of life is just a more sophisticated notion of an old belief that life can arise by spontaneous generation. As late as the mid-ninteenth century, laymen and scientists alike believed that the lesser forms of life arose spontaneously and more or less continuously from rotting organic matter. Out of this we get the idea that rotting meat was the parent of maggots and warm mud the parent of frogs.

Another gem was the recipe for the spontaneous generation of mice made by the chemist, J. B. Van Helmont: "If you press a piece of underwear soiled with sweat together with some wheat in an open mouth jar, after about 21 days the odor changes and the ferment, coming out of the underwear and penetrating through the husks of the wheat, changes the wheat into mice."

The common belief in spontaneous generation was stopped dead in its tracks in 1862 by Louis Pasteur with his famous demonstration that nutrient fluids, sterilized and sealed against contamination, could be kept indefinitely without the generation of microbial or other forms of life.

In a sense, Pasteur was almost too good. His experiment made scientists reject the idea that life could have arisen spontaneously at any time under any circumstances. It wasn't until the 1920s when Oparin, a Russian and Haldane, an Englishman, independently developed a hypothesis that forced reconsideration of spontaneous generation. Oparin and Haldane agreed that spontaneous generation of life is not possible under present earth conditions but suggested that the earth's surface and atmosphere were far different during its first millions of years of existence at present. Primordial conditions would favor spontaneous generation of life rather than inhibiting it.

Oparin and Haldane proposed that the earth's primitive atmosphere contained primarily reduced substances such as methane, ammonia, and water instead of high concentrations of oxygen as we have today.

In such an atmosphere, electrons and hydrogens were readily available for conversion of inorganic materials to organic forms. Solar energy and lightning would more than supply the necessary energy. Organic molecules would have accumulated because their main breakdown processes--oxidation and decay by microorganisms--would not have occurred. They hypothesized that as the organic compounds became more and more concentrated, they interacted spontaneously over millions of years to produce what we call life.

These complex substances, according to the Oparin-Haldane hypothesis, constantly aggregated into random collections of molecules, some of which were able to carry out primitive living reactions. Presumably, these were more successful than nonliving assemblies in the competition for space and raw materials and therefore they persisted. Eventually the most successful of these aggregations developed the full qualities of life, including the ability to self-reproduce.

As the chemical activities of the first collections of living matter increased, the store of organic molecules useful for energy sources would have become depleted. Life persisted though, because photosynthesis developed which took advantage of an essentially inexhaustible energy source--the sun.

Photosynthesis and organisms that exhibited it, gradually became more complex until they used water as their source of electrons and hydrogen and they released oxygen.

The release of oxygen in ever greater amounts changed the character of the atmosphere from one of reducing to one of oxidizing. Once this occurred, there was no longer any chance of spontaneous generation of life because organic molecules were quickly oxidized back to inorganic form.

The Oparin-Haldane hypothesis was NOT widely accepted at first because of the weight of evidence against spontaneous generation and the lack of an effective way to test the hypothesis.

In 1953, Stanley Miller, a graduate student working the laboratory of Harold Urey, built an apparatus to demonstrate the feasibility of abiotic synthesis. Miller built an apparatus that simulated the presumed conditions of primeval earth. The conditions were:

1. a gaseous phase containing reduced sources of carbon (methane), nitrogen (ammonia), oxygen atoms (water), and hydrogen atoms from any or all of these precursors as well as hydrogen gas.
2. electrical energy provided by spark discharge.
3. ambient temperature between 0 and 100 C.
4. sterile conditions to begin with (abiotic environment).

A week long experimental run produced a large number of major organic products.

A diagram is available for your viewing.

The observed products were common amino acids, fatty acids and other organic molecules.

This experiment showed that the key assumptions in the Oparin-Haldane hypothesis were feasible and in fact appeared to have taken place. These findings set off a wave of speculation and experimentation about the conditions and reactions necessary for the transition from the non-living to the living state and, coincidentally, won the Nobel Prize for Miller and Urey.

The contemporary conclusion from this effort is that life originated through spontaneous, inanimate processes and that they took place under the conditions that existed on a primitive earth. What you got were molecular assemblages harboring a spark of life that advanced over a very long period of time through a series of intermediate stages, that although not cellular, were able to carry on succesively more complex chemistry. Eventually, the molecular assemblies achieved fully cellular characteristics.

There is an old German proverb that states: "In den Einzelheiten steckt der Teufel," which means, "The devil lurks in the details." And is this ever true.

Let's look at the stages in the evolution of cellular life more closely.

Stage 1: The formation of the earth and atmosphere is considered the first stage in the long trek from inanimate matter to life. This stage provided the inorganic raw materials for the evolution of life and set up the conditions for their interaction.

Stage 2: The second stage produced organic molecules through interactions between inorganic substances, driven by energy sources such as lightning and ultraviolet radiation from the sun.

Stage 3: In the third stage, the organic molecules present assembled randomly into collections capable of chemical interaction with the environment. As the collections formed, interactions taking place within them produced still more complex organic substances, including polypeptides and nucleic acids. Some of these collections of molecules were capable of carrying out primitive living reactions. There is little agreement on the form taken by the first spark of life in these primitive aggregates.

