The Evolution of Life on Earth
Table of Contents
Evolution of Life on Earth
What is meant by "evolution?"
What is meant by "science?"
Evolution and the fossil record
Ideas about evolution: the history of the theory
The origins of life: 3 theories
Aristotle and Spontaneous Generation
Chemistry is more than the study of compounds and molecules; it has many practical applications in the field of biology, where it can be applied to understand living things. The study of chemistry may, in fact, hold the answer to some very profound questions. It was Alexander I. Oparin who first proposed the theory that life originated on Earth via chemistry; that is, life resulted from the straight-forward evolution of matter according the rules of chemistry. Modern-day proponents of this theory include Leslie Orkel and Sidney Fox.
Evolution of Life on Earth
How in time did life come to be on the Earth? There are several types of answers to this question. Here are two of them:
Science provides answers according to certain rules that science chooses to follow.
God put life here. However, this is a statement that cannot be proven or disproven according to the rules of science, and it therefore is not a scientifically valid statement. Science is agnostic on God; it takes no position on God. If you chose science and God (which many people do), then you rephrase the question slightly: How, according to the rules of nature, did God create life on Earth?
This is really the same as "how, according to the rules of nature, did life appear on Earth?", which is the question every evolutionary scientist is trying to answer. According to Oparin's theory, it was either by the simple rules of chemistry, or by the simple rules of chemistry as they were invented by God. Whether one believes in God or not, the results, when conducted scientifically, should come out the same.
The idea of evolution is one of the truly great ideas humankind has ever expressed. Some people erroneously believe that the opposite of evolution is God. This is not correct. Remember, science is agnostic. Science takes no position on God. Some people may push their beliefs (or lack thereof) on you, one way or another. An atheist and a scientist may tell you science implies atheism; it does not. What we're trying to do here (and elsewhere in your college experience) is provide you with ideas; you have to form your own conclusions after examining the ideas
What is meant by "evolution?"
Evolution mean "change in time." In biology, this means "change in biological entities in time." We will eventually discuss species, and ask, how did species come to populate the Earth? Where did the diversity of life that we have upon the Earth happen in time?
If the definition of evolution is changing in time, then the opposite of evolution is, clearly, not changing in time, or "stasis," which means to stay the same in time. Things either change in time, or the don't change in time. Those are the two opposites that must be considered when discussing evolution.
Has the distribution of life on Earth changed in time, or not changed in time? If it's changed in time, then evolution is a fact; if it has not changed in time, then evolution is an error. So all we have to do is identify what has occurred in the past, in order to determine whether or not life has changed over time.
What is meant by "science?"
What is science? Is science a series of "truths?" Not at all. Science is a way of organizing nature in an attempt to answer questions that we can't know for certain; that is, we attempt to take information from nature and to organize it in such a way as to make sense out of nature. These are not "truths." Science can't provide you with a known truth. It can provide an explanation in accord with all known facts, but you can never know if that happens to be a "truth" or not. What you get with science is an explantion; and as long as the explanation is in accordance with the information gathered from nature, it is scientifically credible. As soon as it is out of synch with information provided by nature, it's over-ruled and thrown out. That is the mechanism by which science proceeds; that which new information discredits is discarded. Science is a system of "negation." It works to get rid of the wrong answers, the answers that do not adhere to nature. We get rid of the wrong answers by accepting nature. We essentially accept nature at face value, as infallible. Whatever nature is showing us, is the way it is. However; we cannot know that for sure! Therefore one of the conditions for being human, even for scientists, is to live in doubt.
We can't even be sure of our own existence, as philosophers have for eons been at some pains to point out to us. Here's a theory: God created the universe ten seconds ago. All of your memories of a personal past, all the history of humankind, was created in the same instant. Did this really happen? Maybe it did, maybe it didn't. We can't disprove something like that.
All of us, including scientists, have to recognize what it is that we are assuming. We assume everything around us is real, and that everything around us is a part of nature that we can interact with in some way.
With these caveats in place; what do we get from nature about evolution? What does nature have to tell us on the subject? That's the question science uses in order to organize information. It is not the only way to organize information; theology or philosophy offer alternative questions and alternative ways to organize thoughts and ideas.
