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Chapter 1 In biology nothing makes sense, except in the light of evolution. – THEODOSIUS DOBZHANSKY, 1970 ur solar system originated through physical and chemical processes. After Earth formed, organisms originated and evolved. These first organisms were microorganisms, and they had a profound impa...

Chapter 1 In biology nothing makes sense, except in the light of evolution. – THEODOSIUS DOBZHANSKY, 1970 ur solar system originated through physical and chemical processes. After Earth formed, organisms originated and evolved. These first organisms were microorganisms, and they had a profound impact on Earth and the formation of its biosphere, the shell about Earth where life occurs. Certain bacterial groups played especially crucial roles early on in Earth’s development. For example, geochemical and fossil evidence indicates that the production of oxygen in the atmosphere was due to the photosynthetic activity of cyanobacteria. The evolution of microorganisms that produced oxygen was of monumental significance, because all plant and animal life that exists today requires oxygen. Thus, plants and animals could evolve only because microorganisms evolved first. In this chapter we discuss Earth’s origin, the evolution of life, and the importance of microorganisms to life on Earth. ORIGIN OF EARTH AND LIFE The origin of Earth and the evolution of life on our planet has been a long process. The universe, which is estimated to have an age of 18 Ga (1 Ga, a giga-annum, is 109 years), began with a “Big Bang” that produced two principal elements, hydrogen (1H) and helium (4He), with smaller amounts of other light (low atomic weight) elements. Following the Big Bang, the universe expanded, as it continues to do today. At its periphery the original light elements condensed to form clouds of gases and dust. In the clouds heavier elements evolved from the lighter ones. Our solar system was formed by an accretional process in which micrometer-sized dust particles collided to form centimeter-sized bodies. These particles were located in a planar disk that orbited the sun. The accretional process continued as the dust and rock particles aggregated to form boulders and larger bodies that eventually attained the size of the planets. Thus, ultimately by gravitational contraction, our solar system, with the Sun, Earth, and other planets, formed about 4.5 Ga ago. The final stages of accretion involved collisions between large bodies at high velocities. A major collision between early Earth and a Mars-sized object resulted in the formation of our moon and Earth. The 600 million years following Earth’s formation is called the era of “heavy bombardment” because of the high frequency of collisions between Earth and large asteroids and comets. Some of these collisions, such as the one responsible for the formation of the moon, were so violent that they heated Earth to sterilizing temperatures. Even collisions with bodies only 100 kilometers in diameter could result in sterilization within the planet to depths of several kilometers. Furthermore, the heat from these collisions would have removed volatile substances such as water. 4 CHAPTER 1 During its first 600 million years, Earth was not a hospitable planet for life. Water was not initially available. It was brought to Earth by comets and asteroids that came from farther out in the solar system. Once water was available and the era of heavy bombardment had ended, conditions became conducive to the evolution of living organisms. Scientists have determined the date of Earth’s formation by studying slowly decaying radioactive isotopes, whose decay occurs at a constant rate independent of temperature and pressure. The isotope most relied on for dating such ancient events is potassium (40K), which decays to argon (40Ar) with a half-life, the time required for half the radioactivity to decay, of 1.26 billion years. Radioisotopic methods are also used for dating strata in sedimentary rocks and therefore offer a means of dating fossilized life forms in rocks. Fossil Evidence of Microorganisms By the nineteenth century it was known that fossils were the remains or impressions of plants and animals that had been preserved in sedimentary rocks. Accurate dating methods were not yet available, so the estimated dates were only guesses. We now know that some of the organisms that became fossilized, such as the dinosaurs, lived and became extinct millions of years ago. By examining fossils, paleontologists came to several conclusions about the evolution of life. They noted that fossils nearest the surface, that is, in the most recently deposited sedimentary rocks, were structurally more complex than those in deeper layers. Fossils found in deeper strata were increasingly simple in structure, fossils of very simple, extinct Table 1.1 animals such as trilobites. The gradation of complexity, from simple organisms in the most ancient rocks to more complex forms in the more recent sedimentary rocks, argued for an evolutionary process in which more complex forms of plants and animals arose from simpler organisms. Geologists and paleontologists worked hand in hand to develop time scales for sedimentary rock deposits and named the various time periods of Earth’s history based upon fossil records (Table 1.1). At the time of Charles Darwin (1809–1882) the fossil record was being carefully studied, but there was no means of estimating ages. Today, we realize that the fossil record for plants and animals extends back to a period of 570 million years ago (570 mya). Rocks older than that contain no plant and animal fossils. This time and all earlier times became known as the Precambrian era (Table 1.1). In the 1950s two American scientists, Stanley Tyler and Elso Barghoorn, made a startling discovery. They reported to the scientific world that they had found fossils of microorganisms in sedimentary rocks dated to the Precambrian era (Box 1.1). This important discovery provided the first convincing evidence that the earliest life forms on Earth were microorganisms. The microbial fossils were discovered in laminated sedimentary rocks called stromatolites (Figure 1.1A). Many of the multilayered stromatolite structures contain calcium carbonate along with the fossils of filamentous microorganisms (Figure 1.1B). Living stromatolites still exist on Earth today. The columnar stromatolites occur in intertidal marine areas such as Shark Bay, Western Australia (Figure 1.2). These living Geological timetable on Earth Years Before Present (millions) Eon/era Period Precambrian Hadean Archaean Proterozoic Paleozoic Cambrian Ordovician Silurian Devonian Carboniferous Permian 570 500 440 395 345 280 Land colonization by plants, animals Fish diversify Reptiles evolve; large “fern” forests Mass extinction at end Mesozoic Triassic Jurassic Cretaceous 245 190 145 Early dinosaurs; first mammals Plants and animals diversify Mass extinction at end; 75% of species lost Cenozoic Tertiary Quaternary 4,500 3,800 2,600 65 1.8 Major Events Heavy bombardment period First sedimentary rocks Appearance of O2 Animals evolve Plant and animal radiation Humans evolve EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE (A) (B) 10.0 µm Figure 1.1 Stromatolites and mat communities (A) Fossil columnar stromatolites from Glacier National Park, shown in cross section. A U.S. quarter is shown for size. (B) Filamentous microbial fossils observed