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LIFE 2010–2011 Edition John H. Postlethwait Janet L. Hopson

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CHAPTERC H A P T E R

What Is Life?

Learning Outcomes LO1 Recognize the characteristics of life

LO2 Identify the characteristics of life relating to energy use

LO3 Identify the characteristics of life relating to reproduction

LO4 Identify the characteristics of life relating to evolution

LO5 Identify the characteristics of life relating to environment

LO6 Explain how biologists study life

LO7 Understand how biologists can help solve problems

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Full Circle from Earth to Mars A huge headline in Time magazine screamed unambiguously: “LIFE ON MARS.” In August 1996, NASA scientists created an international media frenzy by claiming that a 4-pound, potato-shaped rock contained the remains of Martian life.

The blackened lump in question is a meteorite from Mars that entered Earth’s atmo- sphere as a brilliant shooting star about 13,000 years ago during our planet’s Ice Age. Prehistoric hunters and farm- ers probably watched that arrival blaze across the night sky. But the stone fell anony- mously to the frozen surface of Antarctica and lay undisturbed for 13 millennia until scientists discovered it in 1984. Twelve years later, a team from Johnson Space Center in Houston published evidence that the charred chunk contains microscopic wormlike structures formed nearly 4 billion years ago (see Fig. 1.1). This evidence, they claimed, was compatible with life on Mars. Within months, many scientists had rebutted the claim: Nothing but mineral structures, they said.

Debate still simmers, even years later. But an answer may be on its way. NASA began a series of Mars missions in 1998 that will retrieve Martian soil and rocks and return them to Earth for study. And recent photographic data suggest that Mars may have liquid water below its surface capable of sustaining life. Once NASA has retrieved the sample of Martian soil, biologists will immediately begin searching it for signs that organisms lived—or still live—on our neighboring red planet. Proof of life on Mars could be one of the most exciting discoveries in the history of life science.

If there was ever a time in our history to understand the central enigma beneath the search for extraterrestrial organisms, it is now and it is this: What is life? This chapter begins our multimedia exploration of biology’s central puzzle. As you read along, you’ll learn why it’s so important for each of us to understand life, and how biologists study every aspect of living organisms, their environ- ments, and their interrelationships. And you’ll fi nd the answer to these questions:

What characteristics do all living things share?

Which characteristics relate to gathering and using energy?

Which characteristics relate to reproduction?

Which characteristics relate to evolving and adapting?

Which characteristics relate to the physical environment?

How do biologists study life and how will we explore it in this course?

How can the study of life help us solve societal and environmental problems?

CHAPTER 1 What Is Life? 3

“After centuries of study and thought, biologists have come to a consensus about when an entity is alive, formerly alive, or nonliving.”

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What do you know? A car uses energy, is highly organized, and is motile . . . but it’s not “alive.” Why not?

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Characteristics of Living Organisms The search for life on Mars and other planets requires that we recognize life when we see it to identify the general properties of life on our planet, and then speculate about the similarities and differences scientists might encounter while searching on Mars and elsewhere in the solar system and universe. After centuries of study and thought, biologists have come to a consensus about when an entity is alive, formerly alive, or nonliving.

The problem of recognizing life is clearly funda- mental to the search for alien organisms. But it has other important applications, too. Today’s biologists and physicians have unprecedented abilities. These include sustaining the human body and individual organs on life-support machines, freezing human and animal embryos for later use, and changing and merging hereditary traits of microbes, plants, and animals. Perhaps one day this list will extend to generating life in a test tube and to creating hybrids between computers and living things. To manipulate life’s most fundamental properties, biological engi- neers need to know exactly what the boundaries are, how far they can be stretched, and what changes would be desirable, practical, and worth pursuing.

At the same time, the public needs to be fully aware of the benefi ts and risks of manipulating life so they can be informed watchdogs and consum- ers of these biotechnologies. In the broadest sense, every course in biological science and every experi-

ment, no matter how simple or complex, probes the question “What is life?” It takes this entire course of study to provide an answer. The citizen who learns about biology—including the college student major- ing in some other fi eld like business, physical educa- tion, psychology, or English—also discovers a realm of intricacy and beauty that helps them understand their environment, their health, their day-to-day functioning, their children’s growth and develop- ment, and the issues they see in the news, including cancer treatments, impotence drugs, habitat destruc- tion, species loss, and global climate change.

What, then, is life? You may be surprised to learn that no one, not basic biologist, bioengineer, or plan- etary protector, has a thumbnail defi nition that lays out the essence of the living state. Instead, they focus on the characteristics of life as a collective, descrip- tive defi nition.

Life’s Characteristics Think for a minute about puppies, roses, dinosaur bones, and motorcycles. Why those things? Puppies are clearly alive, frolicking, rolling around, and beg- ging for dog biscuits, as directed by their highly orga- nized brains. A rose is obviously alive, too, although it hardly moves and never devours dog treats. Still, a rose plant soaks up sunshine and soil nutrients, and it makes beautiful fl owers with seeds that can produce a new generation of roses. A dinosaur bone is cer- tainly not alive, but its close appearance to the bones of living animals confi rms that it was once alive. And the motorcycle? It’s highly organized and requires energy from the environment. It moves under its own power and responds by going faster when the throttle is turned up. It must be alive, too, right? Of course not. The last time we checked, motorcycles couldn’t reproduce, at least not without the help of an assembly line and some good mechanics.

Comparisons like these underlie the list of char- acteristics that living things share (Table 1.1). Living systems have internal order, or a high degree of orga- nization. To maintain that order, living things carry out metabolism: they use energy to transform and organize materials. Living things also use energy to move under their own power, a trait biologists call motility. They also use energy to react to outside stimuli, a trait called responsiveness.

While living things do all of the above, they also do more. Living things have the ability for self- replication or reproduction. They show growth and development or the expansion of young organisms in size and complexity. Living things are related by heredity; that is, organisms give rise to like organ- isms (not dinosaurs from roses or roses from pup- pies). Finally, living things evolve or change over

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Figure 1.1 Fossilized Martian Bacterium?

4 CHAPTER 1 What Is Life?

Could

this be a fossilized Martian?

CHAPTER 1 What Is Life? 5

many generations, and they adapt or change to bet- ter fi t shifting environments.

We can look around the nonliving world and see many of life’s characteristics in action: Waves move, fl ames use energy, crystals grow. Only living organ- isms, however, display all the characteristics we just discussed at some point during their individual life cycle or species history. For example, rose petals move as the rosebud unfolds; the rose plant captures and uses energy from the sun; the plant originally emerged from a seed, then grew, and developed fl ow- ers. Rose bushes can evolve and adapt to changing climates. All in all, they’re alive.

