32.3 All living organisms are grouped into one of a few major categories The Kingdoms of Life The remaining two kingdoms, Archaebacteria and Eu bacteria, consist of prokaryotic organisms, which are vasthy The earliest classification systems recognized only two different from all other living things(see chapter 34).Ar kingdoms of living things: animals and plants(figure chaebacteria are a diverse group including the 32 9a). But as biologists discovered microorganisms and methanogens and extreme thermophiles, and differ from learned more about other organisms, they added kingdoms the other bacteria, members of the kingdom eubacteria in recognition of fundamental differences discovered among organisms(figure 32 9b). Most biologists now use a six-kingdom system first proposed by Carl Woe Dor maIns University of Illinois(figure 329c) As biologists have learned more about the archaebacteria, it In this system, four kingdoms consist of eukaryotic or- has become increasingly clear that this ancient group is ganisms. The two most familiar kingdoms, Animalia and very different from all other organisms. When the full ge- Plantae, contain only organisms that are multicellular dur- nomic DNA sequences of an archaebacterium and a eubac- ing most of their life cycle. The kingdom Fungi contains terium were first compared in 1996, the differences proved multicellular forms and single-celled yeasts, which are triking. Archaebacteria are as different from eubacteria as thought to have multicellular ancestors. Fundamental dif- eubacteria are from eukaryotes. Recognizing this, biologists ferences divide these three kingdoms. Plants are mainly sta- are increasingly adopting a classification of living organ tionary, but some have motile sperm; fungi have e no mot ile isms that recognizes three domains, a taxonomic level cells; animals are mainly motile. Animals ingest their food, higher than kingdom(figure 32.9). Archaebacteria are in plants manufacture it, and fungi digest it by means of se- one domain, eubacteria in a second, and eukaryotes in the creted extracellular enzymes. Each of these kingdoms prob-third ably evolved from a different single-celled ancestor. he large number of unicellular eukaryotes are arbitrar- Living organisms are grouped into three general ly grouped into a single kingdom called Protista(se categories called domains. One of the domains, the chapter 35). This kingdom includes the algae, all of which eukaryotes, is subdivided into four kingdoms: protists are unicellular during parts of their life cycle fungi, plants, and animals. (a)A two-kingdom system-Linnaeus Animalia b)A five-kingdom system-Whittaker Monera Protista Plantae Animalia (c)A six-kingdom system-Woese Eubacteria Archaebacteria Protista Fungi (d) A three-domain system -Woese Bacteria Archaea Eukarya FIGURE 32.9 Different approaches to classifying living organisms. (a) Linnaeus popularized a two-kingdom approach, in which the fungi and the championed splitting the bacteria into two kingdoms for a total of six kingdoms, or even assigning them separate dondee has too photosynthetic protists were classified as plants, and the nonphotosynthetic protists as animals; when bacteria were described, the were considered plants. (b)Whittaker in 1969 proposed a five-kingdom system that soon became widely accepted. (c) Woese Chapter 32 How We Classify Organisms 657
The Kingdoms of Life The earliest classification systems recognized only two kingdoms of living things: animals and plants (figure 32.9a). But as biologists discovered microorganisms and learned more about other organisms, they added kingdoms in recognition of fundamental differences discovered among organisms (figure 32.9b). Most biologists now use a six-kingdom system first proposed by Carl Woese of the University of Illinois (figure 32.9c). In this system, four kingdoms consist of eukaryotic organisms. The two most familiar kingdoms, Animalia and Plantae, contain only organisms that are multicellular during most of their life cycle. The kingdom Fungi contains multicellular forms and single-celled yeasts, which are thought to have multicellular ancestors. Fundamental differences divide these three kingdoms. Plants are mainly stationary, but some have motile sperm; fungi have no motile cells; animals are mainly motile. Animals ingest their food, plants manufacture it, and fungi digest it by means of secreted extracellular enzymes. Each of these kingdoms probably evolved from a different single-celled ancestor. The large number of unicellular eukaryotes are arbitrarily grouped into a single kingdom called Protista (see chapter 35). This kingdom includes the algae, all of which are unicellular during parts of their life cycle. The remaining two kingdoms, Archaebacteria and Eubacteria, consist of prokaryotic organisms, which are vastly different from all other living things (see