in sections of 860 million-year-old stromatolites from the Bitter Springs formation in central Australia. A, courtesy of Beverly Pierson; B, courtesy of William Schopf. Milestones Box 1.1 The Discovery of Microbial Fossils In the early twentieth century an American geologist, Charles Doolittle Walcott, was studying Precambrian sedimentary rocks in Glacier National Park in northwestern Montana. He noted that some had curious undulating wavelike structures (these are now called stromatolites) and postulated that they were fossilized forms of Precambrian reefs. Contemporary scientists doubted his theory, and it remained untested for many years. American micropaleontologists Stanley Tyler from the University of Wisconsin and Elso Barghoorn from Harvard University were the first to test Walcott’s hypothesis. They were studying stromatolites from 1 to 2 billion-year-old Precambrian Gunflint chert deposits from the Great Shield area in the Great Lakes vicinity of North America. When they examined sections of these stromatolites using the light microscope they discovered microbial fossils. Microbiologists were incredulous when Tyler and Barghoorn first reported their observations in the 1950s and 1960s, as most microbiologists did not believe microbial fossils existed. However, Tyler and Barghoorn’s clear photomicrographic evidence convinced a whole generation of skeptical microbiologists. More recently, micropaleontologists have discovered microbial fossils in stromatolites 3.5 Ga old, dating back to about 1 Ga after the origin of Earth. The undulating layers of this sedimentary rock of Glacier National Park are stromatolites containing fossil microorganisms. The lens cap serves as a scale marker. Courtesy of Beverly Pierson. 5 6 CHAPTER 1 Figure 1.2 Living columnar stromatolites, Shark Bay,Western Australia The largest stromatolite shown here is about 1 m in diameter. Courtesy of Beverly Pierson. stromatolites contain microorganisms that deposit calcium carbonate and other minerals, forming the successive layers of the stromatolite structure. Other precursors of fossil stromatolites are microbial mat communities, which occur extensively in intertidal marine environments throughout the world (Figure 1.3A). Photosynthetic microorganisms, including cyanobacteria, and photosynthetic bacteria are found in distinct layers in living stromatolites (Figure 1.3B–D). These mat communities are flatter and broader than the columnar-shaped classical stromatolites, but they are produced in similar saline, intertidal environments by similar microorganisms. Evidently during some major geological events living stromatolites became fossilized and preserved in sedimentary deposits. Fossil microorganisms have been dated at 3.5 Ga before the present and therefore are found in some of the earliest sedimentary deposits on Earth. Other evidence for early microbial life comes from studies of chemicals left by microbial activities in early sedimentary rocks. These chemicals are found in organic materials, called kerogen, deposited in ancient rocks. The Ishua formation in Greenland, which is more than 3.5 Ga old, is one of the oldest sedimentary deposits known. Over the long period of time following the deposition of organic carbon by microorganisms, the organic matter was altered considerably to form the kerogen. Geochemists who have examined the Ishua kerogen note that it has a significantly higher ratio of 12C to 13C than does the associated inorganic carbon from the same strata. This is indicative of a biological process that deposited organic material that was eventually transformed to kerogen. This dates the biological process to 3.5 Ga ago. How can the occurrence of high concentrations of 12C in the kerogen be attributed to biological activity? Here is the reasoning. Some organisms, called autotrophs, use carbon dioxide as a carbon source for growth and from this produce organic cellular material called biomass. These organisms selectively use 12CO2 in preference to its heavier, stable isotopic form, 13CO2, which is also present in the environment. As a result, by a process called isotopic fractionation, the biomass becomes enriched in the lighter isotope (12C) leaving behind the heavier isotope in the environment. Determination of the relative amounts of 12C and 13C isotopes in the kerogen and inorganic carbon deposits of a sample can therefore be used to determine whether biological activity is involved in geochemical processes (see Chapter 24). Thus, both the fossil and the geochemical evidence suggest that microorganisms originated on Earth within a billion years of its formation. In fact, during the 3 billion years between 3.5 to about 0.5 Ga ago, living mat and stromatolite communities covered vast areas of intertidal zones on the planet and were likely the dominant feature of life on Earth. Mat communities are still common in intertidal areas, but the columnar stromatolites are much rarer. Presumably the evolution of predatory animals led to the selection of organisms that preyed on the microorganisms in stromatolite communities, and this led to the demise of these microorganisms in many areas on Earth. So, except in special environments such as Shark Bay, with its high salt concentration that is inhibitory to predators, columnar stromatolites have disappeared. Origin of Life on Earth Early fossils provide evidence that microbial life existed on Earth within a billion years of its formation, but we have many questions about this early period. How did life originate? What were the first forms of life? What were the conditions on Earth that permitted the origin of life? These are important and intriguing questions, but they cannot be answered by direct observation. Nonetheless, from what we know of life and the early history of the planet, the process can be partially reconstructed. For example, we know that life cannot exist without liquid water. This means that, at the time life originated, the temperature somewhere on Earth must have been between 0°C and 100°C (at atmospheric pressure). Furthermore, we know that the atmosphere was anoxic, that is, without free oxygen gas (O2). Oxygen could not have formed chemically in any great amount, EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE (A) 7 (B) (C) Figure 1.3 Microbial mat communities Four layers of photosynthetic organisms are visible (from top to bottom): cyanobacteria, two layers of purple sulfur (of different species), and green sulfur bacteria. (D) (A) This marine intertidal community in Massachusetts, called Sippewisset Marsh, contains a microbial mat community. Some areas are sectioned off by ribbons for research purposes. (B) The mat community just beneath the surface is made visible by cutting through the upper layers of the sand using a razor blade, shown here to provide a size scale. (C) A vertical section of the mat showing the four layers of photosynthetic microorganisms. Each layer is about 1 mm thick. The Sippewisset Marsh mat forms during the summer months; winter storms disrupt it, and a mat re-forms the next summer season. Other mat communities remain stable for many years, such as this one (D) at Laguna Mormona (Laguna Figueroa), Baja California del Norte, Mexico. A–C courtesy of Beverly Pierson; D, courtesy of William Schopf. Multiple years of bacterial buildup are visible. The green surface layer contains living cyanobacteria. and certainly it did not make up 20% of the atmosphere as it does