Biology of Humans

1. Humans in the World of Biology

In this chapter, we see that life has many levels of organization: individual, population, community, ecosystem, and biosphere. Throughout most of this book, we focus on the human individual—how the individual human body functions and the biological principles that govern those functions. However, we also examine many of the larger health, social, and environmental issues that we must be aware of, because they can affect all of us.

Basic Characteristics of All Living Things

We will begin by exploring the Amazon rain forest—a place that is teeming with life. Given the biodiversity of the rain forest, it not surprising that scientists are exploring it in search of any secrets it may reveal, including any plants that may have healing qualities (see the Environmental Issue essay, Medicinal Plants and the Shrinking Rain Forest).

We say that life is abundant in the rain forest, but how do you determine if something unfamiliar to you is alive? In most cases, the question is easy to answer. Although the leaves around you have different shapes and sizes, a brief examination assures you that they are leaves, and the tree whose trunk you are exploring is clearly a tree—thus telling you that the tree specimen you are examining is indeed alive. But what about the gray material adhering to the trunk? Is it also alive?

Defining life might seem to be easy, but it is not. In fact, no single definition satisfies all life scientists. For example, if we say you can tell something is alive if it reproduces, someone is likely to note that a page with a wet ink spot can fall on top of another page and reproduce itself almost exactly. If we say you can tell something is alive if it grows, what should we conclude about crystals? They grow, but they are not alive. And so it goes.

·       Humans are a small, but important, part of the diverse life on Earth.

Environmental Issue

Medicinal Plants and the Shrinking Rain Forest

The healing powers of many plants have been known for centuries. Historically, such knowledge was gained by trial and error and passed along by word of mouth. For example, many cultures have long known that tea made from willow bark relieves pain and reduces fever. Scientists learned that willow bark contains salicylic acid. They isolated the compound and developed it into the drug we know today as aspirin. Similarly, digitalis, a heart medication, was discovered after a patient with an untreatable heart condition was seen to benefit from an herbal drink provided as a folk remedy. The potion contained purple foxglove, which, like willow bark, is frequently mentioned in ancient texts as a healing herb. Broccoli, a more familiar plant, contains the anticancer chemical sulforaphane.

More than 25% of the prescription medicines sold in the United States today contain chemicals that came from plants, and 70% of the newly developed drugs are from natural sources. Many more healing chemicals first discovered in plants used medicinally by native people are now routinely synthesized in laboratories. Unfortunately, scientists have not been able to synthesize the medicinal compounds for many plants.

Most plants that have proved to be medically useful are found in the tropics, regions where the human population is growing rapidly. Unfortunately, the forests in these regions are being cut to create living space and foster economic development. In Madagascar, home of the rosy periwinkle (Figure 1.A), which is the source of two anticancer drugs, humans have destroyed 90% of the vegetation. Experts estimate that roughly nine-tenths of the original tropical rain forests of the world will have been destroyed by 2030. Considering that 155,000 of the known 250,000 plant species are from tropical rain forests, and that less than 2% of the known plant species have been tested for medicinal value, we have no way of knowing what potential new medicines are being destroyed.

FIGURE 1.A. The rosy periwinkle (Catharanthus roseus) is a source of two anticancer drugs.

Questions to Consider

• Should indigenous people be compensated for plants found in their locality if extracts of the plants become drugs?

• What steps might be taken to preserve biodiversity within the rain forest?

No single definition applies to all forms of life, so we find that instead of defining life, we can only characterize it. That is, we can only list the traits associated with life. Most biologists agree that, in general, the following statements characterize life.

1. Living things contain nucleic acids, proteins, carbohydrates, and lipids. The same set of slightly more than 100 elements is present in various combinations in everything on Earth—living or nonliving. However, living things can combine certain of these elements to create molecules that are found in all living organisms. These include nucleic acids, proteins, carbohydrates, and lipids. The nucleic acid DNA (deoxyribonucleic acid) is especially important because DNA molecules can make copies of themselves, an ability that enables organisms to reproduce (Figure 1.1). The molecules of life are discussed in Chapter 2.

FIGURE 1.1. Characteristics of life

2. Living things are composed of cells. Cells are the smallest units of life. Some organisms (called unicellular organisms) have only a single cell; others (multicellular organisms), such as humans, are composed of trillions of cells. All cells come from preexisting cells. Because cells can divide to form new cells, reproduction, growth, and repair are possible. Cells are discussed in Chapter 3 and cell division in Chapter 19.