Themes That Recur as We Explore Life Some of the life characteristics we just listed occur again and again as themes that guide our exploration of biology. Living things use energy at several levels of biological organization: in the smallest cells, in indi- vidual organisms, and in large groups of organisms called biological communities. The members of a spe- cies reproduce and adapt to the environment over generations by means of evolution, and these themes help explain why organisms act as they do. A fun part of biology is learning how people have explored and discovered exactly what organisms do. These discov- eries almost always use a special system of investi- gation called the scientifi c method. We’ll see the fi ve themes of energy, reproduction, environment, evolu- tion, and the scientifi c method interwoven through- out all of our discussions, and these themes will help to organize the sprawling subject at hand, biology.

Life Characteristics Relating to Energy Once NASA scientists suspected that the potato- shaped meteorite found in Antarctica was actu- ally from Mars, they started studying slices of it, looking for hints of life on Mars and perhaps new answers to the question “What is life?” They fi rst sought to confi rm that the meteorite indeed orig- inated on Mars. They did this by showing that tiny bubbles trapped inside the rock contain air with the same chemical composition as the atmosphere of Mars—a mix that was measured directly by the Viking mission in 1976 and is distinctly different from Earth’s. The mixture of chemicals in the rock itself suggested that the meteorite probably formed 4.5 billion years ago, shortly after Mars solidifi ed as a planet. So how did a rock that old from Mars get here, only to be dis- covered on the Antarctic ice cap in 1984? NASA geolo- gists and astronomers surmise that a huge asteroid slammed into the Martian surface about 16 million years ago, blasting dirt and rocks high enough into the atmosphere that some escaped. This material orbited the sun independently for millions of years, and some of it eventually got tugged fi rmly enough by Earth’s gravity to streak into our atmosphere as fi ery mete- orites that fell to the surface. The NASA researchers were excited by the prospect of looking for signs of life in the Martian rock. But what approach would they take to detecting the unmistakable signatures of liv- ing things in this traveling chunk of Mars?

Order First, they could look for order or struc- tural and behavioral complexity and regularity, because living things possess a degree of order far greater than that of the fi nest Swiss clock, the fastest rac- ing car, or anything else in the nonliving world. The eye of a fl y, for example, and the spiral-packed seeds of a sunfl ower head both consist of highly organized units repeated and arranged in precise geometric arrays. We know that order is important because disorder quickly leads to death in a living thing: most weapons of murder, in fact—clubs, knives, guns, and poisons—will disor- ganize you beyond repair.

Knowing that order is a hallmark of life, NASA investigators searched

order a precise arrangement of structural units and activities; also, in taxon- omy, a taxonomic group comprising members of similar families

LO2

LIFE CHARACTERISTIC PROPERTY

1. Order Each structure or activity lies in a specifi c relationship to all other structures and activities.

2. Metabolism Organized chemical steps break down and build up molecules, making energy available or building needed parts.

3. Motility Using their own power, organisms move themselves or their body parts.

4. Responsiveness Organisms perceive the environment and react to it. 5. Reproduction Organisms give rise to others of the same type. 6. Development Ordered sequences of progressive changes result in an

individual acquiring increased complexity.

7. Heredity Organisms have units of inheritance called genes that are passed from parent to offspring and control physical, chemi-

cal, and behavioral traits.

8. Evolution Populations of organisms change over time, acquiring new ways to survive, to obtain and use energy, and to reproduce.

9. Adaptations Specifi c structures, behaviors, and abilities suit life-forms to their environment.

Table 1.1 Characteristics of Life

their slices of the Mars rock for orga- nized structures and found tiny, regular tubes (see Fig. 1.1). The researchers became convinced that these forms are “microfossils,” the small, preserved bodies of ancient organisms, and pub- lished a scientifi c paper claiming so. Many other biolo- gists, however, disagree that the so-called micro- fossils are evidence of past or present Martian life because they are smaller than the smallest known organisms on Earth. Scientists have concluded that life as we know it could not survive in a package any smaller than a sphere 200 nm (200 billionths of a meter) in diameter. The tubules in the Mars meteor- ite, however, were half that long and one-tenth that wide. For this reason, many think they must be sim- ply mineral formations. The tubules indeed looked highly ordered, but that alone doesn’t confi rm that they were once alive.

A Hierarchy of Order

Martian organisms, if they ever existed, are a near- total mystery, including the degree of order they might possess. But organisms on Earth have an order that is readily apparent at several levels. Biologists defi ne an organism as an independent individual possessing the characteristics of life. The elephant in Figure 1.2 is an individual organism. Each organ- ism, in turn, is made up of organ systems, groups of body parts arranged so that together they carry out a particular function within the organism. The skeletal system, for example, supports an elephant’s body.

Organ systems are made up of organs, sets of two or more tissues that together perform specialized

6 CHAPTER 1 What Is Life?

Organ System: A group of body parts that carries out a particular function in an organism

Organism: An individual, independent living entity

Organ: A structure consisting of two or more tissues that performs specialized functions within an organism

Tissue: A group of similar cells that carries out a particular func- tion in an organism

Cell: The simplest entity that has all the properties of life

Organelle: A structure within a cell that performs a specific function Molecule:

A cluster of atoms held together by chemical bonds

Figure 1.2 Order Reigns at Every Level in the Living World

organism an individual that can independently carry out all life functions

organ system a group of organs that carries out a particular function in an organism

organ a body structure com- posed of two or more tissues that together perform a specifi c function

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mission found no surface water on Mars. But later mis- sions did document geologi- cal features (such as deep channels, fl ood plains, wave patterns in sand, and ero- sion in sedimentary rocks) consistent with standing and fl owing water in the past. Recent fi ndings even suggest that water may have fl owed within the past few thou- sand years. These discoveries have encouraged biologists to believe that life may once have fl ourished on Mars and that remnant popula- tions may still survive today in areas that harbor small amounts of liquid water.

Motility There was a NASA joke that while the Viking lander was focusing in on chemical evi- dence for microbes in the Martian soil, it might miss big- ger evidence like footprints or little green aliens walking by. Self-propelled movement, or motility, would certainly have been as good an indica- tor of life on Mars as it is here. Even organisms as simple as bacteria can move on their own. Plants, which cannot move from place to place, do show various subtle movements based on growth. For example, the little organelles that cap- ture sunlight in plant cells are in constant motion. The fl owers of some plants open in the morning, trace the sun’s arc through the sky, then close at night. Animals, of course, have elevated movement to an art form in their pursuit of food, displays of dominance, and escape from enemies.

Responsiveness If you poke a sea slug, it withdraws. If you turn a houseplant around, its leaves move imperceptibly until, in a day or two, they’re once again oriented toward a window. Organisms are responsive: they respond to changes in their environment involv- ing temperature, food, water, enemies, mates, or other elements. The reaction to the change can be

functions for the organ system. An example is a sin- gle bone that supports part of an elephant’s leg. Each organ is made up of tissues, groups of similar cells that carry out the function of the organ. For example, bone tissue—made up of several kinds of cells func- tioning collectively—provides physical support to the elephant’s leg. Tissues are made up of cells, the simplest entities that have all the properties of life. Cells contain within them small structures known as organelles, which perform the functions necessary for the life of the cell. Finally, organelles consist of biological molecules, the building blocks of all biolog- ical structure and activity. The tubules in the Martian meteorite yielded no visual evidence of organelles. Researchers, however, did fi nd some subunits of bio- logical molecules that on Earth can be associated with organelles. No one is sure, of course, whether the biological molecules were contaminants from Earth organisms or arrived in the meteorite itself.