today. Another precondition for the origin of life is the presence of organic compounds. It is inconceivable that cells could have originated de novo in the absence of organic compounds, which are part and parcel of all living organisms and biological processes. Thus, an important question is, can organic compounds such as sugars and amino acids be produced in the absence of organisms, that is, abiotically? The first experiments to address this question were conducted by Stanley Miller in 1953. He constructed an apparatus for the interaction of a mixture of gases thought to be present in Earth’s early atmosphere. The experimental device mimicked prebiotic conditions (Figure 1.4). The sterile apparatus contained 500 ml of water, representing the “ocean,” and an “atmosphere,” consisting of an anoxic gas mixture of methane, hydrogen, and ammonia. The water was boiled, and steam rose into the atmosphere to mix with the gases. A condenser subsequently cooled the gases to produce liquid water, that is, “rain.” Miller included as a source of energy a 60,000 volt spark discharge that represented lightning in the atmosphere. The gases and water were recirculated and the anoxic process was run continuously. In a matter of a few days of operation, Miller’s apparatus yielded a dark tarry liquid. This material was analyzed and found to contain, in addition to tarry hydrocarbons, a variety of other organic compounds such as glycine, alanine, lactate, glycolate, acetate, and formate, 8 CHAPTER 1 (A) (B) 2 Steam joins gas mixture containing CH4, NH3, and H2. 1 Steam produced in boiler passes into the spark chamber. Spark chamber 80°C 3 Spark discharge mimics the effect of lightning. Figure 1.4 Diagram of Stanley Miller’s apparatus (A) Stanley Miller shown observing his apparatus for generating organic material. (B) Using this apparatus, Miller and others produced organic compounds from inorganic sources. Photo ©Roger Ressmeyer/CORBIS. Condenser Boiler as well as smaller amounts of other organic compounds. Thus, organic materials were formed under anoxic, abiotic conditions that resembled those found on the early Earth. However, we know that the gas mixture used by Miller does not best represent that of the early atmosphere; similar experiments have been conducted by other investigators, who used gas mixtures with compositions more closely resembling those of the atmosphere of early Earth. These are the gases, called fumarolic gases, that are released from Earth’s hot mantle by volcanoes. In addition to the gases and water that Stanley Miller used, the fumarolic gases include large amounts of carbon dioxide, nitrogen, sulfur dioxide, and hydrogen sulfide. Ultraviolet light, which was intense on early Earth, has been successfully used as an alternative to Miller’s spark discharge as an energy source. In addition, volcanism was more prevalent on early Earth, because the nuclear reactions in its interior core produced more heat than they now do. Thus, the heat from within Earth’s crust would have influenced many of these early reactions. In all experiments in which conditions were anoxic, as they were on early Earth, organic compounds similar to those found by Miller were synthesized. The overall results of these Miller-type experiments indicate that organic compounds can readily be synthesized from inorganic compounds under conditions that resemble Earth’s prebiotic environment. However, we also know that organic compounds are synthesized in intergalactic space. These organic compounds, including amino acids and polycyclic aromatic hydrocarbons, would have been brought to Earth by comets and meteors. Thus, a large variety of organic compounds would have been present on Precambrian Earth in the so-called primordial soup. Heat 4 Water and gases recirculate through the apparatus; reaction products condense in the collecting chamber. We now realize that it is unlikely that life originated and evolved in shallow aquatic habitats, because these habitats would have been continually susceptible to destruction during the period of heavy bombardment. Many scientists now believe that life evolved either in deep sea environments such as hydrothermal vents (see Chapters 24 and 25) or in subterranean environments, because these environments were less likely to be destroyed by asteroid impacts. The most difficult questions still remain unanswered. How did the first cell originate? What were its characteristics? Was the first cell a progenitor of all life. TRACING BIOLOGICAL EVOLUTION How can we trace biological evolution? Two approaches have been used. The first is to look at the fossil evidence for microorganisms in sedimentary deposits. This approach, discussed earlier in the chapter, requires the examination of sedimentary rocks for evidence of fossilized microorganisms or their chemical traces or for evidence of their geochemical activity. The second approach is to construct an evolutionary tree based on knowledge about current living organisms. This is accomplished by analyzing the sequences of the monomers (smaller units) of large molecules called macromolecules such as deoxyribonucleic acid EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE (DNA), whose monomers are purines and pyrimidines, or protein, with amino acid monomers. The sequences in these macromolecules provide the necessary information to trace the evolutionary history of organisms, as discussed in greater detail in Chapter 17. However, before further discussion of the evolution of life, we need to provide some background on the characteristics of organisms that live on Earth today. The first characteristic of all organisms is that they are composed of one or more cells, this is the cell theory of life. Cell Theory: A Definition of Life Microorganisms can be divided into four groups on the basis of form and function: bacteria, fungi, and algae and protozoa (protists). Like plants and animals, all microorganisms consist of one or more cells. Or to put it another way, if something is living, it must be cellular. Viruses, also discussed in this book, are not cellular and therefore they are not regarded as living organisms. Nonetheless, they are important biological agents that develop only as intracellular parasites of organisms, including microorganisms. The cell is the fundamental unit of organisms and has characteristic functional and structural features. These functions include metabolism, the chemical reactions and physical activities by which cells obtain and transform energy and synthesize cell material for growth. Metabolism is accomplished by biochemical reactions catalyzed by proteins called enzymes. The other basic function of cells is reproduction, the process by which cells duplicate themselves to produce progeny. Cells have three major groups of structural components (Figure 1.5): 9 • A central nuclear area that contains deoxyribonucleic acid (DNA), the hereditary material that is duplicated during reproduction. • A cell membrane, or plasma membrane, the boundary between the cell’s cytoplasm and its environment. The cell membrane consists of lipids and proteins. Many microorganisms also contain a layer external to the cell membrane that is referred to as the cell wall, a rigid structure that confers shape on the cell. All fungi have cell walls, as do most algae and bacteria. Most protozoa lack cell walls, so their bounding structure is the cell membrane. The chemical composition and structure of the cell