Evolution and the fossil record
The history of life that nature presents to us is tied up in the fossil record. Until the 1800s, humans didn't know how to make any sense of the fossil record at all. It was not even agreed upon what fossils were. Almost everyone has seen fossils; they can be seen here in San Diego at the cliff bases in Torry Pines State Reserve, where many sea shells are embedded in the rock. Clams and the like don't usually exist inside solid rock; what are they doing there, how did they get there? When people first described fossils, they were called "figured stones." Some people even then argued that they were a record of living things that had been captured in rock somehow. Others argued that the "figured stones" were tricks, placed by God as a test of faith.
As far as science is concerned, these two opinions have been settled in favor of the former; these are organisms that were present in the past, that are not present now. It is clear what has happened; change in time has occurred. If change in time occurs, then, by definition, evolution has occurred. This is why, as far as science is concerned, evolution is a fact, not a theory. Science requires that you argue from nature, and what nature has to say on the subject, despite what some people who call themselves scientists might try to tell you, is incontrovertible. Evolution has occurred.
Does this incontrovertible scientific fact mean that evolution is the truth? It may well be, but even if it is, we can't ever know that since we weren't present when it happened. However, if we agree to base our opinions on nature, then we must accept what nature tells us without invoking outside agencies as mediators. It is obvious from the above that whatever "creation science" is, it is not science. This idea of biblical literalism predicts a homogenous fossil record; all species uniformly distributed. Nature says the opposite! No scientific theory can stand in opposition to Nature.
Ideas of evolution: the history of the theory
Now that we've accepted evolution as scientific fact, we can ask how it happened. How did change in time with respect to the distribution of life on Earth occur? How did life ever get here in the first place? These are the questions that we, as scientists, concern ourselves with.
One of the first modes of evolution was proposed by Jean Baptiste de Lamarck. His idea of evolution was that God planted in you the will to survive, and that the will to survive allowed you to "adapt," and that in adapting, you changed your physiology to gather resources. Whatever traits you received as a result of adaptation were passed on to the next generation. This was called "The Inheritance of Acquired Characteristics," and is now called " Lamarckian Evolution." As it happens, it was a brilliant idea -- but it was wrong. It was easily shown over the space of relatively few generations to be wrong. If one worked out in the gym and built up huge muscles from weight lifting, those muscles were not passed on to the offspring; likewise, if a parent lost a limb, that lack of a limb would not show up in the next generation. Lamarckian Evolution was at odds with nature, and therefore was discarded.
Darwin suggested an alternative mechanism, which he called "Natural Selection." It's different from Lamarckian Evolution in that chance acquisition of characteristics plays a huge role. Things either happen, or they don't, in terms of your genetic system, and that genetic system determines who you are. If the genetic system helps you gather resources, you're more likely to succeed and pass those traits on to offspring. If you fail, you go extinct -- just like the dinosaurs.
Darwin is not the last word on a mechanism for evolution. Theories for evolutionary mechanism are continually revised as our understanding of nature increases. One current theory is that of puncuated equilibrium, proposed by S.J. Gould and Niles Eldridge, at Columbia and Harvard Universities. It requires inheritance of chance characteristics, which is straight from Darwin, but it requires that this inheritance happens much faster than Darwin envisioned. Darwin saw gradual changes over long periods of time; Gould and Eldridge theorize that evolution can happen much faster.
The origins of life: 3 theories
So far, we've discussed evidence for life changing over time, rather than how life originated in time. We've looked at how life evolved. But how did life originate on Earth?
The reason evolution was presented first is because the primary theory about the origination of life on Earth is based on Darwin's Theory of Natural Selection. What scientific theory do we have to account for the origin of life on earth?
The primary theory about the origin of life on earth is based on evolution. It wasn't, however, the first theory of life's origin. We've gone through quite a few scientific theories -- theories based on nature -- over the years. We know, for example, that life today comes from pre-existing life. Things of a kind give rise to other things of the same kind. Could that be an answer to have life first came to earth? Clearly not. Life today is based on the idea of "cells." There is a famous quote: " only from cells come cells." This does not, in and of itself, tell us how life originated in the first place.