Metabolism Scientists looking at Mars rocks could—and did— look for evidence of energy use. Living things main- tain order in their organelles, cells, and organs through metabolism: they take energy from the environment and use it, along with materials, in a series of consecutive chemical steps, for repair, maintenance, and growth. By taking energy and materials from the environment and using them for repair, growth, and other survival processes, metabolism helps to combat the disorganization that occurs with time. If you scrape your knee in a fall, for example, metabolism in your cells helps to repair the damage and to generate new, healthy skin, nerves, and blood vessels.

In 1976, NASA sent a lander called Viking to Mars to photograph the planet’s surface and test a scoop of Martian soil. The highlights of that mis- sion were several experiments conducted remotely and designed to detect something in the soil— something alive, perhaps—that could use energy to metabolize (to transform and organize materials) as living things do on Earth. The Viking mission created quite a stir because the “right” byproducts (includ- ing carbon dioxide, CO2) were released during the experiments—as if life were present and actively metabolizing. However, the ways the products were generated cast doubt on a biological origin. Most biologists are now convinced that the results from the 1976 Mars experiments were due solely to non- living soil chemistry and not to metabolism by liv- ing cells.

The chemical reactions of metabolism require water, for reasons we’ll see in Chapter 3. The Viking

tissue a group of cells of the same type performing the same function within the body

cell the basic unit of life; cells are bounded by a lipid-containing mem- brane and are generally capable of independent reproduction

organelle a complex cytoplas- mic structure with a characteristic shape that performs one or more specialized functions

biological molecules molecules derived from living systems; the four major types are carbohy- drates, lipids, proteins, and nucleic acids

metabolism (Gr. metabole, to change) the series of chemical reactions by which cells acquire and use energy and that contribute to repair, growth, and other survival processes

motility the self-propelled move- ment of an individual or its parts

responsiveness the tendency of a living thing to sense and react to its surroundings

istic of reproduction, and two related ones, develop- ment and heredity.

Reproduction Organisms give rise to others of the same kind— roses to roses, robins to robins—by means of a defi n- ing life process, reproduction, or the means by which individuals give rise to other individuals of the same type. In asexual reproduction, a single parent pro- duces offspring identical to it and each other. One- celled microbes, for example, reproduce asexually by splitting into two identical daughter cells (Fig. 1.3a). Most complex organisms reproduce by sexual repro- duction, with genetic information coming from two parents and combining in offspring that are very similar but not identical to the parent or each other. Organisms sometimes go to great lengths for sexual reproduction to occur. A visiting bee must pry open a snapdragon fl ower’s petals to feed from the nectar within (Fig. 1.3b). In doing so it will pick up pollen (sex cells) from the fl ower; some is visible as a fi ne dust on its abdomen. If the bee visits another fl ower of the same type, it can then deliver the pollen to the other fl ower’s sex organ, facilitating the combining of genes and the fl ower’s reproduction.

Development Young organisms usually start out smaller and sim- pler in form than their parents. The offspring then grow in size and increase in complexity, a process

instantaneous: A moth hears the high-pitched whine of a swooping bat and zigzags away on a midnight breeze, and a Venus fl ytrap snaps shut on a tiny, unsuspect- ing frog. The response can be gradual, as well. A trumpeter swan detects the shortening days of autumn and responds by feeding more heavily and then migrating south. Or, a daffodil reacts to the length- ening days of spring by form- ing fl owers.

By metabolizing, moving, and responding, organisms obtain energy and mate- rials from the environment and use them to maintain order in their bodies. Nevertheless, aging inevitably sets in—whether in hours or days for a microbe, a century for a tortoise, or a millennium for a bristle- cone pine. An aged organism can no longer stem the resulting disorganization and death. But life contin- ues to exist because organisms reproduce.

Life Characteristics Relating to Reproduction Like begets like—that’s a central feature of life. If Martian organisms do exist, and if NASA brings them back to Earth, could the aliens reproduce here and multiply out of control? If so, how might we recognize and stop the process? Let’s look at the life character-

reproduction the method by which individuals give rise to other individuals of the same type

asexual reproduction a type of reproduction in which new individu- als arise directly from only one parent

sexual reproduction a type of reproduction in which new individu- als arise from the mat- ing of two parents

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8 CHAPTER 1 What Is Life?

Figure 1.3 Organisms Reproduce

(a) Asexual reproduction (b) Sexual reproduction

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would the same be true for Martian life? NASA will be looking for any minute evi- dence of fossils or changes in cell-like structures in the Mar- tian soil and rock samples.

Life Changes over Time Over time, life forms change. Biologists call this descent with modifi cation evolution, and it is based on changes in the frequencies of genes within populations over time. In part, we can tell that life evolves from our analysis of the fossilized imprints of early organisms. The older a fossil, the less similar it is likely to be to present-day forms. This dissimilarity is good evidence not only of change but also of continued change in liv- ing species. Using fossils, DNA analysis, and other

evidence of changes in gene frequencies, biolo- gists can trace an organism’s family tree. For

example, modern day house cats and tigers (young branches on the feline tree) are closely related. Reaching farther back in time, cats and dogs would share a com- mon ancestor, and going back to life’s early history, cats would share a com- mon ancestor with snakes, fi sh, beetles, mushrooms, trees, and bacteria. Tracing

the evolutionary tree back to life’s fi rst beginnings on Earth, all organisms would

eventually share a common ancestor in the distant past.

Classifi cation of Living Things

T he process of evolution and the tracing of family lineages back in time can

help explain the immense diver- sity of life, which some biologists estimate at upwards of 50 million

species. To help make sense of this vast diversity, biologists have created a system for categorizing organisms into groups accord- ing to their similarities. Brightly colored tropical frogs, for example, are more similar to bullfrogs than to elephants, but frogs and elephants are more similar to each other than to mushrooms or

development the process by which an offspring increases in size and complexity from a zygote to an adult

heredity the transmission of genetic characters from parents to offspring

gene (Gr. genos, birth or race) the biological unit of inheritance that transmits hereditary information from parent to offspring and controls the appearance of a physical, behavioral, or biochemical trait; a gene is a specifi c discrete portion of the DNA mol- ecule in a chromosome that encodes an rRNA molecule

evolution changes in gene frequen- cies in a population over time

known as development. Eventually, the organism may reach sexual maturity and become a parent itself.

Heredity One of the most intriguing questions in all of biol- ogy is how a fertilized egg develops into the millions of cells of various types that function as a viable organism. The answer lies in the remarkable process of heredity, the transmission of genetic characters from parents to offspring.