3. Living things grow and reproduce. Living things grow and ultimately generate new individuals that carry some of the genetic material of the parents. Some organisms, such as bacteria, reproduce simply by making new and virtually exact copies of themselves. Other organisms, including humans, reproduce by combining genetic material with another individual. Many organisms have stages of life. Humans progress from embryo to fetus, child, adolescent, and then adult. Reproduction and development are discussed in Chapters 17 and 18, respectively.

4. Living things use energy and raw materials. The term metabolism refers to all chemical reactions that occur within the cells of living things. Through metabolic activities, organisms extract energy from various nutrients and transform it to do many kinds of work. Metabolism maintains life and allows organisms to grow. Chemical reactions involved in the transformation of energy are discussed in Chapter 2.

5. Living things respond to their environment. A boxer weaves and ducks to avoid the blow of an opponent. A chameleon takes aim at and captures its prey. For a living thing to respond, it must first detect a stimulus and then have a way to react. As later chapters explain, your sensory organs detect stimuli. Your nervous system processes sensory input, and your skeletal and muscular systems enable you to respond. The skeletal and muscular systems are discussed in Chapters 5 and 6, respectively. The nervous system is discussed in Chapter 8, and sensory organs in Chapter 9.

6. Living things maintain homeostasis. Homeostasis is the relatively constant and self-correcting internal environment of a living organism. We generally find that life can exist only within certain limits and that living things tend to behave in ways that will keep their body systems functioning within those limits. For example, if you become too cold, you shiver (a metabolic response). Shivering produces heat that warms your body. Alternatively, if you become too hot, your sweat glands will be activated to cool you down. In addition to these and other physiological responses, the sensation of being hot or cold may motivate you to behave in ways that cool you down or warm you up. We discuss homeostasis in Chapter 4, where we make an initial survey of body systems.

7. Populations of living things evolve and have adaptive traits. Members of a population of reproducing organisms possessing beneficial genetic traits will survive and reproduce better than members of the population that lack these traits. As a result of this process, called natural selection, each of the amazing organisms you see around you has adaptive traits—that is, traits that help it survive and reproduce in its natural environment. For example, we see that most plants in the rain forest have shallow root systems, because the topsoil in the Amazon is thin and nutrients are near the surface. As a result, tall trees have acquired, through evolution, supports like cathedral buttresses to hold them up, while vines climb over both roots and trees to reach the light. Many plants do not grow in the ground at all but live high above it in the canopy for greater exposure to sunlight, which provides energy to produce sugar. These plants, called epiphytes, are rooted on the surfaces of other plants. Rain forest animals also have adaptations—the ability to fly or climb, for example—that enable them to reach the plants for food. Adaptive traits and evolution are discussed in Chapter 22.

Stop and think

Scientists have discovered water and methane on Mars. Water is necessary for life. Solar radiation would quickly destroy methane, so “something” must be producing the methane we detect. If samples of water or soil from Mars were brought back to scientists on Earth, what characteristics of life could they look for to determine whether the samples contain anything that is or was once alive?

What would you do?

Scientists have intentionally crashed a spacecraft onto the surface of the moon and observed a splash described as a significant amount of water. Water is essential for human life, so this opens the possibility of creating a “manned moon.” If it were possible for you to move to the moon, would you go? What factors would weigh in your decision?

Classification by Evolutionary Relationship

At least 10 million species of organisms live on Earth. Organisms are unified because all species descended from the first cells. However, as organisms adapted to different environments through evolution, diversity among species arose.

Scientists organize, or classify, living organisms in a way that shows evolutionary relationships among them. This means, for the most part, that organisms with the greatest similarity are grouped together.

Several classification systems have been proposed over the years. One system recently favored by many biologists recognizes three domains. Two of the domains, Bacteria and Archaea, consist of the various kinds of prokaryotes—all very small, single-celled organisms that lack a nucleus or other internal compartments. All other organisms, including humans, belong to the third domain, Eukarya. Organisms in domain Eukarya have eukaryotic cells, which contain a nucleus and complex internal compartments called organelles. Domain Eukarya is subdivided into four kingdoms—protists, fungi, plants, and animals—as shown in Figure 1.2. Within each kingdom, organisms are further categorized into groups whose members share characteristics that distinguish them from members of other groups in the kingdom. These groups in turn are subdivided into smaller groups to show successively closer relationships.

