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The Search for Extraterrestrial Life Help (page 3)

By — McGraw-Hill Professional
Updated on Apr 25, 2014

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.

Climatic Change

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).

The Search for Extraterrestrial Life What Is Life? The Malthusian Scenario

Figure 12-3. A species faces disaster if the reproduction rate, which grows geometrically, consistently exceeds the death rate.

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.

The Search for Extraterrestrial Life What Is Life? The Malthusian Scenario

Figure 12-4. The maximum obtainable food supply grows arithmetically at best, according to the Malthusian model.

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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