The evolution of life on earth has involved the following sequence of events. The first living things to appear were the simplest creatures, one-celled organisms. From these came more complex, multicellular organisms. Becoming more complex meant more than just an increase in cell number. With more cells came cellular specialization, where certain cells within the multicellular organism carried out specific tasks. Millions, even billions of years of organismal changes led to the living things we now call plants and animals.
Since this basic sequence of events is in accord with that agreed upon by most geologists, paleontologists, biologists, physicists, and even theologians, one might conclude that Moses and Darwin were all keen observers and some were excellent naturalists who were able to logically assess the most probable creation story.
Scientists generally concur that the time from the formation of our solar system until now has been on the order of some 4.5 billion years. Those who believe the world as we know it was created in six days are often called creationists. Their method of inquiry is based on the belief that the Bible is to be accepted as a completely accurate accounting of all about which it speaks. Scientists rely on an approach to understanding the world around us that involves the scientific method, which is how they test hypotheses and theories to learn more while developing new concepts, ideas, and models that can also be tested. Of course, many good scientists are creationists. Even though the two are often compared and contrasted, creationism is not a science. I do not mean to single out creationism. I could speak the same way of so many other fields of endeavor, such as political science, which is probably more like creationism than any hard science I know, because political beliefs are like religious beliefs. While many hold them close to their hearts, these beliefs are based on feelings, and on a camaraderie similar to what sports fans share who root for the same team. Supporting a party or a person is much different than building and testing new ideas, based on proven facts, but all that is for another book.
Not too awfully long ago, people believed that many of the organisms that live around us continually arise from nonliving material in a manner they called spontaneous generation. This concept had many adherents for over a thousand years. Aristotle believed insects and frogs were generated from moist soil. Other elaborations on this basic theme prevailed for centuries. It wasn't until 1668 that Francesco Redi, an Italian, challenged the concept of spontaneous generation when he tested the widespread belief that maggots were generated from rotting meat. He placed dead animals in a series of jars, some of which were covered with a fine muslin that kept flies out while allowing air in. The flies were unable to land on the meat in the covered jars, and no maggots appeared there. Other jars containing dead animals were left open. Maggots appeared only on the meat in the jars that were left open. In these, flies had been able to lay their eggs, which then hatched into fly larvae, or maggots. From this he concluded that maggots would arise only where flies could lay their eggs. This example shows how Redi used the scientific method to test the hypothesis, which is another word for an explanation. This hypothesis that was accepted at that time stated that flies arose from nonliving material. It should be noted that this hypothesis was based on very little evidence. Redi's experiments failed to support his hypothesis.
One last vestige of mysticism in the debate concerning spontaneous generation had to be invalidated before theories regarding the origin of life could move ahead; this was known as the vitalist doctrine. Adherents of the vitalist doctrine maintained that life processes were not determined solely by the laws of the physical universe, but also partly by some vital force, or vital principle.
For theories about life to move forward, scientists would have to agree that all organisms arise from the reproduction of preexisting organisms. For this to happen, the concept of spontaneous generation would have to be laid to rest. During the nineteenth century, many scientists were not yet convinced that microorganisms did not arise spontaneously, and hoped the scientific method would be deployed in ways that would move the debate forward.
It was Louis Pasteur in France, and John Tyndall in England, who used the scientific method to test the theory of spontaneous generation with microorganisms. Through experimentation, they demonstrated that bacteria are present in the air, and if the air surrounding a heat-sterilized nutrient broth is bacteria-free, then the broth remains bacteria-free.
The leading theory for how the universe began is that 13.8 billion years ago, when space, time, matter, and energy as we currently know it had yet to form, from the explosion of a condensed, hypothetic single point. Scientists also believe that billions of years later, after stars had formed, one that exploded created a cloud of gas and dust, and then due to gravitational forces, these gases and dust particles eventually coalesced into the planetary system surrounding a star that we are part of. These planets and our sun formed about 4.5 billion years ago.
In this solar system, the largest mass to coalesce became our sun, and one of the smaller masses became our earth. On earth, the heavier materials sank to the core of the planet, while the lighter substances are now more concentrated at the surface. Among these are hydrogen, oxygen, and carbon – important components for all life that eventually evolved.
