ESSAY: Describing Life: An Impossible Challenge? Unit 1 121

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1 Describing Life: An Impossible Challenge? An alien spaceship orbiting a planet to make observations ejects a special robotic rover toward the planet s surface. Retrorockets fire to slow the robot s speed, and parachutes open to help it land gently. Now the robot settles down on the planet s surface, amid rocks and soil. Its mission: to determine whether life exists on the planet. a At first, the rover sits motionless, a foreign object on the bleak landscape. Then, slowly, it activates its electronic senses. It rotates its twin cameras to scan the horizon. It measures the local weather: temperature, barometric pressure, wind speed, and direction. It uses special equipment to sniff the air to determine its composition. The atmosphere is thin, but it b Figure E3.1 Technology to explore Mars. (a) In July 1997, the rover Sojourner landed on Mars. The white airbags that cushioned its landing are seen on either side of the photo. (b) In 2004, two rovers, Spirit and Opportunity, landed at different places on Mars and sent data back to scientists on Earth. In both pictures, you can see rocks on the surface and hills in the background. ESSAY: Describing Life: An Impossible Challenge? Unit 1 121

2 does contain some water vapor, even a little more than observers had expected. This is a good sign... Finding evidence for the potential of life on Mars has been very exciting, though difficult and inconclusive, so far. Scientists continue to design missions that will collect additional evidence. How should scientists involved in the effort look for life? The landing craft, though technologically very complex, is small. Its designers had to make careful decisions about the equipment that it carries and the activities that it is able to accomplish. Should they look for signs of evolution, of growth and development, of reproduction? All are fundamental characteristics of life, but all probably occur too slowly to be detected by a tiny, robotic craft. Instead, designers decided to test surface samples for more immediate, more easily recognizable signs of life. They will test for those signs that have to do with a living system s requirement to use matter and energy obtained from the environment to maintain its complex organization. Suddenly, a mechanical arm extends from the strange craft and scoops up some of the Martian soil. At last, humans have collected a sample from another planet. It doesn t matter Figure E3.2 Lichens are organisms that consist of a close association between a fungus and a photosynthetic organism such as an alga or a blue-green bacterium. that the rover cannot return to Earth. It will send information about minerals, water, and the climate found on Mars back to scientists on Earth. Technology is extending our hands and eyes. Slowly, the soil sample is deposited in a special chamber. A specially designed piece of equipment adds a mixture of radioactive gases. Some of the smallest forms of life on earth use light energy and certain gases in the environment around them to build more complex molecules that are necessary for them to live. Is there anything in the Martian soil that will use these gases as molecular food? Will these radioactively labeled molecules slowly begin to accumulate in the soil sample, as living things remove them from the air and use them to maintain and build their internal structures? Recognizing life on an alien planet such as Mars is indeed a challenge. At minimum, such an endeavor requires that we start with a good idea of how to recognize life on Earth. But even this task is not simple. Living systems share many characteristics. But do any of these actually distinguish living things from nonliving things? You already have an intuitive sense about life. If you were to ask your classmates to identify a tree, a dog, and a rock as living or nonliving, chances are great that all their answers would be the same. However, if you then asked your classmates precisely how they know what is living and what is not, their responses probably would vary. And suppose you asked them to categorize a less familiar object, such as the scaly, grayish green lichen on a boulder (like the one depicted in Figure E3.2)? This time, some of your classmates might say that this stuff is not alive. How can we describe life so that we always can identify it when we see it? Perhaps the easiest way to begin thinking about life is to consider what happens when an organism dies. Think, for example, about a bird that has just died. The bird can no longer move, or eat, 122 Unit 1 ESSAY: Describing Life: An Impossible Challenge?

3 Figure E3.3 A proposed evolutionary tree. What evidence do you think scientists used to develop this explanation? or keep itself warm. Even if you touch it, it does not respond. Eventually, the dead body will become decayed and disorganized. It will never recover its form or function, nor will it ever again produce offspring. This simple example provides some clues to the nature of life by showing that certain properties of a living system are lost when death occurs. If, however, you try to write a simple description of life that absolutely distinguishes it from nonliving substances, you may find it rather difficult. Rather than trying to describe life precisely, let us first examine the characteristics that we generally observe in living systems. Then, perhaps, we can consider how we might go about recognizing life, both on Earth and on another planet. All forms of life, even vastly different forms such as humans, apple trees, spiders, and microscopic bacteria, share many basic ESSAY: Describing Life: An Impossible Challenge? Unit 1 123