walls of microorganisms differ from one group to another. Chapter 4 covers these structures and their functions in greater detail. Unlike plants and animals, which are all multicellular (containing millions of cells), many microorganisms consist of a single cell and are therefore called unicellular. Most but not all bacteria and protozoa are unicellular. Only one group of fungi is unicellular—the yeasts. Plants and animals are macroscopic; they contain many cells organized into tissues and organs, neither of which are found in microorganisms. The Tree of Life The macromolecules that have been most useful in tracing evolution are found in the ribosomes, which are responsible for protein synthesis in all organisms, microorganisms as well as plants and animals. The ribosome is a complex structure containing RNA and protein (see Chapter 4). Studies of ribosomal RNA (rRNA) molecules indicate that they have changed very slowly during evolution. Because of their highly conserved nature and universal occurrence, rRNA molecules have been • The cytoplasm, the aqueous fluid of the cell in which used in the study of the evolutionary relatedness among most of the enzymatic and metabolic activities occur. organisms. The 16S and 18S rRNA molecules are the most For example, ribosomes, small structures responsible commonly used (where S refers to the Svedberg unit, for protein synthesis, are located in the cytoplasm. which relates to the mass and density of a molecule). As a consequence of these studies, three major domains of organisms are now recognized by biologists: the Bacteria, the Archaea, and the Eucarya (Figure 1.6). Cell membrane: lipid and protein layer The Bacteria contains many of the comsurrounding the cytoplasm. In cells lacking Cell wall: rigid outer layer of the cell, cell walls (some microorganisms, all animal of varying chemical composition. mon microorganisms encountered in typcells), it is the boundary between the cell It is found in many microorganisms ical soil and aquatic environments and and its surroundings. and all fungi and plants. includes those that are known to cause disease. The Archaea comprises a separate group of microorganisms, some of which live in saturated salt environments or Ribosomes Figure 1.5 Cell structure Nuclear material: the hereditary material, DNA. In most cells (but not bacteria) the DNA is contained within a membrane. Cytoplasm: contains organelles, enzymes, chemicals. It is the site of most cellular metabolic activity. The diagram shows the four major components of a typical cell: cell wall, cell membrane, cytoplasm, and nuclear area. 10 CHAPTER 1 BACTERIA Bacteria that gave rise to chloroplasts. Spirochaetes ARCHAEA Cellular Acellular slime slime molds Red algae Entamoebae molds Animals Fungi Chloroflexi Actinobacteria Firmicutes Planctomycetes Verrucomicrobia Chlamydiae Cyanobacteria Proteobacteria Chlorobi Bacteroidetes EUCARYA Euryarchaeota Crenarchaeota Korarchaeota Deinococci Thermotogae Aquificae Plants Heterokonts Ciliates, Dinoflagellates Amoeboflagellates Parabasalians Microsporans Diplomonads Bacteria that gave rise to mitochondria. Figure 1.6 Tree of Life This diagram shows the evolutionary tree of various groups of organisms based on 16S and 18S rRNA sequence analysis. The two prokaryotic domains are the Bacteria and Archaea. All eukaryotic microorganisms are placed in a separate domain, the Eucarya, along with the plant and animal “king- doms.” The Eucarya contains many “kingdoms” of microorganisms, including the fungi and various protists. The Bacteria and Archaea also contain many “kingdoms,” which in this book we call phyla. The Bacteria contains at least 30 phyla, many of which have never been studied in the laboratory. high-temperature environments. The Eucarya contains the microbial groups fungi, algae, and protozoa as well as plants and animals. The major differentiating characteristics among these organisms are shown in Table 1.2. It is noteworthy that the three-domain system is the first truly scientific classification of life (see Chapter 17). Biologists have known for a long time that the cell types of the Eucarya are structurally different from those of the Bacteria and Archaea; the cells of the Eucarya are called eukaryotic and those of the bacteria prokaryotic. In the next section we discuss the differences between eukaryotic and prokaryotic cell structure, or morpholo- gy, and compare other major features of eukaryotic and prokaryotic microorganisms. PROKARYOTIC VERSUS EUKARYOTIC MICROORGANISMS When examined under the light microscope, bacteria appear different from eukaryotic microorganisms (Figure 1.7). Bacterial cells are usually very small and have no apparent nucleus. In contrast, cells of algae, protozoa, fungi, plants, and animals are typically much larger and have a distinct nucleus. These differences noted by observations with the light microscope are borne out by more detailed examinaTable 1.2 Major differentiating characteristics of the three tion using the electron microscope. domains of life Microorganisms can be sliced into very thin sections and examined at Bacteria Archaea Eucarya high magnification with the transmisNuclear membrane No No Yes sion electron microscope (TEM) (see Plastids No No Yes Chapter 4). When viewed in this manPeptidoglycan cell walls Yesa No No ner, the structural differences beMembrane lipids Ester-linked Ether-linked Ester-linked tween bacteria (Figure 1.8) and other Ribosome size 70S 70S 80S microorganisms (Figure 1.9) are strika ing. Bacteria have a much simpler cell Three bacterial groups, the chlamydia, planctomycetes, and mycoplasmas, lack cell wall peptidoglycan (the structure of this material is discussed in Chapter 4). structure and are referred to as pro- EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE (A) Prokaryotes The nuclear material of bacteria is dispersed in the cell and is not evident under the light microscope. Bacillus megaterium Escherichia coli (B) Eukaryotes The nuclear material is surrounded by a membrane, forming a nucleus, which is clearly discernible. Saccharomyces cerevisiae Amoeba 11 Figure 1.7 Drawings of representative microorganisms, as they appear by light microscopy The two examples of bacteria are a large rod, Bacillus megaterium, and a small rod, Escherichia coli. The eukaryotic organisms are an amoeba (a protozoan), a yeast (Saccharomyces cerevisiae), and an alga (Chlamydomonas nivalis). Note the cup-shaped chloroplast and the two flagella of C. nivalis. Chlamydomonas nivalis karyotic (from the Greek meaning “before nucleus”) organisms. In contrast, algae, fungi, and protozoa are called eukaryotic (“good nucleus” or “true nucleus”). Table 1.3 lists the major differences between these two basic types of cellular organization. As the terms imply, the single major difference between these two cell types is related to their nuclear material (Box 1.2). The nucleus of the cell of a eukaryotic microorganism (as well as plants and animals) is bounded by a membrane referred to as a nuclear envelope or nuclear membrane (Figure 1.9). In bacteria or prokaryotic organisms, the nuclear material, which appears as a central fibrous mass in thin sections, is not bounded by a membrane but is in direct contact with the cytoplasm (Figure 1.8). Other differences exist between prokaryotic and eukaryotic cells, some structural, others genetic and physiological. The nuclear material of prokaryotes typically consists of a single type of DNA molecule, called a chromosome. More than one copy of it may be present, depending on how fast the organism is growing. Thus, CM NM N W M Figure 1.8 Cross section of a bacterial cell This electron micrograph of a thin section of Sporosarcina ureae shows the cell wall (W), cell membrane (M), and nuclear material (N), which appears as fibrous matter dispersed in the cytoplasm. ©T. J. Beveridge/Biological Photo Service. M Figure 1.9 Cross section of a eukaryotic cell 10.0 µm A protozoan of the genus Acanthamoeba, showing the cell membrane (CM), nuclear membrane (NM), and mitochondria (M). Courtesy of T. Fritsche. 