chapter 34). Archaebacteria are a diverse group including the methanogens and extreme thermophiles, and differ from the other bacteria, members of the kingdom Eubacteria. Domains As biologists have learned more about the archaebacteria, it has become increasingly clear that this ancient group is very different from all other organisms. When the full genomic DNA sequences of an archaebacterium and a eubacterium were first compared in 1996, the differences proved striking. Archaebacteria are as different from eubacteria as eubacteria are from eukaryotes. Recognizing this, biologists are increasingly adopting a classification of living organisms that recognizes three domains, a taxonomic level higher than kingdom (figure 32.9d). Archaebacteria are in one domain, eubacteria in a second, and eukaryotes in the third. Living organisms are grouped into three general categories called domains. One of the domains, the eukaryotes, is subdivided into four kingdoms: protists, fungi, plants, and animals. Chapter 32 How We Classify Organisms 657 32.3 All living organisms are grouped into one of a few major categories. Animalia Plantae Animalia Plantae Archaebacteria Protista Fungi Monera Protista Fungi Plantae Animalia (a) A two-kingdom system—Linnaeus (b) A five-kingdom system—Whittaker (c) A six-kingdom system—Woese (d) A three-domain system—Woese Bacteria Archaea Eukarya Eubacteria FIGURE 32.9 Different approaches to classifying living organisms. (a) Linnaeus popularized a two-kingdom approach, in which the fungi and the photosynthetic protists were classified as plants, and the nonphotosynthetic protists as animals; when bacteria were described, they too were considered plants. (b) Whittaker in 1969 proposed a five-kingdom system that soon became widely accepted. (c) Woese has championed splitting the bacteria into two kingdoms for a total of six kingdoms, or even assigning them separate domains (d)
Domain Archaea(Archaebacteria) Domain Doma The term archaebacteria(Greek, archaic, ancient) refers to Bacteria Archaea Eukarya (Eubacteria (Archaebacteria) the ancient origin of this group of bacteria, which seem to have diverged very early from the eubacteria(figure 32.10). This conclusion comes largely from comparisons of genes that encode ribosomal RNAs. The last several years have seen an explosion of DNA sequence informa tion from microorganisms, information which paints a more complex picture. It had been thought that by se- quencing numerous microbes we could eventually come up with an accurate picture of the phylogeny of the earliest organisms on earth. The new whole-genome DNA se- quence data described in chapter 19 tells us that it will not be that simple. Comparing whole-genome sequences leads Common ancestor evolutionary biologists to a variety of trees, some of which FIGURE 32.10 contradict each other. It appears that during their early An evolutionary relationship among the three domains evolution microorganisms have swapped genetic informa- Eubacteria are thought to have diverged early from the tion, making constructing phylogenetic trees very difficult ga, a thermophile found on Volcano Island off Italy. The of one of its RNAs places it squarely within the eubacteria near an ancient microbe called aquifex. Recent DNA sequencing, however, fails to support any consistent thermophilic archaebacteria form the basis of food webs ment as to the serious effect of gene swapping on the abil around deep-sea thermal vents where they must with- stand extreme temperatures and pressures. Other types ity of evolutionary biologists to provide accurate phyloge nies from molecular data. For now, we will provisionally like Sulfolobus, inhabit the hot sulfur springs of Yellow stone National Park at 70 to 75C. The recently de accept the tree presented in figure 32. 10. Over the next few scribed Pyrolobus fumarii holds the current record for ears we can expect to see considerable change in accepted viewpoints as more and more data is brought to bear heat stability, with a 106C temperature optimum and 113C maximum--it is so heat tolerant that it is not Today, archaebacteria inhabit some of the most extreme killed by a one-hour treatment in an autoclave(121C) environments on earth. Though a diverse group, all archae bacteria share certain key characteristics(table 32.1). Their Halophiles(“ salt lovers”)liv e In vel ry salty places like the ell walls lack peptidoglycan(an important component of Great Salt Lake in Utah. Mono lake in California, and the cell walls of eubacteria), the lipids in the cell the Dead Sea in Israel. Whereas the salinity of seawater is around 3 %. these bacteria thrive in, and indeed re branes of archaebacteria have a different structure than nose in all other organisms, and archaebacteria have dis quire, water with a salinity of 15 to 20% tinctive ribosomal RNA sequences. Some of their genes pH-tolerant archaebacteria grow in highly acidic(ph possess introns, unlike those of other bacteria. 