The primordial atmosphere on earth was considerably different from that which currently exists. The present atmospheric gases are composed primarily of molecular nitrogen (N2, about 78%) and molecular oxygen (O2, almost 21%), with a small amount of water vapor (H2O about 1% at sea level and about 0.4% on average throughout the entire atmosphere), as well as much smaller amounts of argon (Ar, almost 0.1%), carbon dioxide (CO2, about 0.04%) and many other gases, such as helium (He), methane (CH4), neon (Ne), and nitrous oxide (N2O) which is more commonly called laughing gas. These last gases occur in only trace amounts. Water vapor is a greenhouse gas, as are carbon dioxide, methane, and nitrous oxide. The concentration of water vapor increases as the average temperature of the earth's atmosphere increases. The concentration of nitrous oxide in the atmosphere increases due to agriculture (from farm animals and fertilizer), fossil fuel combustion. And sewage.
The composition of today's atmosphere differs markedly from that found here when life was just beginning to evolve. At that time, the atmosphere contained far more hydrogen, and unlike now, there was very little oxygen. In such an atmosphere, the nitrogen probably combined with hydrogen, forming ammonia (NH3); the oxygen was probably found combined with hydrogen in the form of water vapor (H2O), and the carbon occurred primarily as methane (CH4). The moderately high temperatures of the earth's crust continually evaporated liquid water from rain into water vapor. As the earth cooled, rainwater accumulated in low-lying areas. These rains also washed dissolved minerals into the bodies of water, which depending on size and salinity, are defined either as lakes, seas, or oceans. In addition, volcanic activity erupting in the oceans and on land brought other minerals to the earth's surface, many of which eventually accumulated in the oceans, such as the various types of salts. It should also be mentioned that long before there was any life on earth, the seas contained large amounts of the simple organic compound methane. Most of the compounds necessary for the development of the initial stages of life are thought to have existed in these early seas. Other studies have indicated that suitable environments for the first steps leading to living material could have existed elsewhere as well. But these environments are still poorly understood, and their potential connection with the origin of life is unclear. It was only after cyanobacteria evolved over 2 billion years ago and later algae and more modern plants that the concentration of atmospheric oxygen began to increase precipitously. You might say plants polluted the earth's early atmosphere by releasing so much oxygen. And yet, if it were not for the plants that continually produce oxygen into the atmosphere, organisms like us that need oxygen to survive could not have evolved and flourished. Now that humans are burning carbon sources that were stored in and under the ground for thousands, and sometimes for millions of years, we are adding this carbon to the atmosphere in the form of carbon dioxide. This is good for plants, which thrive in an atmosphere with elevated levels of carbon dioxide. However, many people are concerned that the increasing quantities of carbon dioxide may affect the weather. (For more on this topic, see Chapter 16, entitled Ecology.)
In the early twentieth century, J.B.S. Haldane, a scientist who was born in Britain and died in India, and S.I. Oparin, a Russian biochemist, investigated how life could have evolved from the inorganic compounds that occurred on earth billions of years ago. Their work is credited with leading to important later advances, most prominent of which were Stanley Miller's experiments during the 1950s. Miller duplicated the chemical conditions of the early oceans and atmosphere and provided an energy source, in the form of electric sparks, which generated chemical reactions. When warm water and gases containing the compounds presumed to be found in the early oceans and in the earth's primordial atmosphere were subjected to sparks for about a week, organic compounds formed.
Experiments that followed, such as those performed by Melvin Calvin and Sydney Fox (both American), found important so-called building blocks of life, the amino acids that make up proteins, readily form under circumstances similar to those that were first established experimentally by Stanley Miller.
The thin film of water found on the microscopic particles that make clay has been shown to possess the proper conditions for important chemical reactions. Clays serve as a support and as a catalyst for the diversity of organic molecules involved in what we define as living processes. Ever since J. Desmond Bernal presented (during the late 1940s) his ideas concerning the importance of clays to the origin of life, additional prebiotic scenarios involving clay have been proposed. Clays store energy, transform it, and release it in the form of chemical energy that can operate chemical reactions. Clays also have the capacity to act as buffers and even as templates. A.G. Cairns-Smith analyzed the microscopic crystals of various metals that grew in association with clays and found that they had continually repeating growth patterns. He suggested that this could have been related to the original templates on which certain molecules reproduced themselves. Cairns-Smith and A. Weiss both suggest clays might have been the first templates for self-replicating systems.
Some researchers believe that through the mutation and selection of such simple molecular systems, the clay acting as template may eventually have been replaced by other molecules. And in time, instead of merely encoding information for a rote transcription of a molecule, some templates may have been able to encode stored information that would transcribe specific molecules under certain circumstances.
Other scenarios have been suggested to explain how the molecules that make more molecules could have become enclosed in cell-like containments. Sydney Fox and coworkers first observed that molecular boundaries between protein-nucleic acid systems can arise spontaneously. They heated amino acids under dry conditions and ascertained that long polypeptide chains were produced. These polypeptides were then placed in hot-water solutions, and upon cooling them, the researchers found that the polypeptides coalesced into small spheres. Within these spherical membranes, or microspheres, certain substances were trapped. Also, lipids from the surrounding solution became incorporated into the membranes, creating a protein-lipid membrane.