FIGURE 1.2. One classification scheme showing three domains and four kingdoms of life

As humans, we belong to a subdivision of the animal kingdom called vertebrates (animals with a nerve cord protected by a backbone), and more specifically to the group called mammals. Two characteristics that make us mammals are that we have hair and that we feed our young milk produced by mammary glands. However, we are further defined as belonging to the primates, along with lemurs, monkeys, and apes, because we share a suite of features that includes forward-looking eyes and a particularly well-developed brain. Humans, monkeys, and apes also have opposable thumbs (a thumb that can touch the tips of the other four fingers). Smaller details, such as tooth structure and skeletal characteristics, serve to divide the primates into smaller subgroupings.

What most sets humans apart from all other living species? Human characteristics include a large brain size relative to body size and a two-legged gait. But nothing distinguishes humans more than culture. Culture may be regarded as a set of social influences that produce an integrated pattern of knowledge, belief, and behavior (Figure 1.3). Other animals have social interactions, from various forms of cooperation and mating behavior to territoriality, competition, and social hierarchies. But social influences are much less pronounced in other animals than in corresponding human interactions. Consider, for example, our rituals—weddings, graduation, burial of the dead—and the way our lives are enriched with art, music, and dance.

FIGURE 1.3. Social interactions are an important thread in the fabric of human life.

Of course, there is not one human culture but many. If you were a scientist following native guides through the Amazon rain forest, you would quickly learn that their culture is different from ours. In fact, there are many different cultures within the rain forest. Separate groups of people in the same environment do not adapt to it in the same way. If you were to describe your culture to someone from a rain forest tribe, your description might elicit astonishment and even howls of laughter.

Stop and think

If a new organism were discovered in the rain forest, what characteristic would you look for to decide whether the animal was a mammal?

Levels of Biological Organization

As we study human biology, we learn that life can be organized on many levels (Figure 1.4). Cells, the smallest unit of life, are themselves composed of molecules. A multicellular organism may consist of different tissues, groups of similar cells that perform specific functions. Organs also may consist of different types of tissue that work together for a specific function. Two or more organs working together to perform specific functions form an organ system. Humans are described as having 11 organ systems, as we see in Chapter 4.

FIGURE 1.4. Levels of organization of life

Life can also be organized at levels beyond the individual organism. A population is individuals of the same species (individuals that can interbreed) living in a distinct geographic area. Examples of a population include yellow-bellied marmots living in an alpine meadow or four-eyed butterfly fish living in a coral reef. A community is all living species that can potentially interact in a particular geographic area. Examples of a community include all the species that live and interact in an alpine meadow or all the species living in a coral reef.

An ecosystem includes all the living organisms in a community along with their physical environment. The size of the locality that defines the ecosystem varies with the interest of the person studying it. In other words, an ecosystem can be defined as the whole Earth, a particular forest, or even a single rotting log within a forest. Whatever its size, an ecosystem is viewed as being relatively self-contained.

The biosphere is that part of Earth where life is found. It encompasses all of Earth's living organisms and their habitats. In essence, the biosphere is the narrow zone in which the interplay of light, minerals, water, and gases produces environments where life can exist on Earth. The biosphere extends only about 11 km (7 mi) above sea level and the same distance below, to the deepest trenches of the sea. If Earth were the size of a basketball, the biosphere would have the depth of about one coat of paint. In this thin layer covering one small planet, we find all of the life we currently know of in the entire universe.

Scientific Method

Humans are an irrepressibly curious species, constantly asking questions about the things they observe. Science is a systematic approach to answering those questions, a way of acquiring knowledge through carefully documented investigation and experimentation—the scientific method.

There is no such thing as the scientific method in the sense of a single, formalized set of steps to follow for doing an experiment. Instead, the scientific method is a way of learning about the natural world by applying certain rules of logic to the way information is gathered and conclusions are drawn (Figure 1.5). It often begins with an observation that raises a question. Next, a possible explanation is formulated to answer the question, but that explanation must be testable. Generally, the tentative explanation will lead to a prediction. If the prediction holds true when it is tested, the test results support the explanation. If the original explanation is not supported, an alternative explanation is generated and tested.