0. 7)and very basic (pH= 11)environment The archaebacteria are grouped into three general cate- Pressure-tolerant archaebacteria have been isolated from gories, methanogens, extremophiles, and nonextreme ar- ocean depths that require at least 300 atmospheres of chaebacteria, based primarily on the environments in which pressure to survive, and tolerate up to 800 atmospheres they live or their specialized metabolic pathway?, hyarogen Methanogens obtain their energy by using Nonextreme archaebacteria grow in the same envi ronments eubacteria do. As the genomes of archaebacteria as(H2)to reduce carbon dioxide(CO2) to methane gas have become better known, microbiologists have been able CHA). They are strict anaerobes, poisoned by even traces of oxygen. They live in swamps, marshes, and the intestines to identify signature sequences of DNa present in all ar- of mammals. Methanogens release about 2 billion tons of chaebacteria and in no other organisms. When samples methane gas into the atmosphere each from soil or seawater are tested for genes matching these Extremophiles are able to grow under conditions that signal sequences, many of the bacteria living there prove to seem extreme to us. be archaebacteria. Clearly, archaebacteria are not restricted to extreme habitats, as microbiologists used to think. Tbermopbiles ("heat lovers") live in very hot places cally from60°to80°C. Many thermophiles are Archaebacteria are poorly understood bacteria that totrophs and have metabolisms based on sulfur. inhabit diverse environments, some of them extreme 658 Part IX Viruses and Simple organism
Domain Archaea (Archaebacteria) The term archaebacteria (Greek, archaio, ancient) refers to the ancient origin of this group of bacteria, which seem to have diverged very early from the eubacteria (figure 32.10). This conclusion comes largely from comparisons of genes that encode ribosomal RNAs. The last several years have seen an explosion of DNA sequence information from microorganisms, information which paints a more complex picture. It had been thought that by sequencing numerous microbes we could eventually come up with an accurate picture of the phylogeny of the earliest organisms on earth. The new whole-genome DNA sequence data described in chapter 19 tells us that it will not be that simple. Comparing whole-genome sequences leads evolutionary biologists to a variety of trees, some of which contradict each other. It appears that during their early evolution microorganisms have swapped genetic information, making constructing phylogenetic trees very difficult. As an example of the problem, we can look at Thermotoga, a thermophile found on Volcano Island off Italy. The sequence of one of its RNAs places it squarely within the eubacteria near an ancient microbe called Aquifex. Recent DNA sequencing, however, fails to support any consistent relationship between the two microbes. There is disagreement as to the serious effect of gene swapping on the ability of evolutionary biologists to provide accurate phylogenies from molecular data. For now, we will provisionally accept the tree presented in figure 32.10. Over the next few years we can expect to see considerable change in accepted viewpoints as more and more data is brought to bear. Today, archaebacteria inhabit some of the most extreme environments on earth. Though a diverse group, all archaebacteria share certain key characteristics (table 32.1). Their cell walls lack peptidoglycan (an important component of the cell walls of eubacteria), the lipids in the cell membranes of archaebacteria have a different structure than those in all other organisms, and archaebacteria have distinctive ribosomal RNA sequences. Some of their genes possess introns, unlike those of other bacteria. The archaebacteria are grouped into three general categories, methanogens, extremophiles, and nonextreme archaebacteria, based primarily on the environments in which they live or their specialized metabolic pathways. Methanogens obtain their energy by using hydrogen gas (H2) to reduce carbon dioxide (CO2) to methane gas (CH4). They are strict anaerobes, poisoned by even traces of oxygen. They live in swamps, marshes, and the intestines of mammals. Methanogens release about 2 billion tons of methane gas into the atmosphere each year. Extremophiles are able to grow under conditions that seem extreme to us. Thermophiles (“heat lovers”) live in very hot places, typically from 60º to 80ºC. Many thermophiles are autotrophs and have metabolisms based on sulfur. Some thermophilic archaebacteria form the basis of food webs around deep-sea thermal vents where they must withstand extreme temperatures and pressures. Other types, like Sulfolobus, inhabit the hot sulfur springs of Yellowstone National