Oparin said “the path followed by nature from the original systems of protobionts to the most primitive bacteria … was not in the least shorter or simpler than the path from the amoeba to man.” His point was that although the explanations intended to show how organic molecules could have been manufactured in primitive seas or on clays seem quite simple, and although one can see how such molecules could have been enclosed inside lipid-protein membranes, taking these experimental situations and actually creating living cells is a tremendous leap that may have taken, at the very least, hundreds of millions of years, perhaps considerably longer.
Recent information concerning the origin of life has opened new avenues of research. To the surprise of many, spacecraft that flew past Halley's Comet in 1986 sent back information showing the comet was composed of far more organic matter than expected. From that, and additional evidence, some have concluded that the universe is awash with the chemical precursors of life. Lynn Griffiths, chief of the life sciences division of the National Aeronautics and Space Administration (NASA), said, “everywhere we look, we find biologically important processes and substances.”
We have known for years, from fossil evidence, that bacteria appeared on earth about 3.5 billion years ago, a little more than 1 billion years after the solar system formed. The great challenge has been to learn how, within that first billion years, simple organic chemicals evolved into more complex ones, then into proteins, genetic material, and living, reproducing cells.
As this current theory stands, it is felt that some 4 billion years ago, following the formation of the solar system, vast quantities of elements essential to life, including such complex organic molecules as amino acids, were showered onto earth and other planets by comets, meteorites, and interstellar dust. Now seen as the almost inevitable outcome of chemical evolution, these organic chemicals evolved into more complex molecules, then into proteins, genetic material, and living, reproducing cells.
Unfortunately, no traces of earth's chemical evolution during the critical first billion years survive, having all been obliterated during the subsequent billion years. Biologists and chemists now feel, however, that clues concerning the first stages in the origin of life on earth can be found by looking elsewhere in the solar system. Planetary scientists are to be launching new probes that will eventually investigate these questions, looking for evidence revealing the paths of chemical evolution that may have occurred, or may still be occurring, on planets, moons, comets, and asteroids.
Although most modern theorists do not accept the idea that living organisms are generated spontaneously, at least not under present conditions, most do believe that life could have and probably did arise spontaneously from nonliving matter under conditions that prevailed long ago, as described above. Other hypotheses have also been suggested for the origin of life on earth.
In 1821, the Frenchman Sales-Guyon de Montlivault described how seeds from the moon accounted for the earliest life to occur on earth. During the 1860s, a German, H.E. Richter, proposed the possibility that germs carried from one part of the universe aboard meteorites eventually settled on earth. However, it was subsequently found that meteoric transport could be discounted as a reasonable possibility for the transport of living matter because interstellar space is quite cold (−220 °C) and would kill most forms of microbial life known to exist. And even if something had survived on a meteor, reentry through the earth's atmosphere would probably burn any survivors to a crisp.
To counter these arguments, in 1905 a Swedish chemist, Svante Arrhenius, proposed a comprehensive theory known as panspermia. He suggested that the actual space travelers were the spores of bacteria that could survive the long periods at cold temperatures (some bacterial spores in Carlsbad, New Mexico, survived for 250 million years and were recently revived), and instead of traveling on meteors that burned when plummeting through the atmosphere, these spores moved alone, floating through interstellar space, pushed by the physical pressure of starlight.
The main problem with this theory, overlooked by Arrhenius, is that ultraviolet light would kill bacterial spores long before they ever had a chance to reach our planet's atmosphere. This explains the next modification to the theory. However, it is conceivable that life exists throughout the universe and is spread through space by asteroids, comets, and meteoroids. Also, it should be noted that a NASA scientist published a report that fossilized bacteria-like organisms have been found on three meteorites. He said these fossil life forms are not native to earth.
Another possible way life has spread is due to spacecraft. Francis Crick, who along with James Watson received the Nobel Prize for discovering the structure of DNA, coauthored an article with Leslie Orgel, a biochemist, in 1973. Their article, “Directed Panspermia,” was followed by the book Life Itself, in which Crick suggests that microorganisms, due to their compact durability, may have been packaged and sent along on a spaceship with the intention of infecting other distant planets. The only link missing from Crick's hypothesis was a motive. However, it is possible that microorganisms are unintentionally introduced to planets and moons each time a spacecraft lands on one.
| chemical evolution | scientific method | vital force |
| creationist | scientist | vital principle |
| microspheres | spontaneous generation | vitalist doctrine |
| panspermia |