3 theories on the origins of life:
Aristotle and Spontaneous Generation
1. Aristotle and Spontaneous Generation
Aristotle (384-322 B.C.E.), who was not a scientist but a philosopher, argued from the idea of hierarchy. He believed in a place for everything, and everything in its place, including humans. This included the idea that males were superior to females because, said Aristotle., they had more teeth than females. If he had been a scientist, he would have counted teeth, and realized that males and females had the same number. But he was prejudiced by the idea of hierarchy. He believed in universal principles, principles that have been since been discredited.
Aristotle's explanation for the origin of life was "Spontaneous Generation." He held that life springs spontaneously from decaying organic matter; basically, life is generated from garbage. Anyone observing garbage for a period of time can see where he got this idea. For example: if meat was left out, as it decayed maggots appeared in it. Those were spontaneous; no one saw where the maggots came from.
The most important development there was in 1668, when an Italian scientist named Francesco Redi (1626-1697) studied decaying meat. He put the wormy meat in a covered jar, and found, after a while, flies in the jar. Redi reasoned that the flies must have come from the worms. Further, he reasoned that if the flies came from the worms, perhaps the worms came from the flies. This established a "life cycle." Flies lay eggs, eggs hatch, go through an intermediate form, and become flies. Redi eventually worked out the complete stages, and was the first person to suggest that organisms could go through different stages in a life cycle.
Redi didn't stop there. He did another experiment: he put two identical pieces of meat in jars, one covered, one uncovered. What he found was that worms always, and only, occurred in the uncovered jar. From this he concluded that spontaneous generation wasn't occurring; what was happening was some sort of an infection of the meat. With this experiment, spontaneous generation begins to disappear as an acceptable explanation for the origin of life.
Spontaneous generation had one more heyday, when the Dutch scientist Antony van Leeuwenhoek (1632-1723) invented the microscope, which he did by combing lenses and looking through them. He looked into areas that people assumed were sterile, like water, and found critters he called "animalcules." Today, we call these "microorganisms." People who believed in spontaneous generation latched onto these organisms, saying they were what, originally, spontaneously generated, and then life evolved further. This proved to be a difficult idea to disprove. Redi-like experiments, leaving covered and uncovered jars of vegetable broth out, produced mixed results; the uncovered container always produced animalcules, but a significant proportion of the time, so did the covered containers. Scientists argued that this showed spontaneous generation was a rare event, but that it did occur.
In the 1800s, chemist Louis Pasteur developed the "germ theory of medicine." He argued that people became ill because they were infected by invisible things in the surrounding environment called "germs." This idea was beyond controversial at the time; it was widely derided as ridiculous. Invisible germs were not acceptable to most medical experts of the early 1800s. However, Pasteur was able to demonstrate the existence of germs indirectly, by having hospitals follow proceedures that would eliminate germs from their environment, whereas the patients didn't get sick. He achieved tremendous fame as a result, and became a consultant to France's main industry, wine-making. He studied metabolism and critters, and re-discovered that life, indeed, comes from life, leading hime to hypothesize that micro-organism were somehow sneaking into the "covered" containers. If the containers weren't immediately sealed in an sterile environment, the microbes in the air would simply fall in and contaminate the sample before it was sealed. Pasteur performed several successful experiments, discrediting the idea of spontaneous generation of wholly formed organisms forever. He could show that organisms were in the air by sucking air through a material called "gun cotton"; what collected on the material was a collection of micro-organisms.
Pasteur's most famous experiment was the "swan necked flask" experiment, where he developed a series of long-necked containers that were left open to the air. David V. Cohn explains how the swan-necked flask experiment was set up:
"The experimental design that clinched the argument was the use of the swan-neck flask. In this experiment, fermentable juice was placed in a flask and after sterilization the neck was heated and drawn out as a thin tube taking a gentle downward then upward arc -- resembling the neck of a swan. The end of neck was then sealed. As long as it was sealed, the contents remained unchanged. If the flask was opened by nipping off the end of the neck, air entered but dust was trapped on the wet walls of the neck. Under this condition, the fluid would remain forever sterile, showing that air alone could not trigger growth of microorganisms. If, however, the flask was tipped to allow the sterile liquid to touch the contaminated walls and this liquid was then returned to the broth, growth of microorganisms immediately began."