4 characteristics. That should not surprise you. In fact, it is because living things share much in common that you can make some judgments about whether an unknown object is likely to be alive or not. Understanding these common characteristics is fundamental to understanding biology. In fact, these characteristics are so important that we have summarized them as six unifying principles of biology, and we have organized the flow of topics in this course around them. You already have encountered evolution, one of these principles. As you begin to develop a deeper understanding of the remaining five principles, you will be developing a rich understanding of how all living systems, including humans, function. Evolution: Patterns and Products of Change in Living Systems. As you already have seen in this unit, one significant characteristic of living systems is that they evolve, or change, across time. Through natural selection, some individuals have characteristics that make them best suited to their environment. These individuals are most likely to grow to maturity and reproduce. Those same individuals then pass their traits on to their offspring. As a result, those adaptive characteristics become more common in the population. Figure E3.4 This anole lizard can change color in response to its environment. In some species, such as the horseshoe crab, characteristics remain unchanged for long periods of time. More often, however, natural selection results in gradual change in populations. These changes eventually lead to distinctly different populations of organisms that display an amazing range of diverse characteristics. Figure E3.3 illustrates how scientists think one type of organism may have evolved. Evolution represents the first unifying principle considered in this course. Although evolution is important in understanding life, living organisms, and their interactions, it is not particularly helpful in determining whether a particular object is living or not. Homeostasis: Maintaining Dynamic Equilibrium in Living Systems. A second characteristic of life and a second unifying principle of biology has to do with a living system s ability to maintain an internal balance, referred to as homeostasis. All organisms regulate their internal systems in response to changes in their surroundings. When you are startled, your heart beats faster, sending blood through your body at a faster rate. This response ensures that your body will continue to have a good supply of oxygen and nutrients (which are carried in the blood) during a possibly stressful or dangerous time. In fact, all organisms show a similar type of internal regulation. Bacteria adjust their production of certain key products in response to changes in the nutrient levels in their environments. Plants respond to changes in humidity by opening or closing tiny holes in the underside of their leaves. And some animals can change their coloration in response to their environment (see Figure E3.4). Energy, Matter, and Organization: Relationships in Living Systems. Another common characteristic of life is organization. All living systems are highly organized forms of matter. This matter is made from atoms held together in ways that form large, complex molecules. Scientists have identified more than 100 different types of atoms. One of the most remarkable 124 Unit 1 ESSAY: Describing Life: An Impossible Challenge?

5 similarities among all living things, however, is that they are made predominantly from only a few types of atoms. These are notably carbon, nitrogen, oxygen, hydrogen, phosphorous, and sulfur. The molecules of living materials are organized into complex structures known as cells. Cells are the basic structural units of living matter. Because most cells are too small to see with the unaided eye, scientists did not see cells until 300 years ago, after the invention of the microscope. As Figure E3.5 illustrates, cells are baglike structures made of a membrane that encloses and protects the contents. A related property of all living systems is that they require energy to build and maintain their highly organized structures and to carry out all of their activities. Recall that the bird, once dead, eventually will decay and disintegrate. It will lose its distinctive shape and appearance and become increasingly indistinguishable from the matter around it. The loss of the bird s characteristically high organization follows the more basic loss of its ability to obtain matter and energy from its environment and to use that matter and energy to keep its body (its matter) repaired and functional. Together, the ideas of matter, energy, and organization represent the third unifying principle of biology. Continuity: Reproduction and Inheritance in Living Systems. The organization and the function of living systems depend on specific plans that are encoded in each organism s genetic material, or DNA. For example, maple trees display a characteristic structure and function because they possess DNA characteristic of maple trees. Humans grow and function in ways that we recognize as distinct from other life-forms because humans possess DNA characteristic of humans. DNA is a long and complex molecule that stores information (see Figure E3.6). One of the most significant characteristics that unifies living systems is the universal nature of this DNA. Although the instructions that direct an organism s cellular activities and developmental events are specific for its species, all organisms, from bacteria to humans, use the same DNA to communicate those instructions. The ability to transfer those instructions through DNA to the next generation during reproduction represents a fourth important unifying principle of life, that of continuity. Development: Growth and Differentiation in Living Systems. The ability to grow and develop represents the fifth unifying characteristic of living systems. Growth is an important activity in the early life of a human. Growing requires the body to assemble new tissue. As the organism s size increases, the way in which the organism s tissue is organized also changes. Human adults not only are larger than children are, but they also are shaped differently. And they can do a variety of things that human infants cannot do, such as walk and talk. Many Figure E3.5 The cell is the basic unit of living matter. Most prokaryotic cells (cells without membrane-enclosed nuclei or organelles) are 1 10 µm in diameter. Most eukaryotic cells (cells with a membrane-enclosed nuclei and organelles) are µm in diameter, but protoctists can be much larger. Notice the organization of the interior of this Thecamoeba cell, which is 10 µm. ESSAY: Describing Life: An Impossible Challenge? Unit 1 125