Park at 70º to 75ºC. The recently described Pyrolobus fumarii holds the current record for heat stability, with a 106ºC temperature optimum and 113ºC maximum—it is so heat tolerant that it is not killed by a one-hour treatment in an autoclave (121ºC)! Halophiles (“salt lovers”) live in very salty places like the Great Salt Lake in Utah, Mono Lake in California, and the Dead Sea in Israel. Whereas the salinity of seawater is around 3%, these bacteria thrive in, and indeed require, water with a salinity of 15 to 20%. pH-tolerant archaebacteria grow in highly acidic (pH = 0.7) and very basic (pH = 11) environments. Pressure-tolerant archaebacteria have been isolated from ocean depths that require at least 300 atmospheres of pressure to survive, and tolerate up to 800 atmospheres! Nonextreme archaebacteria grow in the same environments eubacteria do. As the genomes of archaebacteria have become better known, microbiologists have been able to identify signature sequences of DNA present in all archaebacteria and in no other organisms. When samples from soil or seawater are tested for genes matching these signal sequences, many of the bacteria living there prove to be archaebacteria. Clearly, archaebacteria are not restricted to extreme habitats, as microbiologists used to think. Archaebacteria are poorly understood bacteria that inhabit diverse environments, some of them extreme. 658 Part IX Viruses and Simple Organisms Domain Bacteria (Eubacteria) Domain Archaea (Archaebacteria) Common ancestor Domain Eukarya (Eukaryotes) FIGURE 32.10 An evolutionary relationship among the three domains. Eubacteria are thought to have diverged early from the evolutionary line that gave rise to the archaebacteria and eukaryotes
BACTERIA ARCHAEA EUKARYA Halobacterium Entamoebae Purple bacteria Methanobacterium Slime molds Fungi Cyanobacteria Pyrodictium lasma Flavobacteria CoCCUS Ciliates Aquifex FIGURE 32.11 A tree of life. This phylogeny, prepared from rRNA analyses, shows the evolutionary relationships among the three domains. The base of the tree was determined by examining genes that are duplicated in all three domains, the duplication presumably having occurred in the common ancestor. When one of the duplicates is used to construct the tree, the other can be used to root it. This approach clearly indicates that the root of the tree is within the eubacterial domain. Archaebacteria and eukaryotes diverged later and are more closely related to each other than either is to eubacteria Domain Bacteria (Eubacteria) Table 32.1 Features of the Domains of life The eubacteria are the most abundant organisms on earth. Domain There are more living eubacteria in your mouth than there Feature Archaea Bacteria Eukarya are mammals living on earth. Although too tiny to see with the unaided eye, eubacteria play critical roles throughout Amino acid Methionine Formyl Methionine the biosphere. They extract from the air all the nitrogen that initiates used by organisms, and play key roles in cycling carbon and sulfur. Much of the worlds photosynthesis is carried out by synthesis eubacteria. However, certain groups of eubacteria are also Introns Present in Absent sone genes responsible for many forms of disease. Understanding their metabolism and genetics is a critical part of modern medi- Membran Absent bounded organelles There are many different kinds of eubacteria, and the Membrane Branched Unbranched Unbranched evolutionary links between them are not well understood While there is considerable disagreement among taxono- structure mists about the details of bacterial classification most rec- Nuclear Absent ognize 12 to 15 major groups of eubacteria. Comparisons envelope of the nucleotide sequences of ribosomal RNA (rRNA) Number of Several n Several molecules are beginning to reveal how these groups are re- different lated to one another and to the other two domains. One view of our current understanding of the "Tree of Life"is polymerases presented in figure 32. 11. The oldest divergences represent Peptidoglycan Abse Present Absent the deepest rooted branches in the tree. The root of the n cell wall tree is within the eubacterial domain the archaebacteria R Growth Growth Growth not and eukaryotes are more closely related to each other than to the to eubacteria and are on a separate evolutionary branch of antibiotics the tree, even though archaebacteria and eubacteria are and chloral both prokaryotes Eubacteria are as different from archaebacteria as from eukaryotes Chapter 32 How We Classify Organisms 659