Some of these containers are still sitting in the Pasteur Institute, just as Pasteur left them more than 100 years ago, and are still uncontaminated. Thus Pasteur demonstrated, once and for all, that organisms appeared in meat and broth via contamination, not via spontaneous generation. Spontaneous generation could not be used to explain the origin of life on Earth.
So if it can't be spontaneous generation, what can it be?
With spontaneous generation discredited, scientists were left with no credible explanations for the origin of life on Earth. A second theory of life came into being at this time, a theory called "Panspermia," a theory that is today gaining some renewed interest and credibility. This was first articulated by a man named Svante Arrhenius, an extremely famous physical chemist from Sweden who was awarded the 1903Nobel Prize in Chemistry. Arrhenius believed that life came to earth as an infection of bacterial spores from outside our solar system. When meteorites bearing these spores hit Earth, the bacterial spores found an environment suitable for life, and began to develop and adapt to their new home. One problem with this theory is that it doesn't explain the origin of life. Stating that life came to Earth on meteorites still doesn't explain how it initially originated, or where in the Universe it originated. Further, mathematicians promptly demonstrated that this theory was mathematically impossible; given the huge distances between stars in the Universe, it could be shown that the likelihood of living material from other stars or solar systems getting to this solar system, and then impacting this planet, was highly unlikely.
This theory has experienced a modern-day resurrection. Several people believe that Panspermia is still a possible explanation if, instead of accidentally landing on Earth, the bacterial spores were sent here. This is called "Directed Panspermia.", and has been proposed by Leslie Orgel and Francis Crick. They suggest the bacterial spores could have been sent here by a dying civilization. It is, basically, the Super-Man theory; except that instead of sentient organisms, the dying civilization sent bacteria in order to preserve life in the Universe. There are reasons that this latter version of Panspermia is a scientifically credible theory, including the ubiquitous of the genetic code.
Since this theory postulates that the origin of life on Earth actually occurred someplace other than Earth, most people don't like this idea. However, just because it is an unpopular theory doesn't mean it is an incorrect theory. Right now, we're looking around Mars to see if life originated there. We may someday discover life on Mars and Earth shared a common origin.
That's the gist of the second theory: either life came from another star and was directed here; or, possibly, life arose elsewhere in this solar system and came here in an undirected fashion.
This is one of the reason Space is an important objective for scientific research; the only way we'll prove or disprove this theory by looking at other planets or moons.
3. Chemical evolution
The theory of chemical evolution was first suggested in 1920s by the Russian scientist Alexander I. Oparin, whose work was suppressed because he was a Soviet citizen. J.B.S. Haldane, a British scientist, came up with the same idea independently a few years after Oparin. The theory of chemical evolution postulates that life occurs as the natural result of the evolution of inorganic matter. Matter, as it appears on the Earth, is the same as matter that occurs throughout the Universe. Because of the rules of chemistry, matter naturally tends to organize itself and changes through time into bigger macromolecues that are the pre-cursors to life; when those molecule occur, they tend to aggregate into the molecules that will eventually organize themselves in living cells. That is the theory of chemical evolution.
For chemical evolution to work, it requires an extensive amount of time. Here is probably the most agreed amount concept in science: Time is huge. It is not simply a matter of our solar system or of our universe; it is not a few hundred years or a few hundred million years; billions of years at least are required. How big is a billion? If you started counting from the moment, the second of your birth, and you continued to count every waking moment of your life, you could not reach a billion in a normal lifetime. All of recorded human history is 5 to 6 thousand years; a tiny fragment of a billion. That's what we're thinking of when we're thinking about scientific time spans. If that idea is wrong, then almost every idea in science is wrong. All of geology, all of biology, all of astronomy is predicated on the immenseness of time. The idea that time is huge is one fact that all scientists seem to agree on.
This vast time is needed for chemical evolution to occur.
