The Search for Extraterrestrial Life Help (page 2)

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

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.

The Search for Extraterrestrial Life What Is Life? Properties Of Living Things

Figure 12-1. A geometric progression such as takes place when molecules repeatedly copy themselves or when biological cells divide.

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.

The Search for Extraterrestrial Life What Is Life? Natural Selection

Figure 12-2. The process of replication is not perfect. Mutations, however rare, are inevitable.

Natural Selection

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.

View Full Article
Add your own comment

Ask a Question

Have questions about this article or topic? Ask
150 Characters allowed