FIGURE 1.5. The scientific process consists of observation, creating testable hypotheses, experimentation, drawing conclusions, revising hypotheses, and designing new experiments.

How would you modify this diagram to indicate that testing alternative hypotheses is also part of the scientific process?

Add another arrow from “Observation" indicating Hypothesis 2. All the steps that are currently shown leading from “Hypothesis” would be repeated for Hypothesis 2.

1. Make careful observations and ask a question about the observations. The process of science usually begins with an observation that prompts a question. Questions should be reasonable and consistent with existing knowledge.

2. Develop a testable hypothesis (possible explanation) as a possible answer to your question. The next step is to make an educated guess about the answer to that question, called a hypothesis. The hypothesis should be a statement, not a question. It should be possible to test a hypothesis and to prove it false. Keep in mind that although a hypothesis can be shown to be false, it can never be proved to be true. You can collect data that supports a hypothesis, but you must also rule out other possible explanations (hypotheses).

Generally, the hypothesis leads to one or more predictions that will support the hypothesis if they hold true when tested. Depending on the hypothesis, the test may involve further observations or experimental manipulation.

Different hypotheses can sometimes lead to identical predictions and then both hypotheses are supported or refuted, depending on the outcome of the test. In this event, it is necessary to make other predictions that will allow us to reject one of the hypotheses. When we find that the results of various tests are more consistent with one hypothesis than with others, we must still be cautious. New evidence may come to light that will disprove the hypothesis or a new hypothesis may be proposed that is also consistent with the observations.

3. Make a prediction based on your hypothesis and test it with a controlled experiment. Now you make a prediction regarding what should occur if the hypothesis is correct. This prediction will determine the experiment or observations that are necessary to test the hypothesis.

Ideally, your experiment will be designed in such a way that there can be only one explanation for the results. In such an experiment, called a controlled experiment, the research subjects are randomly divided into two groups. One group is designated as the control group, and the other one is designated as the experimental group. Both groups are treated in the same way except for the one factor, called the variable, whose effect the experiment is designed to reveal.

In a scientific study, additional variables that have not been controlled for, and which may have affected the outcome, are called confounding variables. When there are confounding variables, we cannot say for sure which variable or variables caused the effect.

Let's see how the scientific method works. An advertisement on television proclaims that eating a daily bowl of oatmeal lowers blood cholesterol levels. Lower blood cholesterol is desirable because elevated cholesterol is related to atherosclerosis, a condition in which fatty deposits clog blood vessels. In turn, atherosclerosis increases one's risk of having a heart attack or stroke.

What observation led to this claim? Oatmeal contains the soluble fiber β-glucan. We begin to gather information about soluble fiber and learn the following:

• Soluble fiber binds to bile in the intestines, preventing bile from being reabsorbed into the body.

• Bile is high in cholesterol.

• Bile bound to soluble fiber is removed from the body in feces.

• The liver then removes cholesterol from the blood to synthesize new bile.

What experiment would support the claim that oatmeal lowers blood cholesterol? Scientists would first form a hypothesis. In this case, the hypothesis might be that β-glucan in oatmeal lowers blood cholesterol levels. They would then make a prediction that would hold true if the hypothesis is correct: If oatmeal consumption lowers blood cholesterol levels, then a person's cholesterol level will be lowered by eating a bowl of oatmeal a day for 6 weeks (Figure 1.6).

FIGURE 1.6. The design of an experiment to test the prediction that oatmeal lowers blood cholesterol levels

Researchers gathered volunteers who were between 30 and 65 years old and who had high levels of cholesterol. The volunteers were instructed to follow a low-fat diet for 8 weeks. The component of total cholesterol measured was the low-density lipoprotein (LDL) cholesterol carrier, because this "bad" form of cholesterol promotes atherosclerosis. Measurements were made every 3 weeks throughout the study.

Volunteers were randomly chosen for each group studied. Three experimental groups of volunteers consumed one, two, or three 1 oz packets of oatmeal per day. The control group ate a 1 oz packet of farina, a wheat cereal lacking β-glucan. After 6 weeks of treatment, none of the volunteers ate cereal during the following 6 weeks.