Do you know any sets of identical twins? Twins are proof that some type of hereditary information directs each individual’s development with such amazing precision—so much that two separate organisms can go through all the steps of growth and increas- ing complexity over years of time and still wind up looking virtually alike. Contrast identical twins with their different-looking brothers and sisters, however, and you can see that hereditary information must also contain variations so that the offspring in one family can have similar noses but different heights, or similar eyes but different hair color. Biologists have identifi ed the units of inheritance that control an organism’s traits and call them genes.

Genes, made of a remarkable molecule called DNA (deoxyribonucleic acid), determine such things as whether a person’s hair is red, black, brown, blond, or gray. As we will see, genes also direct the day-to-day metabolic activities within cells. If Martian organisms exist, would they have DNA? DNA is so crucial to Earthly life that some of the upcom- ing NASA tests of the Mars sample will look for evidence of DNA and similar molecules.

Life Characteristics Relating to Evolution Four and a half billion years ago, Earth and Mars were newly formed planets and these neighbors—the third and fourth “rocks” from the sun—were probably very similar. The radius of Mars is only about half that of Earth, but the two planets have similar compositions and both probably had stable water near the planet surfaces for much of their history. Biologists think that if life did arise on Mars, it was probably similar to early life here. Fossil evidence tells us that life on our planet has changed or evolved over the millennia. So

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CHAPTER 1 What Is Life? 9

Frogs and elephants are

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species is called “binomial nomenclature.” For exam- ple, the house cat Felis catus is related to but clearly distinct from the ocelot in the same genus Felis but the different species pardalis. (Together, then, the offi cial name is Felis pardalis.) A lion, which is obvi- ously still a cat but quite different in size, color- ation, and habits from house cats and ocelots, is in a different genus, Panthera, and its species name is Pan- thera leo. After once mentioning the complete two- part name, biologists often abbreviate the genus, referring to F. catus or P. leo. You’ve probably seen the term E. coli in newspaper articles about outbreaks of food poisoning. In this case, the “E.” stands for the unwieldy bacterial genus name Escherischia.

Just as biologists group related species into genera, they also group similar genera into families, similar families into orders, similar orders into classes, sim- ilar classes into phyla (or, in plants, divisions), simi- lar phyla into kingdoms, and similar kingdoms into domains. Biologists recognize just three domains, each containing millions of life-forms (Fig. 1.5). Two of the domains, Bacteria and Archaea, consist of microscopic, mostly single-celled organisms that differ in funda- mental ways (see Chapter 11 for details). The domain Bacteria includes the species that cause strep throat, for example, and that recycle decaying matter in soil and at the bottom of ponds. Members of the domain Archaea often live in harsh environments that are

grass. If they discover Martian or other extraterrestrial organ- isms, biologists can begin to catalog them as well, perhaps based on our own existing sys- tem for Earth organisms.

Species

Species are groups of individu- als with similar structures that descended from the same ini- tial group and that have the potential to breed successfully with one another in nature. House cats are one species, and ocelots, small jungle cats whose habitats range from steamy Amazon rain forests to

the dry chaparral of Texas, are a different but related species (see Fig. 1.4).

Genus

A genus (plural, genera) contains several related and similar species. Biologists refer to each species by a two-part name beginning with a term denoting the genus, followed by a separate term denoting the spe- cies. This two-word system for naming genus and

species a taxonomic group of organisms whose members have very similar structural traits and who can interbreed with each other in nature

genus (pl. genera) a taxonomic group of very similar species of common descent

family a taxonomic group comprising members of similar genera

order a precise arrangement of structural units and activities; also, in taxonomy, a taxonomic group comprising mem- bers of similar families

class a taxonomic group comprising members of similar orders

phylum (pl. phyla) a major taxo- nomic group just below the kingdom level, comprising members of similar classes, all with the same general body plan; equivalent to the division in plants

division a taxonomic group of similar classes belong- ing to the same phylum, which is often called a division in the king- doms of plants or fungi

kingdom a taxonomic group composed of mem- bers of similar phyla, i.e., Animalia, Plantae, Fungi, and Protista

domain a taxonomic group composed of members of similar kingdoms

Figure 1.4 A Hierarchy of Cat Species

House cat (F. catus)

Ocelot (F. pardalis)

Lion (P. leo)

Felis Panthera

10 CHAPTER 1 What Is Life?

CHAPTER 1 What Is Life? 11

years and then started divid- ing and growing again when provided with appropriate nutrients. This, too, has excit- ing implications for the Mar- tian soil samples NASA is planning to retrieve.

Earth’s third domain of liv- ing organisms, Eukarya (eu=true karya=nucleus), con- sists of larger, more complex cells containing a nucleus, a special compartment that contains the cell’s DNA. The domain Eukarya contains four kingdoms: Plantae, Animalia, Fungi, and Protista (Fig. 1.5). You’re familiar with the plant and animal kingdoms, of course. Fungi include mushrooms, molds, and yeast. Protists are less familiar because they are often microscopic, and include amebas and other organisms with a single, but complex, cell.

The Unity and Diversity of Life With 50 million living species or more, life on Earth is obviously diverse, but the fact that all this multiplic- ity arose from a single group of ancestral cells pres- ent at the dawn of life gives it unity as well—unity of origin, of cell structure, of genetic material, and of basic day-to-day functioning. How can life be both diverse and unifi ed? What mechanisms can foster not only vast diversity of form but also unity at the level of genes, cells, and basic function? The answer is evolution, the unifying theme for all life science.

Adaptation Different species have different ways of extracting energy and materials from their surroundings. Think, for a minute, about organisms living in the

bitter cold environment of Antarctica in the icy continent’s McMurdo Dry Valleys.

This area receives very little moisture, and tem- peratures are below freezing nearly all year round.

The rocks that line the valleys, however, are some- what porous, and they trap water. As sunlight hits the

rocks, they can warm up above freezing, and the water can become liquid. Amazingly, some

bacterial species have become adapted to live within the rocks, extracting energy from the sunshine, minerals from the rocks, and molecules necessary for life from the

air. Specializations that help an organism adapt to its own special way of life are called

adaptations. Some biologists hypothesize that if life did evolve on Mars in the distant past, Martian organ- isms may have become adapted to ways of living very similar to Earth’s Antarctic life-forms.

very hot, cold, acidic, or salty, such as thermal springs, salty lakes, or Antarctic ice. After Earth formed, envi- ronments like these would have been quite common, and the Archaea alive today probably share similarities with some of Earth’s earliest organisms. (The name “Archaea” refl ects the supposed “archaic” nature of these cells.)