What makes scientists believe in this theory? It was first tested scientifically in the 1950s. Stanley Miller, when he was a graduate student in Harry Urey's lab, demonstrated the validity of this theory. He choose some gases: methane, ammonia, water, hydrogen; the four required elements and compounds for life thought to be present in the early Earth. Simple put, these gases were put in a big pot, and subjected to lighting-type energy. What was found at the end of the experiment was phenomenal; about 12% of the carbon material in the "pot" had converted into an organic acid, and about 9% of that was an alpha-amino-acid. What that means is that chemicals evolve in a direction; it is not random molecules that are formed, but a significant amount of the same molecules.
What's special about alpha-amino-acids? It's a major component of most forms of life on earth called "protein." Protein is made up of alpha amino acids. Therefore the material we're all composed of is easily made from the conversion of these gases under conditions likely to have taken place in the early Earth. Miller's experiment becomes a possible experimental verification of the Oparin theory of chemical evolution.
There are pitfalls with this theory. As it happens, organisms are also critically dependent upon nucleic acids, which control proteins. DNA is how nucleic acids are packaged in humans. Unfortunately, precursors for DNA have not been found with this experiment. However, small molecules for formaldehyde and cyanide have been found. These molecules can undergo chemical processes to form the amino acids; they can also be recombined in slightly different ways to form other things, things that would provide the basis for nucleic acids; purine and pyrimidine bases, and sugars. Sugars can be made from formaldehyde; purine and pyrimideins can be made from cyanide-type molecules. Therefore, the simple substances resulting from this experiment can be subjected to other conditions, and coverted to other molecules necessary for life. This is how we now believe things happened. Now, of course, once again, we can't know this because we weren't there. We are hoping, however, to find someplace (such as a moon of Jupiter, perhaps) where the conditions are such that this is occurring right now, so that we can check the credibility of this theory.
Crick and the other scientists who have suggested that Directed Panspermia is how life came to Earth believe that chemical evolution is what generated life in the universe to start with, even if it isn't what happened on Earth itself. That makes chemical evolution the predominant scientific theory for the origin of life in the universe: by the rules of chemistry, life generated itself.
There has been a twist. DNA and protein are the two major components of life. DNA directs the protein, which works to generate new DNA. This is not a simple two-step process; DNA generates intermediates called RNA, a key step in the transition between DNA and protein. The new theory that has evolved to take advantage of this is "RNA-World," an offshoot of the chemical evolution theory. Proponents of this theory believe that the original nucleic acid was a molecule of RNA, not DNA. Thomas Cech, the 1989 Nobel laurelate, discovered that ribozymes have catalytic abilities like proteins, and genetic abilities like DNA. So they can serve both roles. Jerry Joyce is a proponent of RNA-World, with RNA as the first of the nucleic acid molecules to form. DNA developed from RNA; DNA was more stable, and took over the genetic abilities, while proteins took over the catalytic function because they could do the job better.
The Creation of the World Project: A brief explanation of the different theories about the origin of life on Earth, including Creationism, the Oparin-Haldane chemical evolution theory, and the Crick infection-from-space theory.
The Center For Scientific Creation: Arguments against the theory of evolution.
Cosmic Ancestry: The modern-day version of Panspermia.
The Slow Death of Spontaneous Generation: From Redi to Pasteur; how scientific experiments over two centuries discredited the predominant theory of the day.
Lamarck and His Theory of Evolution: The historical and theoretical impact of the precursor to the theory of Natural Selection.
The Origin of Life on the Earth: Lesley Orgel looks at the role catalytic RNA may have played in the origin of life on this planet.
The Big Ear Radio Observatory: Find out about SETI (Search for Extra-Terrestrial Intelligence) and Radio Astronomy.
From Primordial Soup to the Prebiotic Beach: An interview with exobiology pioneer, Dr. Stanley L. Miller: Dr. Miller explains both his ground-breaking experiment that supports the theory of chemical evolution and the field of "Exobiology." An animation of the experiment takes you through the different stages.
Exobiology Strategy: What does the possibility of microbes on Mars tell us about the pre-life chemistry here on Earth? This NASA site compares the atmosphere of early Earth and early Mars.
The Beginnings of Life on Earth: What is "RNA-World?" This American Scientist article explains.