Domain Bacteria (Eubacteria) The eubacteria are the most abundant organisms on earth. There are more living eubacteria in your mouth than there are mammals living on earth. Although too tiny to see with the unaided eye, eubacteria play critical roles throughout the biosphere. They extract from the air all the nitrogen used by organisms, and play key roles in cycling carbon and sulfur. Much of the world’s photosynthesis is carried out by eubacteria. However, certain groups of eubacteria are also responsible for many forms of disease. Understanding their metabolism and genetics is a critical part of modern medicine. There are many different kinds of eubacteria, and the evolutionary links between them are not well understood. While there is considerable disagreement among taxonomists about the details of bacterial classification, most recognize 12 to 15 major groups of eubacteria. Comparisons of the nucleotide sequences of ribosomal RNA (rRNA) molecules are beginning to reveal how these groups are related to one another and to the other two domains. One view of our current understanding of the “Tree of Life” is presented in figure 32.11. The oldest divergences represent the deepest rooted branches in the tree. The root of the tree is within the eubacterial domain. The archaebacteria and eukaryotes are more closely related to each other than to eubacteria and are on a separate evolutionary branch of the tree, even though archaebacteria and eubacteria are both prokaryotes. Eubacteria are as different from archaebacteria as from eukaryotes. Chapter 32 How We Classify Organisms 659 BACTERIA Purple bacteria Common ancestor Cyanobacteria Flavobacteria Thermotoga Pyrodictium Thermoproteus Methanobacterium Methanopyrus Thermoplasma Methanococcus Thermococcus Halobacterium Aquifex Gram-positive bacteria Entamoebae Slime molds Animals Fungi Plants Ciliates Flagellates Diplomonads Microsporidia ARCHAEA EUKARYA FIGURE 32.11 A tree of life. This phylogeny, prepared from rRNA analyses, shows the evolutionary relationships among the three domains. The base of the tree was determined by examining genes that are duplicated in all three domains, the duplication presumably having occurred in the common ancestor. When one of the duplicates is used to construct the tree, the other can be used to root it. This approach clearly indicates that the root of the tree is within the eubacterial domain. Archaebacteria and eukaryotes diverged later and are more closely related to each other than either is to eubacteria. Table 32.1 Features of the Domains of Life Domain Feature Archaea Bacteria Eukarya Amino acid Methionine Formyl- Methionine that initiates methionine protein synthesis Introns Present in Absent Present some genes Membrane- Absent Absent Present bounded organelles Membrane Branched Unbranched Unbranched lipid structure Nuclear Absent Absent Present envelope Number of Several One Several different RNA polymerases Peptidoglycan Absent Present Absent in cell wall Response Growth Growth Growth not to the not inhibited inhibited inhibited antibiotics streptomycin and chloramphenicol
Domain Eukarya(Eukaryotes) Because of the size and ecological dominance of plants, ani- mals, and fungi, and because they are predominantly multi- For at least 2 billion years, bacteria ruled the earth. No cellular, we recognize them as kingdoms distinct from Pro- other organisms existed to eat them or compete with tista, even though the amount of diversity among the them, and their tiny cells formed the world s oldest fossils. protists is much greater than that within or between the The third great domain of life the eukaryotes, appear in fungi, plants, and animals the fossil record much later, only about 1.5 billiOn N ago. Metabolically, eukaryotes are more uniform than bacteria. Each of the two domains of prokaryotic organ- Symbiosis and the Origin of Eukaryotes isms has far more metabolic diversity than all eukaryotic The hallmark of eukaryotes is complex cellular organiza organisms taken together. However, despite the metabolic tion, highlighted by an extensive endomembrane system similarity of eukaryotic cells, their structure and function that subdivides the eukaryotic cell into functional compart allowed larger cell sizes and, eventually, multicellular life ments. Not all of these compartments, however, are de tions, all modern eukaryotic cells system. With few excep- Four Kingdoms of Eukaryotes organelles, the mitochondria, and some eukaryotic cells possess chloroplasts, which are energy-harvesting or- The first eukaryotes were unicellular organisms. A wide ganelles. Mitochondria and chloroplasts are both believed variety of unicellular eukaryotes exist today, grouped to- to have entered early eukaryotic cells by a process called gether in the kingdom Protista on the basis that they do endosymbiosis(endo, inside). We discussed the theory of not fit into any of the other three kingdoms of eukaryotes. the endosymbiotic origin of mitochondria and chloroplasts Protists are a fascinating group containing many organ in chapter 5; also see figure 32.12. Both organelles contain isms of intense interest and great biological significance. their own