Mars and Earth had similar beginnings, but Mars is very different now, with its frigid surface tempera- tures of 15°F during the day and 125°F at night. (The Martian day, by the way, is 24 hours and 37 minutes long.) Some biologists think that if Martian organ- isms once existed, or exist now, they would prob- ably resemble members of the domain Archaea. Those terrestrial organisms therefore are a model in many ways for the search for extraterrestrial life. In addition, recent research on soil samples from many meters below Earth’s surface turned up living Archaea that had been dormant for over a million

Figure 1.5 The Domains of Life

Animal

Fungi Plant

Protists

BACTERIA

UNIVERSAL ANCESTOR

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adaptation (L. adaptere, to fi t) a par- ticular form of behavior, structure, or physiologi- cal process that makes an organism better able to survive and reproduce in a particular environment

Many adaptations relate to taking in energy and materials, but there are others, too, that improve an organism’s ability to grow, to reproduce, to move, to live in a group, or to attract a mate more successfully.

Natural Selection

Adaptations usually arise by natural selection, a “weeding out” process that depends on individuals’ hereditary differences in their abilities to reproduce and to obtain energy. To see how this works, let’s go back to the example of cells living in the McMurdo Dry Valleys. Some of these bacteria may have heredi- tary factors that allow them to manufacture sugars at a slightly lower temperature than other nearby cells. In such a bitterly cold climate, this ability could help these particular bacterial cells survive and repro- duce under conditions in which other cells become dormant or die. As time passes, this advantage may mean that most or all of the cells in this better- surviving population and their descendants have the favorable trait while the nearby cells, lacking the trait, are out-competed. In 1859, Charles Darwin published the landmark book On the Origin of Species, and coined the term natural selection, for the mechanism under- lying change in living species over time. He called it “natural selection” because nature was “selecting” the individuals with the most suitable variations to survive and become the parents of the next genera- tion. A contemporary of Darwin’s, Alfred Russell Wal- lace, also published papers naming natural selection as the prime mechanism of evolution.

During the century and a half since Darwin’s and Wallace’s work, biologists have delved deeply into both the principles of, and the evidence for, evolution by nat- ural selection. Some of the most important advances

pinpointed the sources of variation from which nature selects individuals: Variations usually arise through mutations or alterations in gene structure. Largely because of modern molecular genetics, today’s biolo- gists can explain both the diversity of life and the unity of its origins and shared characteristics. They can account for life’s remarkable diversity because differ- ent environments require unique adaptations. Looking at foxes, for example, some of the characteristics that help an arctic fox survive in the frozen north are very different from those that help a desert fox survive in the desert. The arctic fox has small ears and a short muzzle that help conserve body heat and a white coat that serves as camoufl age against snow and ice (Fig. 1.6a). In contrast, the desert fox has large ears and a long muzzle that radiate extra body heat and a tan coat that disappears against the surrounding sand and rocks (Fig. 1.6b). An arctic fox in the desert would over- heat, stand out against the earth-toned background, and likely exhaust itself before catching rabbits or other prey. A desert fox in the arctic would have frost-bitten ears, would stand out against the snowy background, and would probably die of exposure or hunger before catching enough prey. The diversifying action of the environment, working through natural selection over millions of years, could have produced the remarkable variety of life-forms that have existed on our planet.

Likewise, evolution—descent with changes over time—can account for life’s unity, including the com- mon characteristics we’re surveying in this chapter. Biologists can trace the ancestry of both arctic and desert foxes back to a single fox species that lived in North America millions of years ago. They can trace that fox back to a common ancestor with all dogs. They can trace this group, the canids, back to a com- mon ancestor with all mammals, and so on back to the origin of life itself. As we will see in more detail in later chapters, the metabolic machinery within all liv- ing cells is very similar despite wide differences in cell

Figure 1.6 Fox Adaptations to Arctic and Desert

12 CHAPTER 1 What Is Life?

natural selection the increased survival and reproduction of individuals bet- ter adapted to the environment

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shape, size, and function. Biologists think all organ- isms alive today inherited this underlying machinery from cells that appeared at the dawn of life, and this helps explains the characteristics they share today.

Evolution by natural selection is so grand an orga- nizing principle for all biology that it will resurface repeatedly in this book, in the many magazine and newspaper articles you may read on life science, and in any future biology courses you may take.

Life Characteristics Relating to Environment Was life widespread on Mars early in its history? People used to believe the planet was cov- ered in canals built by little green aliens until mod- ern space probes showed that at least some of the “canals” were the remains of giant river beds. We know from the geological evidence of fl owing water and the current absence of liquid water on the plan- et’s surface that Mars was once wet and must have changed drastically to its current extreme aridness. If life was present and if it survived at all, then it would have had to change too. Ecology is the branch of biology that studies the relationships between living organisms and their environment, and it is an interesting and pervasive part of exploring life.

The Hierarchy of Life Organisms interact with their living and nonliving environments at several different levels. These levels extend the continuum of order we discussed earlier: molecules to cells to organisms. Take, for example, life in the African savanna environment—a splendid collection of plants, animals, fungi, microbes, and habitats that includes elephants, acacia trees, tussock grasses, and arid plains. The hier- archy of life on the savanna proceeds from small to large in the following sequence: Organisms, as we saw earlier, are individual, independent living things; an elephant is an organism and so is an acacia tree. Groups of a particular type of organism that live in the same area and actively interbreed with one another are called populations, for example, an elephant herd or a fi eld of grass. All the populations that live in a particular area, including the plants, animals, and other organisms that share the savanna, for example, make up a community. The living community together with its nonliving physical surroundings is called an ecosystem. The savanna ecosystem includes ele- phants, the egrets that pick insects off their skin, and the coarse grass they chew and trample, as well as

the water in clouds, the sandy soil underfoot, and the hot African sunshine. All the eco- systems of the Earth make up the biosphere, that portion of the Earth on which life exists, including every body of water; the atmosphere to a height of about 10 km (6 mi); the Earth’s crust to a depth of many meters; and all living things within this collective zone. The biosphere encompasses unimaginably remote places that nevertheless still teem with life, such as the deepest parts of the ocean fl oor, deep- sea vents spewing superheated water, the frozen ice of Antarc- tic lakes, and porous hot rocks a mile and a half (2.7 km) down toward the center of the Earth. A huge variety of organisms exists in these and the more

familiar and hospitable forests, meadows,

lakes, marshes, and grasslands we know in our own surroundings. The surpris- ing abundance of life-forms in Earth’s extreme environments gives scientists good reason to think that Mars could also

harbor life. The search for life throughout the solar system and beyond is fueled by our understanding of life’s diversity and

tenacity. We’ve seen that organisms take their

energy from the environment and that over time evolution fi ts organisms to their envi- ronments. Because this is just as true for

Earth’s extreme environments as for its tem- perate ones, there is good reason to think that

at least some environments on ancient Mars were inhabited by living organisms.

How Biologists Study Life So far, we’ve examined the basic life char- acteristics on our planet. But we haven’t talked much about how biologists coax, prod, and pry the secrets of nature from living organisms. It’s impor- tant to keep in mind that behind every fact and concept in this course, there were people in labora- tories or fi eld stations engaged in the often joyful

ecology the scientifi c study of how organisms interact with their environment and with each other and of the mechanisms that explain the distribu- tion and abundance of organisms

population a group of individuals of the same species living in a particular area

community two or more populations of different interacting species occupying the same area

ecosystem a community of organ- isms interacting with a particular environment

biosphere (Gr. bios, life + sphaira, sphere) that part of the planet that supports life; includes the atmosphere, water, and the outer few meters of the Earth’s crust

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Earth, then the thickness of its taut rubber skin would be proportional

to the biosphere.