12 CHAPTER 1 Table 1.3 Major differentiating characteristics of prokaryotes and eukaryotes Characteristic Prokaryote Eukaryote Nuclear structure and function Nucleus with membrane Chromosomes Mitosis Sexual reproduction No One No Rare; only part of genome involved No Yes Two or more Yes Common; all chromosomes involved Yes occurs (Figure 1.10). This elaborate physiological and morphological orchestration does not occur in prokaryotes. Other Morphological Differences Ribosomes appear as granules (about 5 nm in diameter) in the cytoplasm. Prokaryotic ribosomes are called 70S ribosomes. Eukaryotes, with rare exceptions in some Meiosis protozoa, have slightly larger 80S Cytoplasmic structures ribosomes (see Chapter 4). MoleMitochondria No Yesa cular differences in the RNA and Chloroplasts No Yes (if photosynthetic) protein of ribosomes account for Ribosomes 70S 80Sb the differences in size. Typical cell volume <5 µm3 >5 µm3 One of the striking features of a eukaryotes is their organelles, A few lack mitochondria. b Some rare, primitive eukaryotic microorganisms have 70S ribosomes. small structures in the cytoplasm. There are several types of organelles. All are distinct compartrapidly growing cells might have two or four copies of the ments surrounded by one or more membranes, which, like the cell membrane, contain both protein and lipid. DNA molecule, but all copies are identical. Some bacteria The most common organelle of this type, found in also have nonchromosomal DNA in their cells called plasalmost all eukaryotic cells, is the mitochondrion (Figure mids, which are discussed in greater detail in Chapter 15. Plasmids are smaller than the chromosome but contain 1.11). The mitochondrion is the site of respiratory activgenes that are often significant to the bacterium. ity in eukaryotes. Mitochondria have their own interIn contrast, the nucleus of eukaryotic organisms connal DNA, cytoplasm, and ribosomes. One exciting fact tains many separate chromosomes, each with its own of cell biology is that the DNA of the mitochondrion is genetic material. Thus, bacteria can be regarded as typsimilar to prokaryotic DNA, that is, it has no nuclear ically having a single chromosome and eukaryotic envelope. Furthermore, the ribosomes of the mitomicroorganisms as having more than one chromosome. chondrion are 70S in size, like those of prokaryotes. To ensure orderly, accurate, and precise delivery of their These features and other lines of evidence (see multiple chromosomes during the process of cell divi“Evolution of Eukaryotes” on page 17) suggest that the sion, eukaryotic organisms undergo mitosis. In this mitochondrion evolved from a bacterium that develprocess, each chromosome replicates and aligns along oped a close interdependence or symbiotic association the division axis of the cell before asexual cell division with another cell over 1 billion years ago. Milestones Box 1.2 Separating the Organisms of Earth into Two Categories on the Basis of Cell Structure Although his views were largely ignored in the 1930s, the French biologist E. Chatton noted the differences in cellular structure between “higher” and “lower”forms of life. He coined the terms “eukaryotic”and “prokaryotic”based on his light microscopic observations of the differences between the cells of higher organ- isms and bacteria (see Figure 1.7). Only after the invention of the electron microscope (in the late 1930s) and the subsequent development of appropriate procedures to thin-section organisms (1950s to 1960s) did other biologists confirm the detailed differences between these two types of cellular organization. In addition to these morphological features, a number of other differences were also discovered that permitted the clear distinction of these two types of cells. The major features that distinguish prokaryotic from eukaryotic cells were eloquently stated in an important publication by Roger Stanier and C. B. van Niel in 1962. EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE Interphase 13 Prophase Metaphase Anaphase Telophase Interphase The cell has two sets of chromosomes (shown in two different colors), one from each parent. Each of the four chromosomes has replicated; the two “daughters” of each chromosome are still joined together. The chromosomes align along the central axis of the cell. The two daughter chromosomes separate. The cells divide. The two daughter cells are now ready to replicate their own chromosomes again to repeat the process. Figure 1.10 Mitosis In mitosis, a dividing eukaryotic cell duplicates its chromosomes and distributes one copy to each of the newly forming daughter cells. This particular cell has two sets of chromosomes. The chloroplast is the site of photosynthesis in eukaryotes. Like the mitochondrion, the chloroplast is a membrane-bounded organelle found in the cytoplasm. It also resembles the mitochondrion in that its DNA has no nuclear envelope and its ribosomes are 70S in size. However, unlike mitochondria, it also has internal membranes containing the chlorophyll pigments involved in photosynthesis. Chloroplasts are found in algal and plant cells. They, too, are thought to be derived through an evolutionary process from a prokaryotic organism, in this case an organism from the photosynthetic group called the cyanobacteria (see Chapter 21). The organelles of motility of eukaryotic cells—the flagellum and cilium—are larger and more complex than the flagellum of prokaryotes. A cross section of the eukaryotic flagellum reveals an elaborate fibrillar system called the “9 + 2” arrangement, with nine outer doublets of fibrils called microtubules and an inner pair (Figure 1.12). In contrast, the prokaryotic flagellum has a single fibril when viewed in cross section; the thread is of such a fine diameter that a single flagellum cannot be seen by light microscopy. Eukaryotic flagella and cilia, in contrast, are readily observed with the light microscope (see the alga in Figure 1.7). Figure 1.11 Mitochondrion Electron micrograph of a mitochondrion from a eukaryotic microorganism, showing the outer membrane, the inner membrane folded into cristae, and the enclosed fluid, the matrix. ©Barry F. King/Biological Photo Service. 