The Search for Extraterrestrial Life Help (page 3)
What Is Life?
We of the human species have entered the third millennium. We have wondered for a long time whether or not life exists on other worlds, but we do not yet know the answer. There are tantalizing clues, and some astronomers believe that extraterrestrial life exists, but as of this writing, such beliefs are a matter of faith.
In our quest to find life in other parts of the Cosmos, we have assumed, perhaps unconsciously, that such life is similar to life on Earth. We do this partly as a mental crutch to help us get and keep a vision of what we’re looking for. We also do it to keep to a scientific course of thought so that we don’t fall into pure speculation or into nonscientific thought modes.
What If . . . ?
Some people suggest that the scientific method forces our minds to take a narrow and conceited view of reality. What if life is “out there” in a form entirely different from life as we know it? Suppose, for example, that some of the science-fiction authors’ stories have been true to the mark and that energy-field life forms dwell in the vast tracts between the stars and galaxies? Suppose that the stars and galaxies themselves are life forms that have evolved to levels of sophistication far higher than we humans—so much loftier that we are no more aware of their existence than a bacterium is aware of the elephant in whose ear it dwells? Are there life forms like this? We do not know the answer to this question. We have no idea of how to communicate with such beings. The closest we can come in this respect is to turn things over to the clergy and to approach the problem from a spiritual standpoint.
There exists a psychological split between the church and the scientific community that at times leads one group to criticize the other. Let’s not question the beliefs of people who have faith in the existence of life on loftier planes than ours. Nor should we make any claims as to the absolute truth of any scientific theory. Theories are just that. History is full of examples of people or groups of people who turned theory into dogma and later were proven wrong. Our job, as scientists, is to look for good evidence of life on a level similar to life on this planet. By taking this attitude, we have some hope of finding such extraterrestrial life, assuming that it exists, and communicating with it in a meaningful way.
With this in mind, and guarding against the danger of using statistics inappropriately (the “probability fallacy” mentioned at the beginning of Chap. 9), a special group of astronomers is engaged in a pursuit known as SETI (pronounced “SET-eye” or “SEE-tie”), an acronym that stands for Search for Extraterrestrial Intelligence . The purpose of SETI is exactly what its name implies: to find another technologically advanced civilization in our galaxy or beyond. We’ll encounter estimates of the “probability” or the “chance” that life exists elsewhere in the galaxy or in the Universe. We play this mind game with ourselves to make the nature of our quest comprehensible. In truth, however, we can only say this: Either we will discover life of extraterrestrial origin someday or else we won’t. We have a better chance (oops) of finding extraterrestrial life if we search for it than if we don’t.
Properties Of Living Things
The evolution of intelligent life is a complex process. The molecules of certain chemical compounds, given the right circumstances, develop the ability to make copies of themselves. This process, according to scientific thought, is a necessary basis for life and is part of a common definition of life.
Another necessary property of living beings is the ability to create order from chaos, acting against the entropy process at work in the Universe. Entropy constantly tries to get order to fall into chaos and to distribute all the energy in the Cosmos uniformly so that energy-transfer processes, vital to the existence of life, cannot go on. Living things can reproduce. Living things can gather energy and concentrate it and process it in an orderly fashion. Living things are orderly. Examples of this abound in all human civilizations. Look at the buildings our species has created out of rocks and metal!
A single particle capable of splitting into two other particles identical to itself will rapidly spread over a planet, as long as there is enough raw material and enough energy to sustain the process. In this scenario, the number of such particles increases in a geometric progression: First 1, then 2, then 4, then 8, 16, 32, and so on (Fig. 12-1). This sort of sequence grows rapidly to enormous size. Imagine an average reproduction rate of one particle-duplication per day for several weeks. The resulting population, assuming that none of the particles deteriorate or are otherwise destroyed, would dominate the host planet.
In a real-world scenario, the process of replication cannot go on forever; something eventually will limit the number of particles in the population. This “something” is another important property of living things: They die. Death can occur for various reasons: a limited food supply, disease, limited physical space in which to live, changes in the environment, and the effects of cosmic radiation.
Life On Earth
To understand how life can be expected to have developed and evolved on distant planets, we must first understand how the process took place on Earth. Then we might get an idea of whether or not our planet represents a one-of-a-kind miracle, the sole oasis of life in an otherwise sterile Universe.