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to a broad, unifying generalization is a creative, intui- tive, exciting moment every bit as original as writing a sonnet or sculpting a form from clay.

The second type of reasoning starts with general principles and then goes in the opposite direction to analysis of specifi c cases. For example, biologists knew that living organisms had turned up in places previously thought to be lifeless. Based on that knowl- edge, they reasoned that living cells might be found within the permanent ice that covers Antarctic lakes. They took ice samples, tested them, and found that, indeed, bacteria do survive around dark specks of dust incorporated in the ice. These particles absorb sunlight, warm up, and melt a small halo of water around themselves. This moisture and the pale Ant- arctic summer sunlight are enough to provide a brief growing period each year for the bacteria.

These two types of reasoning—from the specifi c to the general, and from the general to the specifi c—help shape how scientists think, but they aren’t unique to science. What is unique to the scientifi c process is a particular approach to testing generalizations. The steps may sound regimented, but they’re really just organized common sense.

Testing Generalizations: The Scientifi c Method

You may not know it, but you already use scientifi c reasoning. Say you come home late one night and fl ip on the light switch in the hall, but the overhead fi xture stays dark. You think to yourself, “I guess the bulb’s burned out.” On the basis of that hypothesis (guess), you predict that a bulb you know to be working (because it lights up a nearby fl oor lamp) will fi x the hall light. You test your hypothesis by screwing the

working bulb into the hall socket and fl ip the switch again. Still no light. You have just dis- proven your initial hypothesis (“burned-out bulb”) and need a new hypothesis (“broken switch” or “broken socket”). Biologists use this approach of hypothesis and testing—the scientifi c method—in much the same way.

First, they ask a question or identify a prob- lem to be solved based on observations of the natural world. Your observation was a dark hall, and your question was, “Why won’t the light turn on?”

Second, they propose a hypothesis, a pos- sible answer to the question or a potential

and exciting, but sometimes tedious and frustrating, pur- suit of knowledge about living things.

Natural Causes and Uniformity of Nature A lightning bolt fl ashes in a cloud-darkened sky. A man stands on a street corner

shouting that alien invaders have placed probes in his head. Modern scientists assume that events like these are due to natural causes. The ancient Greeks, on the other hand, believed that thunderbolts arose when the god Zeus hurled them at the Earth and that men- tal illness was due to evil spirits. Today’s scientists do not yet fully understand what causes Alzheimer’s dis- ease, for instance, or the El Niño climate fl uctuation. But they fi rmly believe these are based upon natural causes they will someday discover by applying the scientifi c process.

Scientists consider the fundamental laws of nature to be uniform and to operate the same way at all places and at all times. For example, biologists assume that the fi xed speed of light, the laws of grav- ity, and the properties of chemical elements work the same way in Ohio today as they did in East Africa 1 million years ago or on Mars 3 billion years ago. The events that led to life’s origin and diversity on Earth, and perhaps on Mars and other planets, occurred long before humans lived to observe them. Yet biologists are confi dent that today’s natural laws functioned the same way at the dawn of time, as life began, and all during its evolution.

The Power of Scientifi c Reasoning The search for life on Mars reveals the two kinds of scientifi c reasoning biologists use. In one type, the biologist collects specifi c cases and then gen- eralizes from them to arrive at broad principles. For example, after observing that cells can live inside rocks in Antarctica’s frigid des- erts, biologists proposed a generaliza- tion: that Mars rocks could perhaps harbor life, as well. The instant when the scientist’s mind leaps from previously isolated facts

scientifi c method a series of steps for understanding the natural world based on experimental testing of a hypothesis, a possible mechanism for how the world functions

hypothesis a possible answer to a question about how the world works that can be tested by means of sci- entifi c experimentation

work swit prov bulb swit this scien

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e Power of entifi c Reasoning earch for life on Mars reveals the two kinds entifi c reasoning biologists use. In one type, iologist collects specifi c cases and then gen-

zes from them to arrive at broad principles. xample, after observing that cells can nside rocks in Antarctica’s frigid des- biologists proposed a generaliza- that Mars rocks could perhaps

or life, as well. The instant n the scientist’s mind leaps

previously isolated facts

14 CHAPTER 1 What Is Life?

Scientifi c Method:

1. identify a problem 2. propose a hypothesis 3. make a prediction 4. test the prediction 5. draw a conclusion

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and they studied grains of a magnetic substance in the Mars rock similar to grains found inside certain bacteria that can orient the cells to Earth’s magnetic fi eld. Their hypothesis predicts each of these factors, and indeed, the researchers found these items in the rock.

We saw that to interpret any experiment, scientists need a control or known stan- dard for comparison. In tests of the Martian rock, NASA researchers found little tubu- lar structures that look like fossilized bacteria (see Fig. 1.1). For the control, they chose fossilized Earthly bacteria of a size and shape similar to the tubular structures and in rocks of a similar age and compo- sition to the Martian meteorite. Despite their best choices thus far, however, all of the fossilized bacte- ria from Earth have been much larger than the Mar- tian tubular “fossils.” Because of such comparisons with Earthly controls, most biologists doubt that the tubules in the Martian rock are fossilized cells, since there would have been so little space inside them for genes and the machinery of life as we know it.

In their second test, the NASA scientists did fi nd molecules in the Martian meteorite similar to those produced by Earthly life. Their hypothesis pre- dicted this fi nding. Remember, though, that while an incorrect prediction disproves a hypothesis, a correct prediction does not automatically prove it to be correct. For example, the chemicals they found in the Mars rock can be produced by living organisms, but they can also be produced by purely chemical means, as well. In our previous light bulb example, screw- ing in a working bulb didn’t fi x the overhead fi xture. But if it had, it would have been consistent with the hypothesis (burned-out bulb) yet still not proven it. Other hypotheses could have been true, as well — for example, maybe the original bulb still worked, but was not screwed in tightly enough. How would you test that hypothesis?

In their third test, the NASA researchers found tiny crystals of an iron compound called magnetite that some bacteria on Earth use as an internal compass. Crystals of this exact shape and size are not known to be formed outside of living cells. Other shapes are, however, and many researchers think that the mag- netite crystals in the Mars rock may have a nonliving origin. Again, the NASA team’s hypothesis predicted correctly, but they were not able to rule out other competing hypotheses.

solution to the problem. A hypothesis is a guess. Yours was “burned-out bulb.”

Then they make a prediction, a statement of what they will observe in a specifi c situation if the hypoth- esis is correct. You predicted that a working bulb would fi x the problem.