14 CHAPTER 1 (A) (B) Inner microtubules Outer microtubules Plasma membrane The arrangement of two central microtubules surrounded by nine pairs of microtubules (9 + 2) is characteristic of eukaryotic cilia and flagella. Figure 1.12 Eukaryotic flagella (A) This electron micrograph is a cross section through a eukaryotic flagellum. (B) A bacterial flagellum has a very different structure: its single fibril is smaller than one of the microtubules shown here. Photo ©W. L. Dentler/Biological Photo Service. Reproductive Differences All prokaryotes reproduce by asexual cell division. Cells simply enlarge in size, replicate their DNA (i.e., produce a second identical copy of their DNA), and divide to form two new cells, each containing a copy of the DNA molecule (Figure 1.13). Thus, prokaryotes have only one copy of DNA and are called haploid. Sexual reproduction is relatively rare in prokaryotes. Though many bacteria are able to exchange genetic material between mating types, this is not known to be a universal characteristic. As discussed later (see Chapter 15), this rarely results in the formation of a diploid cell, with one copy of the DNA molecule from each of the mating cells. A diploid cell has two copies of each chromosome, that is, two copies of each DNA molecule. In contrast to prokaryotes, most eukaryotes exist as diploid organisms or have diploid stages in their life cycles. Thus, their cells have two sets of chromosomes, one set from the “male” and another set from the “female” mating types. For example, human body cells have 46 chromosomes. These exist as 23 paired chromosomes. Half, or one set of 23, is derived from the father and the other 23 from the mother. To generate reproductive cells, the number of chromosomes and amount of DNA are reduced by half. Meiosis is the process whereby, for example, the 46 human chromosomes are reduced to 23 in preparation for sexual reproduction. Meiosis results in the formation of haploid mating cells, called gametes—the sperm and the ovum, produced by male and female mating types, respectively (Figure 1.14). During sexual reproduction the gametes fuse together during fertilization to form a diploid zygote (the fertilized egg). Therefore, the zygote contains a full genetic complement from each of the parental mating cells. Sexual reproduction is very common among eukaryotic organisms. Except for haploid gametes, the cells of most higher eukaryotes are diploid. Because prokaryotes contain only a single copy of each gene, genetic studies are much simplified. There are no dominant and recessive characteristics, which means that any genetic change is expressed fully and immediately in progeny cells. In contrast, mutations in eukaryotic cells may not show up in the next generation, because the diploid cells have two copies of each gene. Thus prokaryotes are model organisms for the study of genetics. Cell Size: Volume and Surface Area As mentioned earlier, cell size is an important characteristic for an organism. Most eukaryotic organisms have larger cells than prokaryotic organisms (Figure 1.7)—but there are some exceptions. For example, although typical bacterial cells range in diameter from 0.5 to 1.0 µm, some wider than 50 µm have been reported (Box 1.3). The cells of typical eukaryotes range in diameter from 5 to 20 µm, with most about 20 µm, although some species have larger ones. Specialized cells in multicellular organisms can be much larger. A human neuron can be as long as 1 m. DNA 1 The bacterial cell elongates and the DNA (the single bacterial chromosome) replicates. 2 The cell begins to divide, enclosing one DNA molecule in each new cell. 3 The two daughter cells have identical DNA molecules. Figure 1.13 Prokaryotic cell division Though this process is analogous to mitosis in eukaryotic organisms (compare with Figure 1.10), mitosis involves complex structural features that are absent in bacteria. EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE Interphase Prophase I Metaphase I Anaphase I Metaphase II The original two pairs of homologous chromosomes; each chromosome has replicated. Each pair aligns close together in the center of the cell. While the two chromosomes are paired “crossovers” may occur–an exchange of DNA between the chromosomes. The homologous chromosomes separate. Two cells form, each containing two sets of chromosomes. Anaphase II Figure 1.14 Meiosis Telophase II These daughter cells divide again by mitosis to form four cells, each with half the complement of chromosomes; these haploid cells are gametes, either eggs or sperm. In meiosis, a process occurring in organisms that undergo sexual reproduction, a diploid cell undergoes two rounds of division to form four haploid cells, the gametes. In this case, two pairs of chromosomes are shown. Research Highlights Box 1.3 Although most bacteria are very small, some are amazingly large.The largest bacterium we know of is Thiomargarita. Individual cells of this bacterium can be seen by the naked eye.The bacterium lives in the intertidal area off the coast of Namibia, in southwest Africa. Although it has not yet been isolated in pure culture, Thiomargarita is known to be a sulfur bacterium that lives by the oxidation of reduced sulfur compounds. You Can’t Tell a Bacterium by Its Size Alone! Three cells of Thiomargarita A chain of spherical cells of Thiomargarita is lying next to a fruit fly, indicating their huge (for bacteria) size. Reprinted with permission from Science, Vol. 284, pp. 493–495 ©1999 AAAS. 6 mm 15 16 CHAPTER 1 Many bacteria grow and reproduce at very Figure 1.15 Surface area and volume Hypothetical cubical cells, showing how rapid rates. Some can double in size or in numthe ratio of surface area to volume ber of cells in less than 10 minutes under opti(SA/V) varies with cell size. The larger mal growth conditions. This implies that cell has a much smaller SA/V ratio. The metabolic processes can be extremely rapid in text explains the implications of this. these organisms. The rapid metabolic rate is due in part to the small size of bacteria. Their small size ensures that all the cytoplasm is in close proximity to the surrounding environment from which they derive their nutrients. 10.0 µm 1.0 µm The greatest distance between the cytoplasm Surface area (SA) 600 µm2 6.0 µm2 and the growth environment is only 0.5 µm in 3 Volume (V) 1.0 µm 1,000 µm3 a bacterium with a diameter of 1.0 µm, whereSA/V 6 0.6 as it is 10.0 µm in a eukaryotic organism with a diameter of 20.0 µm. Another way to consider the close spatial relationship between the cytoplasm of a cell and its enviwell known for their ability to use simple sugars and ronment is to calculate the ratio of its surface area to its other dissolved substances as carbon sources. Some volume (Figure 1.15). Let’s assume that a bacterial cell is fungi can also degrade particulate organic materials, cubical, with sides 1.0 µm in length (actually, one such as cellulose, by excreting enzymes that solubilize extreme salt-loving bacterium is a cube!); its surface area the organic material outside the cell; they then transport is 6.0 µm2 and its volume is 1.0 µm3. Thus, the ratio of its the dissolved compounds into the cell. surface area to its volume (SA/V) is 6.0. In comparison, a Like fungi, protozoa are chemoheterotrophic organhypothetical eukaryotic microorganism of the same isms, using organic compounds as sources of carbon shape with 10.0 µm sides has an SA/V of 0.6. This smalland energy for growth. Typical protozoa, which lack cell er value for the eukaryote indicates that it has a tenfold walls, engulf bacteria and other microorganisms in greater amount of cytoplasm per unit of cell membrane much the same way that higher animals eat food. The surface than does the smaller prokaryote. Given that protozoa’s source of food is particulate organic materinutrients for growth must enter the cell by crossing the al; this type of feeding is called phagotrophic. cell membrane, more nutrients are available per unit of As a group, bacteria are exceedingly diverse in their cytoplasm in the prokaryote (6.0) than in the