Let’s take an imaginary journey back billions of years in time to the very beginnings of life on our planet. The following is an oversimplification, but it represents a scientific hypothesis for events following the Earth’s formation along with the Sun and the other planets in our Solar System.
The First Life Forms
Several billion years ago, the Earth was much different than it is now. The atmosphere was a noxious mixture of chemicals we would find impossible to breathe: hydrogen, ammonia, methane, and water vapor. There was little or no oxygen. The oceans were less salty than they are today, and they were sterile. Some aspects of Earth would look and sound familiar if we could travel back in time and stand there. Ocean waves broke on rocky shores with the same booming and crashing sounds we know so well, and the land looked like that we see in some places today, such as on newborn volcanic parts of the Big Island of Hawai’i. However, not a single tree graced the horizon. No birds soared over the land or the sea. No grass grew. Somehow, out of this environment, the Earth developed, in about 3 billion years, to a place where life abounds. It is hard to say, after giving the matter serious thought, that this is anything other than the outcome of a miracle. However, if we hope to find life on other worlds, we must hope that such a chain of events is a commonplace thing in the Universe.
According to modern science, life on Earth began with complex groups of particles. Some molecules, called ribonucleic acid (RNA) and deoxyribonucleic acid (DNA), developed the ability to make copies of themselves. Not long after the first of these molecules appeared, the oceans teemed with them. The presence of a hospitable fluid (water) was an essential ingredient in the proliferation of these particles. No one knows the exact process from which the first of these molecules formed, but it is believed to have involved a burst of energy such as a lightning strike, the impact of a comet or meteorite, or a volcanic eruption. According to some scientists, this process took place in several or many diverse locations on Earth, not necessarily all from the same type of energy burst.
Somehow, the replicating molecules organized themselves into groups and formed animate matter with complex functions. The exact way in which this happened is another mystery. The development of the first living cells took many millions of years. Two major types of cells appeared after about 2 billion years. One type of cell was able to convert sunlight into the stuff necessary to carry on its life processes. This process is known as photosynthesis , and the earliest cells capable of it were the first plants on Earth. Among the waste products of such cells was oxygen. Another type of cell developed that was able to use oxygen for its own life processes. These cells were the first animals. The animal cell found, in the oxygen, a more efficient source of energy than sunlight because oxygen is a reactive element. It readily combines with many other types of atoms.
Some of the cells began to stick together in colonies of two, four, five, dozens, or hundreds. The reason why some cells clung to each other and others did not is unknown. Apparently, some cells underwent mutations that caused their outer membranes to be sticky or rough. Mutations occur because the reproductive process is not always perfect (Fig. 12-2). Radioactivity, which is always present to some extent everywhere in the Universe, can cause errors in RNA and DNA duplication. As things turned out, large groups of cells were better able to survive adverse conditions than individual cells. Sometimes an error in the reproductive process actually results in an improvement in the offspring.
It’s good that reproductive “accidents” happen. Otherwise, according to a popular theory, the Earth would harbor only the rudimentary beginnings of life. Biologist Charles Darwin is credited with developing this idea, known as the theory of natural selection .
The congregations of cells grew larger. Eventually, groups of cells evolved in which not all the cells were identical. The outer cells became ideally suited to protecting the inner ones from damage or injury. The inner cells were better able to act as food and energy processors. The natural selection process dictated that the outer cells must be physically tough, but this was not required of the inner cells. Congregations with soft outer cells died, whereas those with hardier outer cells survived longer and produced more offspring like themselves.
The theory of natural selection, given sufficient time to operate, results in the evolution of life forms that are better and better suited to the particular environment in which they live. The process is evident only after many generations have passed. However, the available time on Earth is on the order of billions of years; there’s no shortage of time! The Sun and the Earth have been around for more than 4 billion years, and our parent star is expected to shine reliably for several billion more years.
Some stars are not as stable as the Sun and do not give their planets time enough for evolution to take place before they use up their fuel and burn out. Other stars are not hot enough to allow for the development of, or the evolution of, life. However, there are plenty of stars that resemble the Sun enough that some of their planets—assuming planetary systems are commonplace in the Cosmos—have temperatures similar to those on Earth. How common are such planets? This is another question that we cannot yet answer definitively because we have no hard evidence. We can only surmise that there are at least a few such planets in our galaxy, and we can see easily enough that there exist millions upon millions of galaxies in the observable Universe. It is difficult to imagine that the known Cosmos does not harbor millions or even billions of planets whose climates resemble that of the Earth.