They test the prediction by performing an experi- ment or making further observations. You tested a bulb in a fl oor lamp to make sure it glowed, then screwed that working bulb into the hall light socket and fl ipped the switch again. The fl oor lamp pro- vided a control, a standard for comparison based on keeping all factors the same except for the one being tested. The hall light socket provided the experimental situation—the carefully planned and measured test of the hypothesis.

Finally, they draw a conclusion. If the hypothesis predicts incorrectly, then they must discard it as wrong. In your case, you said, “Nope. Not the bulb.” If the hypothesis predicts correctly—let’s say the light did go on—then they devise more tests to see whether the hypothesis might still be incorrect in some way. If they can never design a situation that shows the hypothesis to be wrong, then they begin to accept it. (Here’s where we differ from scientists in our daily lives. If the bulb goes on, we think, “Solved!” and go about our business. We don’t dream up more tests for why it worked!)

The Scientifi c Method at Work How do biologists approach scientifi c puzzles? Let’s review how they applied the scientifi c method to the question, “Does life exist on Mars?”

Searching the Mars Rock for Signs of Life

NASA researchers investigating the Mars rock found in Antarctica began by posing a question: Are there signs of past life inside this Martian meteorite? Next they stated an assumption they were making: that Martian life is or was similar to Earthly life. With- out knowing anything about possible life on Mars, they had to make some assumptions about what it would be like and what they’d be looking for, and so logically they chose the characteristics of life on Earth. (Explicitly stating underlying assump- tions is an important, but sometimes overlooked, part of the scientifi c method.) Then they created a hypothesis that life did exist at one time in the Mar- tian rock and left fossilized remains. Based on that hypothesis and their assumption, they carried out several tests: They looked for shapes similar to fos- silized Earthly bacteria; they searched for traces of chemicals similar to those formed by life on Earth;

prediction in the scientifi c method, an experimental result expected if a particular hypothesis is correct

control a check of a scientifi c experiment based on keeping all factors the same except for the one in question

experimental during the application of the scientifi c method, the phase involving the carefully planned and measured test of the hypothesis

16 CHAPTER 1 What Is Life?

The search for life in the Mars rock illustrates sev- eral points about the scien- tifi c method, but differs in a couple of key ways from how

biologists often learn about the natural world:

1. The scientists could not produce controls or do direct experiments, since they were observing a unique specimen.

2. Scientists usually carry out the same experi- ment or observation many times before draw- ing a fi rm conclusion. To do this would require searching for life on many different Earthlike planets and/or getting many samples from Mars. Martian soil and rocks may help provide controls and duplicate the experiments.

A Word About Theories Eventually, a theory can emerge from a broad general hypothesis that is tested repeatedly but never disproved. But what is a theory? A theory is a general principle about the natural world, like the theory of gravity, the cell theory, or the theory of evolution. People often say “It’s just a theory, ” meaning something that’s an untested idea. But scientists don’t use “theory” in that way; to them, a theory is a highly tested and never disproven prin- ciple that explains a large number of observations and experimental data.

The scientifi c method is a powerful tool for understanding the natural world, but it does not apply to matters of religion, politics, culture, ethics, or art. These valuable systems for approaching the world rely on different lines of inquiry and expe- rience. There will always be a place for scientifi c reasoning, though, because so many of the world’s complex problems have underlying biological bases, and we can’t solve them without biological facts and principles.

Biology Can Help Solve World Problems The search for life elsewhere in the universe is an obvious application of biology and of the question, “What is life?” But biology can do more than just prepare us to look for life on other worlds. It can contribute solutions to a long and growing list of problems here on Earth.

In the chapters that follow, you’ll explore how biology is helping to solve world problems. We are, in fact, in the midst of a revolution in the biologi- cal sciences, with exciting new information surfac- ing weekly in the fi ghts against cancer, heart disease, AIDS, infertility, and obesity. Researchers are making rapid advances in gene manipulation to create new drugs, crops, and farm animals; in exercise physiol- ogy to improve human performance; in the diagno- sis of genetic diseases; and in the transplantation of organs, including brain tissue. The discoveries are so frequent and fast-moving, in fact, that many of them will appear only on this book’s companion Web site and not in this text. Across all frontiers of biological science, at all levels of life’s organization—from mol- ecules to the biosphere—scientists are learning the most profound secrets of how living things survive day to day and reproduce new generations. You’re about to embark on an adventure of exploration and discovery that will not only excite your imagination and enrich your appreciation of the natural world, but will also allow you to contribute intelligently to the diffi cult choices all human societies must make in the future.

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Who knew? While an automobile might exhibit some of the charac- teristics of life, it does not exhibit all of the characteris- tics found in a living organism. It is not able to acquire energy and nutrients from its environment (it has no metabolism), and it cannot grow or reproduce.

theory a general hypothesis that is repeatedly tested but never disproved

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SPEAK UP!

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LIFE was built on a simple principle: to create a new teaching and learning solution that refl ects the way today’s faculty teach and the way you learn.

Through conversations, focus groups, surveys, and interviews, we collected data that drove the creation of the current version of LIFE that you are using today. But it doesn’t stop there – in order to make LIFE an even better learning experience, we’d like you to SPEAK UP and tell us how LIFE worked for you.

What did you like about it? What would you change? Are there additional ideas you have that would help us build a better product for next semester’s non-major biology students?

At 4ltrpress.cengage.com/life you’ll fi nd all of the resources you need to succeed in introductory biology – animations, visual reviews, fl ash cards, interactive quizzes, and more!

Speak Up! Go to 4ltrpress.cengage.com/life.

of students surveyed feel that LIFE is a better value than other textbooks.87%

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They are written in concise, down-to-earth language. There are tons of pictures and interest- ing blurbs of information. It’s very relevant to my life. It’s nice to have a book/website that seems to reach out to students and actually care about how we learn and try to tailor to our needs as much as possible. Thank you for this.

– Alice Brent, Student at Arizona State University

Chapter in ReviewChapter in Review 1

WHAT IS LIFE?

order a precise arrangement of structural units and activities; also, in taxonomy, a taxo-

nomic group comprising members of similar families

organism an individual that can independently carry out all life functions

organ system a group of organs that carries out a particular function in an organism

organ a body structure composed of two or more tis- sues that together perform a specifi c function

tissue a group of cells of the same type performing the same function within the body

cell the basic unit of life; cells are bounded by a lipid-containing membrane and are generally capable of independent reproduction

organelle a complex cytoplasmic structure with a char- acteristic shape that performs one or more specialized functions

biological molecules molecules derived from living systems; the four major types are carbohydrates, lipids, proteins, and nucleic acids

metabolism (Gr. metabole, to change) the series of chemi- cal reactions by which cells acquire and use energy and that contribute to repair, growth, and other survival processes

motility the self-propelled movement of an individual or its parts

responsiveness the tendency of a living thing to sense and react to its surroundings

reproduction the method by which individuals give rise to other individuals of the same type

asexual reproduction a type of reproduction in which new individu- als arise directly from only one parent

sexual reproduction a type of reproduction in which new individu- als arise from the mating of two parents

development the process by which an offspring increases in size and complexity from a zygote to an adult

heredity the transmission of genetic characters from parents to offspring

gene (Gr. genos, birth or race) the biological unit of inheritance that transmits hereditary informa- tion from parent to offspring and controls the appearance of a physical, behavioral, or biochemical trait; a gene is a specifi c discrete portion of the DNA molecule in a chromosome that encodes an rRNA molecule

Introduction In planning missions to search for life on Mars and other planets, NASA has to consider planetary protection: safeguarding Mars against accidental release of Earth organisms,

and safeguarding Earth against the possible retrieval and escape of alien (nonnative) organisms

from Mars. An important part of planetary protection is recognizing the characteristics of life on

Earth and predicting what life might be like elsewhere in the universe.