Stop and think

Why is randomly dividing the volunteers into groups a better experimental design than allowing the volunteers to choose their group?

4. Draw a conclusion based on the results of the experiment. Next, you arrive at a conclusion, which is an interpretation of the data. The results of a scientific experiment are often summarized in a graph, such as the one shown in Figure 1.7, which presents the results of the experiment we just described. When you read a graph, first look at the axes. The horizontal line, or x-axis, shows the independent variables—the variables altered by the researcher. In this case, the independent variable is the amount of soluble fiber consumed over time. The vertical line is the y-axis; it presents the dependent variable, that is, the variable that was changed by the independent variables. In this experiment, the dependent variable was the blood level of LDL cholesterol. Always read the labels on the axes to see what the graph pertains to, and notice the scale to appreciate the extent of variation. This is a line graph where each data point is indicated. The data points are then connected to show the trend of the results. Each treatment group is represented by a different color line. A line graph is appropriate here because the variable (blood level of LDL cholesterol) varies continuously over time. (A bar graph is appropriate when each treatment is a discrete category).

FIGURE 1.7. Cholesterol level in blood decreases with increased consumption of oatmeal.

From “The hypocholesterolemic effects of β-glucan in oatmeal and oat bran" in JOURNAL OF THE AMERICAN MEDICAL ASSOCIATION, 206:1833-1839,1991. Copyright © 1991 by the American Medical Association. All rights reserved. Used by permission.

Notice in Figure 1.7 that during the treatment phase of the experiment, blood levels of LDL cholesterol of all treatment groups decline. After treatment, blood levels of LDL cholesterol rise slightly but not to pretreatment levels. However, when a person eats farina, a cereal that doesn't contain β-glucan, LDL cholesterol levels remain fairly constant. Thus, a conclusion might be that eating oatmeal lowers blood LDL cholesterol.

Could these results be due to chance alone? Scientists base conclusions on the statistical significance of the data, which is a measure of the possibility that the results were due to chance. A probability of less than 5% (written as p < 0.05) that the results are due to chance is generally acceptable. The lower the number, the more confidence we have in the accuracy of the results. In this experiment, the differences in blood LDL cholesterol at the end of the treatments phase were statistically significant from the starting value, but the differences at the end of 12 weeks were not.

Stop and think

Given the level of statistical significance at the end of 6 weeks and the end of 12 weeks, how would you modify a conclusion?

Another requirement of scientific inquiry is that experiments must be repeated and yield similar results. Other scientists following the same procedure should obtain a similar outcome. Note, however, that it can be very difficult to identify all the factors that might affect the outcome of an experiment.

The testing and refinement of a hypothesis represents one level of the scientific process. As time passes, related hypotheses that have been confirmed repeatedly can be fit together to form a theory—a well-supported and wide-ranging explanation of some aspect of the physical universe. Because of its breadth, a theory cannot be tested by a single experiment but instead emerges from many observations, hypotheses, and experiments. Nevertheless, a theory, like a hypothesis, leads to additional predictions and continued experimentation. Among the few explanations that have been tested thoroughly enough to be considered theories are the cell theory of life (which says all cells come from preexisting cells) and the theory of evolution by natural selection (which you learn about in Chapter 22).

Inductive and Deductive Reasoning

Scientific investigation usually involves two types of reasoning: inductive reasoning and deductive reasoning.

In inductive reasoning, facts are accumulated through observation until the sheer weight of the evidence allows some logical general statement to be made. You use inductive reasoning to develop a testable hypothesis.

Deductive reasoning begins with a general statement that leads logically to one or more deductions, or conclusions. The process can usually be described as an "if-then" series of associations. We used deductive reasoning when we predicted that "If oatmeal consumption lowers blood cholesterol levels, then a person's cholesterol level will be lowered by eating a bowl of oatmeal a day for 6 weeks." This prediction helped us decide whether the results of the experiment supported or refuted the hypothesis.

Clinical Trials

Before testing a new drug or treatment on humans, scientists must take steps to ensure that it will not do more harm than good (Table 1.1). Usually a drug is tested first on animals such as laboratory rodents. Rats and mice are mammals, so some aspects of their physiology are similar to, and can be generalized to, human physiology. The advantages of using rodents for testing drugs include that they are relatively inexpensive to use, have short life spans, and reproduce quickly. Research on animals also helps determine how the drug is handled by the body, which helps determine dosage. Most medical advances, including vaccinations, chemotherapy, new surgical techniques, and organ transplants, began with animal studies. Strict rules safeguard the care and use of animals in research and testing.