ribosomes, which are more similar to bacterial ri- They vary from the relatively simple, single-celled bosoms than to eukaryotic cytoplasmic ribosomes. They amoeba to multicellular organisms like kelp that can be 20 manufacture their own inner membranes. They divide in meters long dependently of the cell and contain chromosomes similar Fungi, plants, and animals are largely multicellular king- to those in bacteria. Mitochondria are about the size of ns, each a distinct evolutionary line from a single-celled bacteria and contain DNA. Comparison of the nucleotide estor that would be classified in the kingdom Protista. sequence of this dNa with that of a variety of organisms Ancestry eukaryotic Original Brown algae Mitochondria Plantac Photosynthetic bacteria Eubacteria Other bact FIGURE 32 12 Diagram of the evolutionary relationships among the six kingdoms of organisms. The colored lines indicate symbiotic events. 660 Part IX Viruses and Simple organism
Domain Eukarya (Eukaryotes) For at least 2 billion years, bacteria ruled the earth. No other organisms existed to eat them or compete with them, and their tiny cells formed the world’s oldest fossils. The third great domain of life, the eukaryotes, appear in the fossil record much later, only about 1.5 billion years ago. Metabolically, eukaryotes are more uniform than bacteria. Each of the two domains of prokaryotic organisms has far more metabolic diversity than all eukaryotic organisms taken together. However, despite the metabolic similarity of eukaryotic cells, their structure and function allowed larger cell sizes and, eventually, multicellular life to evolve. Four Kingdoms of Eukaryotes The first eukaryotes were unicellular organisms. A wide variety of unicellular eukaryotes exist today, grouped together in the kingdom Protista on the basis that they do not fit into any of the other three kingdoms of eukaryotes. Protists are a fascinating group containing many organisms of intense interest and great biological significance. They vary from the relatively simple, single-celled amoeba to multicellular organisms like kelp that can be 20 meters long. Fungi, plants, and animals are largely multicellular kingdoms, each a distinct evolutionary line from a single-celled ancestor that would be classified in the kingdom Protista. Because of the size and ecological dominance of plants, animals, and fungi, and because they are predominantly multicellular, we recognize them as kingdoms distinct from Protista, even though the amount of diversity among the protists is much greater than that within or between the fungi, plants, and animals. Symbiosis and the Origin of Eukaryotes The hallmark of eukaryotes is complex cellular organization, highlighted by an extensive endomembrane system that subdivides the eukaryotic cell into functional compartments. Not all of these compartments, however, are derived from the endomembrane system. With few exceptions, all modern eukaryotic cells possess energy-producing organelles, the mitochondria, and some eukaryotic cells possess chloroplasts, which are energy-harvesting organelles. Mitochondria and chloroplasts are both believed to have entered early eukaryotic cells by a process called endosymbiosis (endo, inside). We discussed the theory of the endosymbiotic origin of mitochondria and chloroplasts in chapter 5; also see figure 32.12. Both organelles contain their own ribosomes, which are more similar to bacterial ribosomes than to eukaryotic cytoplasmic ribosomes. They manufacture their own inner membranes. They divide independently of the cell and contain chromosomes similar to those in bacteria. Mitochondria are about the size of bacteria and contain DNA. Comparison of the nucleotide sequence of this DNA with that of a variety of organisms 660 Part IX Viruses and Simple Organisms Thermophiles Halophiles Methanogens Purple bacteria Photosynthetic bacteria Photosynthetic protists Nonphotosynthetic protists Brown algae Animalia Fungi Protista Plantae Eubacteria Red algae Green algae Other bacteria Archaebacteria Ancestral eukaryotic cell Original cell Mitochondria Chloroplasts FIGURE 32.12 Diagram of the evolutionary relationships among the six kingdoms of organisms. The colored lines indicate symbiotic events