eukaryote nutritional capabilities. Some are similar to algae in being (0.6). The larger SA/V ratio enables faster metabolism photoautotrophic. Others are photoheterotrophic, that is, and growth. they can obtain energy from sunlight but use organic compounds as carbon sources. Most prokaryotic organisms Microbial Nutrition are chemoheterotrophic, deriving both energy and carbon Algae and several groups of bacteria are photosynthetfrom organic compounds. One especially interesting ic, that is, like plants, they obtain their energy from sungroup of bacteria can obtain energy by the oxidation of light. Also, like plants, they use carbon dioxide as their inorganic compounds, such as ammonia or hydrogen sulprincipal source of carbon for growth. This type of nutrifide, and use carbon dioxide as their principal carbon tion, which is based entirely on inorganic compounds, source. This type of nutrition is termed chemoautois referred to as autotrophic (self-nourishing or selftrophic, a nutritional category unique to these specialized feeding). Algae are therefore called photoautotrophic, bacteria. The four principal groups of microbes and their to indicate that they obtain their energy from sunlight types of nutrition are shown in Table 1.4. Microbial nutriand their carbon from carbon dioxide. tion is discussed in more detail in Chapter 5. In contrast to algae, fungi obtain their energy directWith this background in microbiology, we are ready ly from chemical compounds, not sunlight. This type of to address more specifically the early evolution of nutrition is referred to as chemotrophic (chemical feedorganisms. ing). Fungi require organic chemical compounds as their sources of energy and carbon. Such nutrition is termed MICROBIAL EVOLUTION AND heterotrophic (other or different feeding, as distinct BIOGEOCHEMICAL CYCLES from autotrophic) or organotrophic. Thus, fungi are chemoheterotrophic—they use chemical compounds as Although we do not know which organisms were the energy sources and organic compounds as carbon first biological entities on Earth, various theories have sources. Only dissolved organic carbon sources can pass been presented. Most scientists believe that the first through the cell walls of fungal cells. Thus, fungi are forms of life were anaerobic (living in the absence of fpo EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE Table 1.4 Microorganisms and their nutritional types Microbial Group Number of Cells per Organism Cell Walls Algae Protozoa Fungi Usually one, some filamentous One Filamentous, except yeasts (unicellular) Usually one, some multicellular Yes No Yes Photoautotrophic Chemoheterotrophic Chemoheterotrophic Yesa Photoautotrophic, Photoheterotrophic, Chemoautotrophic, or Chemoheterotrophic Bacteria Bacteria and Archaea 17 Nutritional Type a A few bacteria, namely, the mycoplasmas and thermoplasmas, lack cell walls. O2), based on evidence that early Earth was anoxic. Many also believe that the earliest bacteria were thermophilic (heat-loving), living in high-temperature environments such as hydrothermal systems or deep within Earth’s crust where it is hot. Evidence supporting this hypothesis is that the earliest branches in the Tree of Life contain thermophilic Bacteria and Archaea (Figure 1.6; see also Chapter 17). Carl Woese from the University of Illinois calls the progenitor of microbial life the progenote, the prototypical precellular “organism” that gave rise to both the Bacteria and the Archaea, and ultimately the Eucarya as well. The progenote likely had a cell membrane that conferred the ability to concentrate important chemicals and carry out simple reactions. Wolfram Zillig, a German biochemist, has proposed that the Progenote populations must have separated physically, possibly geographically, into two communities early in Earth’s history and that this separation led to the evolution of the two main lines of descent—the Bacteria and the Archaea. However, all theories on the origin of life and the first microorganisms are speculative and will not be addressed in great detail here. Possible Early Metabolic Types The Russian evolutionist A. I. Oparin argued that the initial metabolic type was likely a simple heterotrophic bacterium. He reasoned that autotrophic organisms are inherently more complex, so they would not have evolved first. As he noted, although autotrophs can live on simple nutrients, they are more complex in that they need not only metabolic pathways for the generation of energy but also additional pathways to carry out carbon dioxide fixation, that is, the conversion of CO2 into organic material. In contrast, simple fermentative heterotrophs require only a few enzymes for energy generation, and they could have lived on the organic compounds formed abiotically early in Earth’s history. Others have argued that early life forms might have been hydrogen bacteria, those that obtain energy from the oxidation of hydrogen gas. Both bacterial and archaeal hydrogen users are known. These organisms have simple nutritional requirements. Some grow autotrophically, generating energy from the oxidation of hydrogen gas and using carbon dioxide as a sole source of carbon (see Chapter 8). Furthermore, many are anaerobic and could have existed in an anoxic environment like that of early Earth. Although photosynthetic bacteria may not have been the first organisms, it is believed that they evolved early. The ability to carry out photosynthesis using chlorophyll-type compounds probably evolved shortly after the split between the Bacteria and the Archaea. Several groups of the Bacteria carry out photosynthesis, whereas none of the Archaea produce chlorophyll compounds. The first photosynthetic organisms may have resembled the photosynthetic Proteobacteria or Chlorobia, two of the major lineages of Bacteria; this is consistent with recent molecular evidence on the origin of photosynthesis. These bacteria carry out photosynthesis anaerobically using hydrogen sulfide or elemental sulfur for carbon dioxide fixation (see Chapters 9 and 21): (1) CO2 + H2S → (CH2O)n + S0 (2) CO2 + S0 → (CH2O)n + SO42– where (CH2O)n represents organic material (equations are not balanced). The volcanic atmosphere of early Earth would have been ideal for these organisms, because it provided abundant quantities of carbon dioxide and hydrogen sulfide, the essential “ingredients.” This type of photosynthesis is termed anoxygenic photosynthesis, because it proceeds in an anoxic environment without the 18 CHAPTER 1 production of oxygen gas. Most photosynthesis that occurs today, however, generates oxygen. Cyanobacteria are the only prokaryotic organisms that can carry out this oxygenic (oxygen-producing) photosynthesis. Cyanobacteria and the Production of Oxygen To understand the importance of cyanobacteria in the production of oxygen, we must first review their metabolism. The metabolism of the cyanobacteria is similar to that of the anoxygenic photosynthetic bacteria (see reaction (1) above). However, there is one major difference: cyanobacteria use water in place of hydrogen sulfide as the hydrogen donor. Thus, the overall equation for cyanobacterial photosynthesis is: (3) CO2 + H2O → (CH2O)n + O2 This process is termed oxygenic photosynthesis, because oxygen is produced. C. B. van Niel, a Dutch-born American microbiologist who studied photosynthetic bacteria, noted that the O2 produced in reaction (3) must be derived from the water molecule rather than