The Evolutionary Spiral
Precisely when, in the process of increasing complexity, did the collections of atoms and molecules cease to be a simple matter? At what point can we call something “alive”? Some people say that any RNA or DNA molecule is alive because it can reproduce; other people impose more stringent conditions. There is no well-defined point where we can conclusively say, “Now there is life, but one second ago there was not.” If everyone could agree on this, many arguments taking place in such scientific fields as genetic engineering and some of the political dilemmas involving reproduction would be easier to resolve.
As the eons passed, increasingly complex groups of living cells, known as organisms , evolved. Thus mutations among individual RNA or DNA molecules became more frequent within any given organism. This is statistically inevitable. Mutations can be expected, in general, to take place twice as often in a congregation of 2 million cells as in a group of 1 million cells. As a result of this, life forms became more varied and more sophisticated. This accelerated the process of evolution, which in turn produced still more diversification. It became an evolutionary positive-feedback system, sometimes called the evolutionary spiral .
Some of the cells from the primordial oceans were washed ashore. Most of those cells perished on the dry land for lack of a fluid medium in which to reproduce. Some, however, were able to survive in tidal pools and in puddles left behind by storms. These cells developed into the terrestrial (land-based) plants. As these organisms died, soil was built up on the barren rocky surfaces of the continents. Most of the Earth’s living cells remained in the seas, where they developed into marine plants and fishes. The oxygen-burning cell congregations developed the ability to propel themselves from place to place in the never-ending search for food. From this point, the process of evolution reached a climax. Some of the fish developed the ability to live on dry land. These eventually became the dinosaurs, and they reached such a level of perfection that they dominated the continents. A species had emerged that enjoyed a biological monopoly.
Some animals, much smaller than the dinosaurs, also survived, but with difficulty. They had to scurry around at night in their search for food under cover of darkness to avoid being seen and eaten by dinosaurs. Their small bodies gave up heat more quickly than the bodies of the larger dinosaurs, and this problem was exacerbated by the cool nighttime temperatures. The result was the evolution of a new sort of animal that had an increased rate of metabolism, capable of generating enough internal heat to offset the cooling effects of small bodies and chilly surroundings. The earliest of these creatures are believed to have been small rodents, resembling mice, rats, and chipmunks.
About 65 million years ago, the climate of the Earth cooled. The reason is not known with certainty, but increasingly, scientists suspect that a small asteroid struck the Earth in the Gulf of Mexico. The result was greatly increased volcanic activity for a time, along with the production of ash and dust that was sent into the upper atmosphere, partially blotting out the light and warmth from the Sun. Another theory suggests that the Sun itself cooled off. Still another theory holds that there was a sudden, dramatic change in ocean currents, such as might be produced if the poles of the Earth shifted position or if a land bridge between Siberia and Alaska suddenly appeared or vanished.
Whatever the reason for the climate change, the dinosaurs could not cope with it. The event was too sudden and its magnitude too great for the processes of evolution to keep up, so the dinosaurs perished in a geological “blink of an eye.” Within a few million years, nearly all the dinosaurs were gone. However, the small rodents survived because they had the ability to generate internal body heat. In addition, they possessed a characteristic that might best be called wiliness —a brain that could figure out how to deal with unusual or complicated situations and crises.
Adversity plays an important role in the evolution of intelligent life. Without problems, there is no need for reasoning power. If the environment becomes ideal and stays that way, evolutionary progress comes to a halt. The dinosaurs are an example of a species that lived in harmony with the environment and adjusted to a status quo that would, were it not for the sudden cooling of Earth 65 million years ago, still exist today. We can curse our cold winters, but we owe our existence to them, according to the most popular theories of evolution and natural selection.
The Future Of Life On Earth
Our species, Homo sapiens , has roamed Earth for only a moment on the Cosmic scale of time. Imagine watching time pass from a speeded-up perspective so that 1 billion years is represented by a single day. On this scale, the Universe was born 10 to 15 days ago. Suppose that it is high noon on the thirteenth day of the Universe’s existence. Our Sun and all the planets that orbit around it are 4½ days old. A million years is represented by 86.4 seconds, or a little less than a minute and a half. The sudden cooling of Earth took place at about 10:30 this morning. The earliest known civilizations flourished within the last second.