If no ill effects are discovered in animals receiving the drug, then studies on humans, called clinical trials, may begin. In all phases of clinical testing, the studies are done on people who volunteer. In phase I of a clinical trial, the drug is screened for safety on fewer than 100 healthy people. At this stage, researchers hope to learn whether they can safely give the drug to humans, determine the effective dosage range, and identify side effects.

If the drug is found to be safe, it is tested further. In phase II of a clinical trial, a few hundred people with the target disease are given the drug to see if it works for its intended purpose. If it does, the new drug will be compared with alternative treatments in phase III trials. Thousands of participants are involved in phase III of a clinical trial. The Food and Drug Administration (FDA) approves only those drugs or treatments that have passed all three phases of human-subjects testing.

TABLE 1.1. Tests Performed on a New Drug before It Is Approved by the Food and Drug Administration (FDA)

Tests on laboratory animals

What would you do?

The job of the FDA is to ensure the safety and effectiveness of new drugs and treatments. It must balance the patients' desires for access to new treatments against the government's desire to protect patients from treatments that may be unsafe or ineffective. The drug approval process is painstakingly slow, usually taking more than 8 years. Do you think that the FDA should bypass certain steps of the approval process to make new drugs available to critically ill patients who may not be able to wait? If you do, what criteria should be used to decide the degree of illness that would warrant treatment with a drug that was not yet approved? Who should be held responsible if early access to a drug of unknown safety causes a patient to suffer serious side effects or premature death? If you were ill and there was a drug for your illness in clinical trials, would you participate in those trials? What factors would influence your decision?

Recall that a well-designed experiment has both an experimental group and a control group. Clinical studies are no different. In a drug trial, the experimental group receives the drug under consideration. The control group receives a placebo, an innocuous, nondrug substance made to look like the drug being tested. Study participants are randomly assigned to either the control group or the experimental group and do not know whether they are receiving the treatment or a placebo. When neither the researchers nor the study participants know which people are receiving treatment and which are receiving the placebo, the study is described as being double-blind. It is important that participants do not know whether they are receiving the placebo or the drug, because their expectations about the drug could affect the way they respond. Similarly, researchers should not know which people are in the experimental or control groups, because their expectations or desire for a particular result could affect their interpretation of the data.

Finally, it is extremely important, and legally required, that study participants give their informed consent before participating. An informed consent document lists all the possible harmful effects of the drug or treatment and must be signed before a person can take part in the study. To give informed consent, study participants must be mentally capable of understanding the treatment and risks, so they cannot be mentally impaired due to mental retardation, mental or other illness, or substance abuse.

Epidemiological Studies

Human health can also be studied without clinical trials. In an epidemiological study, researchers look at patterns that occur within large populations. For example, an epidemiological study to investigate the effects of air pollution on asthma (a condition in which airway constriction causes breathing problems) would look for a correlation of some kind between the variable of interest (air pollution) and its suspected effects (worsening of asthma). If the researchers' hypothesis is that air pollution aggravates asthma, they might predict and then look for evidence that the number of people admitted to hospitals for asthma-related problems increases with increased levels of air pollution.

Recent epidemiological studies have asked the question, "Does using a cell phone increase your risk of developing brain cancer?" Cell phones emit radiofrequency waves and are usually used by holding them to one's ear. In 2010, the World Health Organization released the results of the largest study to date exploring a possible link between cell phone use and brain cancer. Researchers tracked nearly 13,000 cell phone users from 13 countries over 10 years. A comparison of brain cancer rates of all people who used cell phones and all people who never used a cell phone did not show a difference in brain tumor rates. However, when the comparison was between the heaviest cell phone users and all others, there was a slight increase of brain cancer among cell phone users. Thus, this study does not conclusively demonstrate a link between cell phone use and cancer. Indeed, nearly all studies to date have failed to show such a link. Although these results are reassuring, additional research must be done. Cell phone use is a relatively new practice, and many cancers take years to develop. You can find current information on cell phones and cancer on the website of the American Cancer Society.