from the carbon dioxide. He concluded this based upon analogy to reaction (1) for anoxygenic photosynthesis. His hypothesis was confirmed when scientists used radiolabeled water, H218O, to show that the label ended up in the oxygen produced (18O2); the label from C18O2 did not. Thus, the oxygen comes from a reaction referred to as the “watersplitting” reaction. This reaction, which is the key reaction of oxygenic photosynthesis, is found in all cyanobacteria, algae, and plants. The cyanobacteria evolved about 2.5 to 3.0 Ga ago, when Earth’s atmosphere still lacked O2. But we know that by 2.5 to 1.5 Ga ago free oxygen was present in the atmosphere, because this was the time that iron oxides were deposited in geological strata called banded iron formations. The bands of these formations are alternating millimeter-thick layers of quartz and iron oxides, partially oxidized forms of iron (FeO and Fe2O3) that could have been produced only in the presence of atmospheric oxygen. Banded iron formations are not formed today, because the concentration of oxygen in the atmosphere and in the oceans is too high. Instead, contemporary iron deposits form red beds, so named because they contain hematites (Fe3O4), a more highly oxidized form of iron that gives them their red color. From the locations of the iron oxides, it is concluded that the banded iron formations were produced during the period in which O2 was first formed but before significant concentrations accumulated in the atmosphere, that is, between about 2.5 and 1.7 Ga ago. The O2 needed to oxidize the iron is believed to have come from oxygenic photosynthesis first carried out by cyanobacteria. Initially, when the cyanobacteria first began produc- ing oxygen, its concentration in the atmosphere would have remained very low. This is because oxygen is highly reactive chemically and would have combined with the large amounts of highly reduced compounds that existed on Earth at the time. These reduced compounds, such as ferrous iron and sulfides, would have reacted with the free oxygen, preventing it from accumulating rapidly in the atmosphere. Therefore, the oxygen concentration in the atmosphere increased very gradually over the past 2 to 3 billion years to reach its present level of about 20% of atmospheric gases. One of the recent exciting discoveries about cyanobacteria is that some of them can also carry out anoxygenic photosynthesis, as in reaction (1) above. This finding suggests that the cyanobacteria may have evolved from anoxygenic photosynthetic bacteria similar to purple or green sulfur bacteria. Indeed, evidence supporting this comes from molecular phylogenetic studies of the two photosystems of photosynthesis (see Chapter 9), one of which is thought to be derived from a member of the Chlorobi and the other from a member of the Firmicutes, a photosynthetic gram-positive group (see Chapter 21). However, some variant must have evolved that could use water in place of hydrogen sulfide as a reductant in photosynthesis and therefore could split water and carry out oxygenic photosynthesis, as in reaction (3). This important process may have evolved by natural selection when hydrogen sulfide became scarce and water for photosynthesis was abundant. The oxygen produced by cyanobacteria would have been toxic to early life forms. Fortunately, for the reasons noted above, free oxygen would not have been available in the atmosphere for many millions of years after the first oxygenic cyanobacterium began producing it. This lengthy time provided favorable conditions for the selection and evolution of enzymes such as peroxidases, which would protect sensitive bacteria from the oxidizing effects. Impact of Bacteria and Archaea on Biogeochemical Cycles Bacterial groups carry out significant reactions, called biogeochemical reactions, that are crucial to the operation of Earth’s biosphere. Examples of these reactions occur in the great cycles of elements such as the carbon cycle, in which autotrophic organisms fix carbon dioxide to form organic carbon that is recycled back to CO2 by heterotrophic organisms. These reactions are discussed in greater detail later in the book, so we present just a brief summary here. Because of the early evolution of microorganisms, particularly Bacteria and Archaea, they were provided with many energy sources 3 billion years before plants and animals evolved. As a result of this long period of EVOLUTION, MICROBIAL LIFE, AND THE BIOSPHERE evolution, they have diversified into many different metabolic groups that uniquely use unusual growth substrates such as methane, ammonia, hydrogen gas, sulfur, and reduced iron. In addition, several different groups of the Bacteria carry out photosynthesis using light energy. All the biological transformations of the nitrogen cycle can be carried out by microorganisms. Likewise, all the biological transformations of the carbon and sulfur cycles can be carried out by microorganisms. Indeed, these cycles as we know them today were in place 1 to 2 Ga ago, at least 1 Ga before plants and animals evolved. The Bacteria and Archaea continue to carry out unique steps in these cycles, such as nitrogen fixation, that eukaryotic microorganisms, plants, and animals cannot perform. More information on these cycles and the important roles of microorganisms in them is provided in Chapter 24. EVOLUTION OF EUKARYOTES 19 terial ancestor had already developed its cell membrane and this feature was thus incorporated into the Eucarya. Although much of this is speculative, it is interesting to note that as scientists further dissect organisms at the molecular level, they are beginning to infer likely, if not actual, evolutionary events that occurred billions of years ago. 1 Progenotes are spatially separated, and the separated forms evolve independently, one line producing Prebacteria, the other producing Prearchaea, with their characteristic cell membrane structures. Progenotes Prebacterium Prearchaean The origin of eukaryotic organisms is obscure at this Spatial separation time. The most popular hypotheses about eukaryotic evolution stem from the ideas of scientists such as Lynn Margulis and Wolfram Zillig. Zillig has proposed that Protobacterium Protoarchaean with eukaryotic organisms, the Eucarya, evolved through a with ester membrane ether membranes fusion event between an ancestor of the Bacteria and an Fusion ancestor of the Archaea (Figure 1.16). According to this Bacterial Archaea theory, the progenote had a permeable cell membrane lineages lineages (phyla) (possibly protein) that allowed genetic communication (phyla) Preeucarya with with other species. As evolution proceeded, some event, ester membrane perhaps geographic isolation, led to the separation of 2 Fusion between a two different populations—the Prebacteria and the Prearchaean and a Prebacterium gives rise Prearchaea. During this period of separation, the two to a preeukaryote groups of organisms developed different metabolic pat(Preeucarya), which Eukarya with terns and genetic systems. Also at this time, the organevolved into the Eucarya. nucleus isms developed their own characteristic lipid cell membranes, ester-linked fatty acid lipids for the Prebacteria and ether-linked isopranyl lipids for the Prearchaea. According to Zillig, it was at about this time that the Eukarya with Alga with fusion event occurred between these two cell types givmitochondrion chloroplast ing rise to the eukaryotic cell. Si