The process of evolution has not changed the human brain very much (some scientists say not at all) since the dawn of civilization. The ancient Babylonians, Egyptians, Africans, and others were just like us! We did not develop our gadgets and conveniences and weapons of mass destruction because we are smarter than those people were. In fact, as historians delve deeper into the nature of ancient civilizations, evidence mounts to suggest that they were in some ways superior to us. The process of evolution generally requires thousands of centuries to make a significant difference.
What will happen to our species in the future? Are we doomed to destroy ourselves, as the purveyors of gloom keep telling us? Or will we venture out to explore the Universe beyond our Solar System and search for other life forms and civilizations? If the former is our fate, is this also the destiny of other technologically advanced civilizations in the Universe? If so, we cannot expect to communicate with extraterrestrials. However, if Homo sapiens can overcome its “suicide seed,” or if some species follows ours that does not have this problem, then there is hope. In any case, SETI goes on. A few dedicated scientists are looking for life “out there,” if for no other reason than that the alternative—not to search—is unthinkable.
The Malthusian Scenario
Predictions for the future of any species whose reproduction rate consistently exceeds its death rate, no matter on what planet, can range from extremely pessimistic (certain doom) to extremely optimistic (they will rule their planet and venture into space). According to a scientist named Thomas Malthus who lived in the 1800s, any population that increases at a fast enough rate inevitably will face one or more crises. We, the members of the species Homo sapiens who dwell on the planet we call Earth, are already feeling some of the real-life manifestations of Malthus’s predictions. There are too many of us, and the population is increasing.
We are intelligent, or so we claim. However, in the collective sense, are we smart enough to control our own numbers and prevent the consequences of unchecked population growth? Until we can overcome this problem, our own Earthly concerns may become so weighty as to overshadow efforts toward reaching for the stars. In fact, an excessive reproduction rate could lead us repeatedly back to stone-age conditions. Malthus’s principle operates with mathematical rigor. It can be expected to apply to any matter-based life forms on any planet anywhere. A species that cannot control its own numbers faces catastrophe, perhaps more than once, until it learns to keep its population below the limit that the environment of its host planet can support (Fig. 12-3).
Malthus showed that any population increase must take place in a geometric manner: The rate of growth gets faster and faster. If we plot the number of people in the world as a function of time, we get a graph that looks like the left portion of Fig. 12-3. If there were nothing to stop the process, the planet eventually would become so crowded that people would have to sit on one another’s shoulders and would occupy every square centimeter of every continent. However, things happen to keep Earth, or any planet with life whose population grows geometrically, from suffering such a fate. Unfortunately, with the exception of voluntary population control, all these limiting processes are horrible.
Malthus believed that the maximum obtainable food supply can, at best, grow at an arithmetic rate, a straight line on a quantity-versus-time graph. Today we know that the food supply can’t increase indefinitely at a steady rate; it must level off sooner or later (Fig. 12-4) because the planet can support only so much agricultural, fish-breeding, and livestock-raising activity. Thus the population eventually will outstrip the food supply. Mass starvation will occur. This will put a limit on the population by increasing the death rate and by reducing the reproduction rate. The world has not reached this point yet everywhere, but in some countries it is getting painfully close.
Another factor that will limit population growth is disease. When masses of people are crowded, epidemics start and spread much more easily than if people live with plenty of space between each other. New bacteria, viruses, and other pathogens (including the prions that cause mad cow disease) develop and evolve rapidly in such conditions, defying attempts at vaccination and literally “learning” how to overcome the effects of antibiotics and other medicines. It is almost as if the planet fights back against further population increase, with disease organisms playing the roles of antibodies and human beings acting as an infectious agent in the “body” of Earth!
Still another limiting factor is the intolerance of humanity for its own kind, manifested in wars and brutal political regimes. New weapons of mass destruction and an increase in the frequency of quarrels leading to wars will tend to limit the population, at least of a species predisposed to violence, as is Homo sapiens .
This is a grim picture, isn’t it? Hunger, disease, and war, with disasters taking place at ever-more-frequent intervals as a planet gets more and more crowded with the species that, were it not for its collective inability to control its reproduction, ought to live lives of peace, comfort, fulfillment, and, to the extent they choose, interstellar adventure. All these things may be ours if only we could learn not to make so many copies of ourselves. If ever an intelligent civilization from another star system sends explorers here, we will know that they got their breeding instinct under control, at least until they found a way to colonize worlds besides their planet of origin.
Practice problems of this concept can be found at: The Search for Extraterrestrial Life Practice Problems
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