indicates clearly that mitochondria are tain forms of multicellular green algae the descendants of purple bacteria that were ancestors of the plants(see chapters were incorporated into eukaryotic cells 35 and 37), and, like the other photosyn early in the history of the group. Chloro- thetic protists, are considered plants in plasts are derived from cyanobacteria ome classification schemes. In the sys- that became symbiotic in several groups tem adopted here, the plant kingdom of protists early in their histor cludes only multicellular land plants, a ome biologists suggest that basa group that arose from a single ancestor odies, centrioles, flagella, and cilia in terrestrial habitats and that has a may have arisen from endosymbiotic unique set of characteristics. Aquatic spirochaete-like bacteria. Even today, plants are recent derivatives. so many bacteria and unicellular pro Fungi and animals arose from unicel- tists form symbiotic alliances that the lular protist ancestors with different incorporation of smaller with characteristics. As we will see in subse desirable features into eukaryotic cells quent chapters, the groups that seem appears to be a relatively common FIGURE 32.13 Colonial bacteria. No bacteria ar have given rise to each of these king oces doms are still in existence truly multicellular. These gliding bacteria, Stigmatella aurantiaca, have Key characteristics ggregated into a structure called a Sexuality. Another major characteris- of Eukaryotes fruiting body; within, some cell tic of eukaryotic organisms as a group is transform into spores. sexuality. Although some interchange of genetic material occurs in bacteria(see Multicellularity. The unicellular hapter 34), it is certainly not a regular, body plan has been tremendously suc predictable mechanism in the same cessful, with unicellular prokaryotes sense that sex is in eukaryotes. The se nd eukaryotes constituting about half of the biomass on ual cycle characteristic of eukaryotes alternates between earth. Yet a single cell has limits. The evolution of mul syngamy, the union of male and female gametes producing cellularity allowed organisms to deal with their environ- a cell with two sets of chromosomes, and meiosis, cell divi ments in novel ways. Distinct types of cells, tissues, and sion producing daughter cells with one set of chromo organs can be differentiated within the complex bodies of somes. This cycle differs sharply from any exchange of ge multicellular organisms. With such a functional division netic material found in bacteria within its body, a multicellular organism can do many Except for gametes, the cells of most animals and plants things, like protect itself, resist drought efficiently, regu are diploid, containing two sets of chromosomes, during late its internal conditions, move about, seek mates and some part of their life cycle. A few eukarya otes co prey, and carry out other activities on a scale and with a their life cycle in the haploid condition, with only one set complexity that would be impossible for its unicellular of chromosomes in each cell. As we have seen, in diploid ancestors. With all these advantages, it is not surprising cells, one set of chromosomes comes from the male parent that multicellularity has arisen independently so many and one from the female parent. These chromosomes seg egate during meiosis. Because crossing over frequently oc True multicellularity, in which the activities of individ- curs during meiosis(see chapter 12), no two products of a ual cells are coordinated and the cells themselves are in single meiotic event are ever identical. As a result, the off- contact,occurs only in eukaryotes and is one of their spring of sexual, eukaryotic organisms vary widely, thus major characteristics. The cell walls of bacteria occasion- providing the raw material for evolution. ally adhere to one another, and bacterial cells may also be Sexual reproduction, with its regular alternation be- held together within a common sheath. Some bacteria tween syngamy and meiosis, produces genetic variation form filaments, sheets, or three-dimensional aggregates Sexual organisms can adapt to the demands of their envi (figure 32. 13), but the individual cells remain independent ronments because they produce a variety of progeny of each other, reproducing and carrying on their meta- In many of the unicellular phyla of protists, sex nial aggregates of many cells with little differentiation or environmental conditions. The first eukaryotes weic i r bolic functions and without coordinating with the other production occurs only occasionally. Meiosis may have cells. Such bacteria are considered colonial, but none are originally evolved as a means of repairing damage to DN truly multicellular. Many protists also form similar colo- producing an organism better adapted to survive cha Integration. ably haploid. Diploids seem to have arisen on a nuns prob ample--have independently attained multicellularity. Cer- then eventually divided by meiosis tf haploid cells, which Other protists-the red, brown, and green algae, for ex- separate occasions by the fusion Chapter 32 How We Classify Organisms 661