Stage 4: In the fourth stage, a genetic code appeared in the primitive living aggregates. This code regulated duplication of information required for reproduction of the aggregates and established the link between nucleic acids and the ordered synthesis of proteins. Things were still pre-cellular, but with these developments (directed synthesis and reproduction), life was fully established in the molecular assemblages.

Stage 5: The fifth and final stage involves conversion of the pre-cellular assemblages into fully organized cells with a nuclear region and a cytoplasm, all enclosed by an outer boundary membrane--a plasma membrane.

The Miller-Urey experiment was reproduced over and over again. An important substance included in many of the later experiments was hydrogen cyanide (HCN), readily produced by action of electrical discharges on inorganic materials assumed to be present in the reducing atmosphere of a primitive earth.

When HCN is present in a Miller-Urey experiment, a greater variety of amino acids are produced, as are purines and pyrimidines (important in the formation of nucleic acids) and porphyrins (important in the formation of pigments such as chlorophyll and hemoglobin).

Why only 20 or so of the many amino acids produced in a Miller-Urey experiment are actually used in protein synthesis by today's living organisms is an unanswered question.

Without laboring the point, experiments with primitive atmospheres and energy sources have produced the organic building blocks for all the major biological molecules.

There are two recurring problems with the Miller-Urey scenario. First, if the newly formed molecules were exposed to the sun's ultraviolet light, they would break apart. The only way to avoid the sun is to shelter the molecules under water or somewhere else. The second problem is to find a way to enclose the newly formed molecules in a membrane to protect them from the environment.

And, if life arose so quickly and naturally, why does it not do so over and over again. The answer here is that the these first cells had some unique advantages--an environment full of energy-rich molecules and NO competition.

Another real tough question is how did the cell acquire a cell membrane?

There are many theories that address this question but they fall into two categories, each of which has a lot of variations. One school of thought requires that DNA or RNA be present; the other school does not require DNA or RNA--these were added later.

A major attraction of the no-nucleic-acid school is that it is a more simple scheme. Here, the full machinery of the modern cell was a later development. A key feature of this scheme is that all living things share a lot of the same basic biochemistry. A clam, a yeast cell, an elephant and a redwood tree all share a lot of similar chemical processes. Cells in both plants and animals contain a pigment protein known as cytochrome c which is involved in cellular respiration. The cytochrome c of humans is 71 percent identical with that of pumpkins.

The other school--the pro-nucleic acid school--is probably accepted by the majority of scientists. The notion here is that reactions in Earth's early oceans produced both DNA and proteins. As time went on, the complexity of things increased but the basic working principles were always there. Those buying this school would say that life originated with cells using some form of nucleic acid--that they evolved together.

There is still the problem with the origin of a cell or plasma membrane.

There are no clear-cut answers to the nucleic acid question or the origin of a cell membrane but there are a lot of theories. Let me mention a few.

There is the RNA world theory. RNA is a close relative of DNA and it has been recently shown that RNA can act in an enzyme-like manner. In the RNA world scenario, RNA came first, playing the role of both DNA and enzyme proteins. This would make the first cell's chemistry very different from today's cells and would require its being superceded by today's cell's chemistry.

Another scenario is the Clay World. Some clays can have a pattern of electrostatic charges on their surfaces which could attract specific molecules in a particular pattern. This could serve as a template and allow more complex associations of molecules to form, leading to perhaps "living" organisms.

The Oil Slick senario. Whatever chemistry occurred in an early world, it may well have produced oils. Oily molecules tend to clump together in a water world to form vesicles. Each vesicle could contain a different mix of whatever chemicals were present in the water.

There is no true consensus about about any of these theories except a certain inevitability. Almost no one argues that if you wait long enough, something will happen. More often you hear that given the right mix of molecules, something will happen and probably pretty quickly. Life did not happen just by chance but happened according to the same kinds of laws that govern ordinary chemical reactions. This kind of notion has an enormous bearing on the question of whether there is life elsewhere in the Universe. Maybe Ponnamperuma (a famous origin of life researcher) was correct when he declared, "The business of the Universe is creating life."

You may take a quiz on the material in this module. No record of the quiz is made. You decide after the quiz if you really know this material.

Footnote:

Paul Davies is a theoretical physicist and a bestselling author of more than 20 books dealing with science for the non-scientist. He is truly amazing in his ability to explain extremely complicated scientific ideas in everyday terms for the layman.

In his latest book, The Fifth Miracle, he addresses the origin of life. He points out that new research hints that the crucible of life lies deep within the Earth's crust and not in a warm little pond at its surface. The new research findings involve microbes discovered dwelling near underwater volcanic vents.

Davies also discusses the discovery of a meteorite from Mars that may contain traces of life. Three and a half billion years ago, Mars resembled Earth. Davies believes that Mars may still harbor microbes in thermally heated rock below the Martian permafrost.

He discusses panspermia, the notion that life on Earth may have been seeded extraterrestrially.

Asking the question, what exactly is life, he shows that the living cell is an information-processing system that uses a sophisticated mathematical code and argues that the secret of life lies not with exotic chemistry but instead with the emergence of information-based complexity.

He entertains intriguing questions such as: Are we alone in the Universe?; Is life inevitable?; Is life pre-ordained toward greater complexity and intelligence?

This book, The Fifth Miracle: the search for the origin and meaning of life, by Paul Davies, is an extremely good book and will be a fascinating read for any thoughtful individual. It is published by Simon and Shuster, 1999; ISBN 0-684-83799-4

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