The Search for Extraterrestrial Life Help (page 2)
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.
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