indicates clearly that mitochondria are the descendants of purple bacteria that were incorporated into eukaryotic cells early in the history of the group. Chloroplasts are derived from cyanobacteria that became symbiotic in several groups of protists early in their history. Some biologists suggest that basal bodies, centrioles, flagella, and cilia may have arisen from endosymbiotic spirochaete-like bacteria. Even today, so many bacteria and unicellular protists form symbiotic alliances that the incorporation of smaller organisms with desirable features into eukaryotic cells appears to be a relatively common process. Key Characteristics of Eukaryotes Multicellularity. The unicellular body plan has been tremendously successful, with unicellular prokaryotes and eukaryotes constituting about half of the biomass on earth. Yet a single cell has limits. The evolution of multicellularity allowed organisms to deal with their environments in novel ways. Distinct types of cells, tissues, and organs can be differentiated within the complex bodies of multicellular organisms. With such a functional division within its body, a multicellular organism can do many things, like protect itself, resist drought efficiently, regulate its internal conditions, move about, seek mates and prey, and carry out other activities on a scale and with a complexity that would be impossible for its unicellular ancestors. With all these advantages, it is not surprising that multicellularity has arisen independently so many times. True multicellularity, in which the activities of individual cells are coordinated and the cells themselves are in contact, occurs only in eukaryotes and is one of their major characteristics. The cell walls of bacteria occasionally adhere to one another, and bacterial cells may also be held together within a common sheath. Some bacteria form filaments, sheets, or three-dimensional aggregates (figure 32.13), but the individual cells remain independent of each other, reproducing and carrying on their metabolic functions and without coordinating with the other cells. Such bacteria are considered colonial, but none are truly multicellular. Many protists also form similar colonial aggregates of many cells with little differentiation or integration. Other protists—the red, brown, and green algae, for example—have independently attained multicellularity. Certain forms of multicellular green algae were ancestors of the plants (see chapters 35 and 37), and, like the other photosynthetic protists, are considered plants in some classification schemes. In the system adopted here, the plant kingdom includes only multicellular land plants, a group that arose from a single ancestor in terrestrial habitats and that has a unique set of characteristics. Aquatic plants are recent derivatives. Fungi and animals arose from unicellular protist ancestors with different characteristics. As we will see in subsequent chapters, the groups that seem to have given rise to each of these kingdoms are still in existence. Sexuality. Another major characteristic of eukaryotic organisms as a group is sexuality. Although some interchange of genetic material occurs in bacteria (see chapter 34), it is certainly not a regular, predictable mechanism in the same sense that sex is in eukaryotes. The sexual cycle characteristic of eukaryotes alternates between syngamy, the union of male and female gametes producing a cell with two sets of chromosomes, and meiosis, cell division producing daughter cells with one set of chromosomes. This cycle differs sharply from any exchange of genetic material found in bacteria. Except for gametes, the cells of most animals and plants are diploid, containing two sets of chromosomes, during some part of their life cycle. A few eukaryotes complete their life cycle in the haploid condition, with only one set of chromosomes in each cell. As we have seen, in diploid cells, one set of chromosomes comes from the male parent and one from the female parent. These chromosomes segregate during meiosis. Because crossing over frequently occurs during meiosis (see chapter 12), no two products of a single meiotic event are ever identical. As a result, the offspring of sexual, eukaryotic organisms vary widely, thus providing the raw material for evolution. Sexual reproduction, with its regular alternation between syngamy and meiosis, produces genetic variation. Sexual organisms can adapt to the demands of their environments because they produce a variety of progeny. In many of the unicellular phyla of protists, sexual reproduction occurs only occasionally. Meiosis may have originally evolved as a means of repairing damage to DNA, producing an organism better adapted to survive changing environmental conditions. The first eukaryotes were probably haploid. Diploids seem to have arisen on a number of separate occasions by the fusion of haploid cells, which then eventually divided by meiosis. Chapter 32 How We Classify Organisms 661 FIGURE 32.13 Colonial bacteria. No bacteria are truly multicellular. These gliding bacteria, Stigmatella aurantiaca, have aggregated into a structure called a fruiting body; within, some cells transform into spores