6 base Figure E3.6 DNA is a complex molecule. It has a double helix (twisted ladder) shape. plants, in a similar manner, begin life as small seedlings that push up through the soil and grow into mature plants that look quite different from the early seedlings. Plant growth also involves the addition of new tissue and the organization of new parts such as leaves and reproductive structures. Ecology: Interaction and Interdependence in Living Systems. Finally, all living systems on earth are part of an interactive and interdependent web of life. Figure E3.7 Organisms along and in this stream interact and depend on each other. Organisms do not exist in isolation, but rather live as one element in a complex community of life (refer to Figure E3.7). Imagine a wooded area alongside a stream on an early summer day. Plants provide shelter and food for a variety of birds. Perhaps a rabbit has dug a burrow nearby and now feeds on wild berries growing in the light shade close to the forest. Not far away a fox has just left her den in search of food for her young. This community of different, yet interdependent, living systems illustrates the sixth unifying principle of biology, the interactive and interdependent nature of life. To the extent that these brief descriptions capture the essence of each of the unifying principles, we might say that in this short list of characteristics, we have described life as it exists on earth. Can we say that any one of these principles defines life, in the sense that it alone is necessary for life and that it alone is an indicator of life? Probably not. Just as a combination of characteristics identifies you as a human, a combination of these principles indicates the presence of life. To this day, curiosity about life on Mars remains high. Unfortunately, although two of the three tests described in Figure E3.8 yielded some interesting results, scientists failed to duplicate the results with subsequent samples. This was disappointing and suggested 126 Unit 1 ESSAY: Describing Life: An Impossible Challenge?

7 lightbulb a gas detector b radiation counter c gas processing tube nutrient solution soil sample 1. Soil sample is suspended in a porous cup. 2. Nutrient solution is added to the soil sample. 3. Changes in gas content are measured by a gas detector. nutrient solution with radioactive carbon atoms soil sample 1. Soil sample is sprayed with radioactively labeled nutrient solution. 2. Any radioactive carbon dioxide that is produced by the soil and released into the air above the sample is detected and counted. gases with radioactive carbon atoms soil sample radiation counter 1. Radioactive gases are introduced into the chamber containing the soil. 2. The light is turned on as a source of energy. 3. The chamber is heated to release newly made substances into the air. 4. The air is processed to separate complex substances from the simple gases that had been introduced earlier. 5. Any radioactive carbon that is contained in these complex molecules is detected and counted. Figure E3.8 Three experiments to test Martian soil. (a) A gas exchange experiment tested the Martian soil for evidence of organisms that took in gases from the Martian atmosphere and nutrients from the soil and gave off gases as wastes. This experiment is then performed using earth s soil. The experiment indicates the presence of microscopic organisms that take in oxygen and nutrients and give off carbon dioxide. (b) Scientists next search for the release of carbon dioxide. The experiment tests the Martian soil for evidence of organisms that could use simple nutrients and give off waste gases (CO 2 ). This experiment was similar to the gas exchange experiment. It served as an important check on its results. Again, this experiment gives strong, positive results when earth s soil is tested. (c) A third experiment tested the Martian soil for evidence of organisms that might build large, complex substances out of simple gases in the Martian atmosphere. This experiment is then performed on earth s soil. The experiment indicates the presence of microscopic organisms that use the energy of sunlight to help them build sugars and other large, complex molecules. caution in interpreting even the changes that the distant instruments did detect. In fact, by 1979, most scientists involved with the project had agreed that although they could not rule out the possibility that life exists on Mars, all the data that they collected in the original experiments could be explained as resulting from purely chemical (not biological) causes. The rovers that landed on Mars in 2004 did not find life. But they did find evidence indicating that water, a necessity for life as we know it, existed on Mars in the past. Describing life... a difficult, but not an impossible challenge. Looking for life, using earth s criteria, in a very different environment more than 40 million miles away... more difficult to be sure, but impossible? What do you think? Five Kingdoms In which of these pairs of illustrations are the organisms most closely related? Figure E3.9 shows two animals that bear little resemblance to each other. In contrast, Figure E3.10 shows two types of cells, each an individual organism and each looking quite like the other. Surprisingly, from an evolutionary point of view, the two animals are much more closely related than are the two single-celled organisms. The animals are an African elephant and a close relative, a small mammal known as a hyrax. What you cannot see in Figure E3.9 is all of the ways in which these organisms are similar, from the basic structures of their cells to the structures of their feet and teeth. ESSAY: Five Kingdoms Unit 1 127