Critical Thinking to Evaluate Scientific Claims

Few of us perform controlled experiments in our everyday lives, but all of us must evaluate the likely validity of scientific claims. We encounter them in many forms—as advertisements, news stories, and anecdotes told by friends. We often must make decisions based on these claims, but how can we decide whether they are valid? Critical-thinking skills can help us analyze the information and make prudent decisions.

The key to becoming a critical thinker is to ask questions. The following list is not exhaustive, but it may help guide your thinking process.

1. Is the information consistent with information from other sources? The best way to answer this question is to gather as much information as possible from a variety of sources. Do not passively accept a report as true. Do some research.

2. How reliable is the source of the information? Investigate the source of the information to determine whether that person or group has the necessary scientific expertise. Is there any reason to think the claim may be biased? Who stands to gain if you accept it as true? For example, the Food and Drug Administration (FDA) is probably a more reliable source of information on the effectiveness of a drug than is the drug company marketing the drug. If a claim is controversial, listen to both sides of the debate and be aware of who is arguing on each side.

3. Was the information obtained through proper scientific procedures? Information gathered through controlled experiments is more reliable than anecdotal evidence, which cannot be verified. For example, your friend might tell you that his muscles have gotten larger since he started using some special exercise equipment. But you cannot be sure unless measurements were taken before and after he began to use it. Even if your friend can prove his muscles are bigger with such measurements, there is no guarantee that exercising with this equipment will build your muscles.

4. Were experimental results interpreted correctly? Consider, for example, a headline advertising capsules containing fish oils: "Fish Oils Increase Longevity." It may be tempting to conclude from this headline that you will live longer if you take fish oil supplements, but in fact the headline is referring to an experiment in which dietary fish oil increased longevity in rats. Rats fed a diet high in fish oils lived longer than did rats on a diet low in fish oil. The claim that taking fish oil supplements will increase longevity is not a valid conclusion based on the experiment. First, the study was done on rats, not on humans. Not all aspects of rat physiology generalize to humans.

For example, rats are more resistant to heart disease than are humans. Second, the amount of dietary fish oil, not the amount of fish oil from capsules that supplement dietary fish oil, was the variable in the study the headline refers to. Supplements of fish oil may not have the same effect as dietary fish oils. It could be that taking fish oil supplements would boost the amount of fish oil in your body to unhealthy levels.

5. Are there other possible explanations for the results? Suppose you learn that the fish oil headline is based on a study showing that people who eat fish at least three times a week live longer than those who eat fish less frequently. In this case, the data indicate that there is a correlation between fish in the diet and length of life. However, a correlation between two factors does not prove that one caused the other. Instead, the two factors may both be caused by a third factor. In this case, the difference in longevity may be due to other differences in the lifestyles of the two groups. For instance, people who eat fish may exercise more frequently or have less stressful jobs or live in areas with less pollution.

Throughout your life you will be asked to make many decisions about scientific issues. Some will affect your community and even beyond. For example, should we eliminate genetically modified food? Should stem cell research be permitted? Should companies polluting the atmosphere be taxed at a higher rate? We will raise these and similar questions throughout this textbook. You will find others every day in the local and national news media.

Although you may never be one of the lawmakers deciding these issues, you are a voter who can help choose the lawmakers and voice your opinions to the lawmakers, who will decide. Scientists can provide facts that may be useful as we all struggle to answer the complex questions facing society, but they cannot provide simple answers. As scientific knowledge grows and our choices become increasingly complex, each of us must stay informed and review the issues critically.

Looking ahead

In this chapter, we considered the characteristics of life. In the | next chapter, we will explore the chemistry of life.

Highlighting the Concepts

Basic Characteristics of All Living Things (pp. 1-4)

• Life cannot be defined, only characterized.

• Living things contain nucleic acids, proteins, carbohydrates, and lipids; are made of cells; grow and reproduce; metabolize; detect and respond to stimuli; and maintain homeostasis. Populations of living things evolve over long periods of time.

Classification by Evolutionary Relationship (pp. 4-5)

• Classifications of living organisms reflect the evolutionary relationships among them. One currently popular classification system recognizes three domains: Bacteria, Archaea, and Eukarya. Domain Eukarya consists of four kingdoms: protists, fungi, plants, and animals.

• Humans are classified as animals, vertebrates, mammals, and primates.