Characteristics of Life Biologists ask “What is life?” and although they can’t answer with a simple defi nition, they can describe the common characteristics all liv-

ing things share at some point in their life histories: order, metabolism, motility, responsiveness,

reproduction, development, heredity, evolution, and adaptation.

Characteristics Relating to Energy Use Living things possess a degree of order, or structural and behavioral complexity and regularity. Biologists categorize the levels

of that order as independent individuals, or organisms; groups of functioning body parts, or organ

systems; groups of two or more tissues that together perform a certain specialized function, or

organs; groups of similar cells, or tissues; the basic units of life, cells; the specialized parts within

cells, or organelles; and the biochemical compounds that make up organelles and other cell

components, or biological molecules. Living things carry out metabolism, taking energy from the

environment and using it to organize materials. Living things have self-propelled movement, or

motility. Living things are responsive—they respond to changes in their environment.

Characteristics Relating to Reproduction Living organisms carry out reproduc- tion, either asexual or sexual. Living organisms usually develop (undergo develop-

ment); that is, they usually grow in size and increase in complexity. Heredity is the transmission

of genetic characters from parent to offspring. The hereditary information contained in genes is

passed on to subsequent generations.

Characteristics Relating to Evolution Species descend from the same initial group and can breed successfully with one another in nature. A genus is a group of

related species. Higher orders include families of similar genera, orders of similar families, classes

of similar orders, phyla or divisions of similar classes, kingdoms of similar phyla, and domains

of similar kingdoms. Living organisms change over time in a process called evolution. Living

organisms display adaptations, or specializations that help them adapt to their own special way

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LIFE CHARACTERISTIC PROPERTY

1. Order Each structure or activity lies in a specifi c relationship to all other structures and activities.

2. Metabolism Organized chemical steps break down and build up molecules, making energy available or building needed parts.

3. Motility Using their own power, organisms move themselves or their body parts.

individual acquiring increased complexity.

7. Heredity Organisms have units of inheritance called genes that are passed from parent to offspring and control physical, chemi-

cal, and behavioral traits.

8. Evolution Populations of organisms change over time, acquiring new ways to survive, to obtain and use energy, and to reproduce.

9. Adaptations Specifi c structures, behaviors, and abilities suit life-forms to their environment.

Table 1.1 Characteristics of Life

evolution changes in gene frequencies in a population over time

species a taxonomic group of organisms whose mem- bers have very similar structural traits and who can interbreed with each other in nature

genus (pl. genera) a taxonomic group of very similar species of common descent

family a taxonomic group comprising members of similar genera

order a precise arrangement of structural units and activities; also, in taxonomy, a taxonomic group comprising members of similar families

class a taxonomic group comprising members of similar orders

phylum (pl. phyla) a major taxonomic group just below the kingdom level, comprising members of similar classes, all with the same general body plan; equivalent to the division in plants

division a taxonomic group of similar classes belonging to the same phylum, which is often called a division in the kingdoms of plants or fungi

kingdom a taxonomic group composed of members of similar phyla, i.e., Animalia, Plantae, Fungi, and Protista

domain a taxonomic group composed of members of similar kingdoms

adaptation (L. adaptere, to fi t) a particular form of behav- ior, structure, or physiological process that makes an organism better able to survive and reproduce in a particular environment

natural selection the increased survival and reproduction of indi- viduals better adapted to the environment

ecology the scientifi c study of how organisms interact with their environment and with each other and of the mechanisms that explain the distri- bution and abundance of organisms

population a group of individuals of the same species liv- ing in a particular area

community two or more populations of different interact- ing species occupying the same area

ecosystem a community of organisms interacting with a particular environment

biosphere (Gr. bios, life + sphaira, sphere) that part of the planet that supports life; includes the atmo- sphere, water, and the outer few meters of the Earth’s crust

scientifi c method a series of steps for understanding the natural world based on experimental testing of a hypothesis, a possible mechanism for how the world functions

hypothesis a possible answer to a question about how the world works that can be tested by means of scientifi c experimentation

of life. Adaptations usually arise through mutations, and nature “selects” the fi ttest individuals in

populations via natural selection.

Characteristics Relating to Environment Ecology is the branch of biology that studies the relationships of living organisms and their living and nonliving environ-

ments. Organisms interact with their environments at several different levels. Actively interbreed-

ing groups in a particular area are called populations. The populations in a particular area make up

communities. Communities and their nonliving physical surroundings are ecosystems. All of Earth’s

ecosystems make up the biosphere.

How Biologists Study Life Biologists sort out events based on natural causes and consider the fundamental laws of nature to be uniform. Biologists use two kinds

of reasoning, forming generalizations from many specifi c cases, and applying general principles to

specifi c cases. Biologists test their generalizations through the scientifi c method: they ask a ques-

tion, propose a hypothesis, make a prediction, design a control and an experimental procedure,

test the prediction, and then draw a conclusion. Based on numerous applications of the scientifi c

method, scientists sometimes develop a theory, a general principle about the natural world.

Biologists Can Help Solve Problems Humans are drastically changing Earth’s landscapes, species, even its temperatures and climates. The burgeoning human

population is a major contributor to global problems, including famine, diseases, ozone depletion,

and the destruction of species. Biological advances can help society grapple with and solve many

of these problems.

LO5

LO6

LO7

Animal

Fungi Plant

Protists

BACTERIA

UNIVERSAL ANCESTOR

EUCARYA

ARCHAEA

Figure 1.5 The Domains of Life

prediction in the scientifi c method, an experimental result expected if a particular hypothesis is correct

control a check of a scientifi c experiment based on keeping all factors the same except for the one in question

experimental during the application of the scientifi c method, the phase involving the care- fully planned and measured test of the hypothesis

theory a general hypothesis that is repeatedly tested but never disproved

For further review, exercises, practice quizzes, and more, log into the LIFE Web site

at 4ltrpress.cengage.com/life.

Ch 1: What Is Life?

Characteristics of Living Organisms
Life Characteristics Relating to Energy
Life Characteristics Relating to Reproduction
Life Characteristics Relating to Evolution
Life Characteristics Relating to Environment
How Biologists Study Life
Biology Can Help Solve World Problems

Chapter in Review 1: What Is Life?