• Humans share traits with other species, but unlike most other animals, they also have many unique features, among the most important of which is culture.

Levels of Biological Organization (pp. 5-6)

• Human biology can be studied at different levels. Within an individual, the levels of increasing complexity are molecules, cells, tissues, organs, and organ systems.

• Beyond the level of the individual are populations, communities, ecosystems, and the biosphere.

Scientific Method (pp. 6-10)

• The scientific method consists of observation, formulation of a question and a good hypothesis (a testable, possible explanation), experimentation (performed with controls), and drawing a conclusion, which may lead to further experimentation.

• As evidence mounts in support of related hypotheses, the hypotheses may be organized into a theory, which is a well-supported explanation of nature.

• Inductive reasoning uses a large number of specific observations to arrive at a general conclusion. Deductive reasoning, in contrast, uses "if-then" logic to progress from the general to the specific.

• There are strict rules concerning experiments on humans and other animals. Drugs are usually tested on animals before they are tested on humans. If no ill effects in animals are observed, the drug is then tested on humans. Phase I trials determine whether the drug is safe for humans, phase II determines whether it works for its intended purpose, and phase III determines whether it is more effective than existing treatments.

• The design of a human experiment often includes an experimental group that receives the treatment and a control group that receives a placebo. In what is known as a double-blind experiment, neither the study participants nor the researchers know who is receiving the real treatment.

• Study participants must sign an informed consent document indicating that they were made aware of the possible harmful consequences of the treatment.

• Epidemiological studies examine patterns within populations to find a correlation between a variable and its suspected effects.

Critical Thinking to Evaluate Scientific Claims (pp. 10-11)

• Critical thinking consists of asking questions, gathering information, and evaluating evidence and its source carefully before drawing conclusions.

Reviewing the Concepts

1. List seven traits that characterize life. p. 2

2. Name two characteristics that classify humans as mammals. Name two that classify them as primates. p. 4

3. Distinguish between a population, a community, an ecosystem, and the biosphere. p. 5

4. What is a hypothesis? How does it differ from a theory? pp. 6-9

5. Define a controlled experiment. p. 7

6. Differentiate between inductive and deductive reasoning. p. 9

7. What is a placebo? p. 10

8. What is meant by a double-blind experiment? p. 10

9. Describe the procedure in an epidemiological study. p. 10

10. A theory is

a. a testable explanation for an observation.

b. a conclusion based on the results of an experiment.

c. a wide-ranging explanation for natural events that has been extensively tested over time.

d. the factor that is altered in a controlled experiment.

11. The maintenance of physical and chemical conditions inside the body within tolerable ranges is called ____.

12. A(n) _____ is a testable explanation for an observation.

13. A trait that increases the chance that an organism will survive and reproduce in its natural environment is described as being _____.

Applying the Concepts

1. Interpret the following graph to answer the questions.

a. What percentage of these 7-year-old children have access to a cell phone?

b. At a young age, these children share a cell phone with their family. At what age does sharing begin to decline?

c. What percentage of 10-year-old children own their own cell phone?

2. A native you met in the rain forest told you that one of the plants you collected brings relief to people who are having difficulty breathing. You suspect that it might be a good treatment for asthma, a condition in which constriction of airways causes breathing problems. You are able to isolate a component of this plant as a drug. Tests on animals show that it is effective. Phase I and phase II of clinical trials on humans show that the drug can be given safely to humans. Design an experiment to test the hypothesis that this drug eases breathing during an asthma attack.

3. Find an article or advertisement that makes a scientific claim. Use your critical-thinking skills to evaluate the claim.

The percentage of Swedish children questioned who answered yes to three questions about cell phone use.

Fredrik Soderqvist and others, “Ownership and Use of Wireless Telephones: A Population-Based Study of Swedish Children Aged 7-14 Years," BMC Public Health 7 (2007): 105-13 (fig. 1, p. 107).

Becoming Information Literate

Humans have an impact on the world around us at every level of the organization of life: molecule, cell, tissue, organ systems, individual, population, community, ecosystem, and biosphere. Use reliable sources of information (books, newspapers, magazines, journals, reliable websites) to identify at least one current issue or concern to humans at each level of life's organization. Provide a citation for each source you use.

Begin by planning a strategy for your search. Keep a log of each source you use, and evaluate it for reliability and helpfulness.