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PALEOBIOLOGY
1. INTRODUCTION
1.1 Life on Earth began early. The oldest undoubted fossils are about 3500 million years old, back in the early Archean—and, as you will see in a later section of this chapter, the earliest fossil organisms are very similar to organisms that are abundant and successful today. These organisms, although primitive compared to advanced metazoans (like us), are, in an absolute sense, rather advanced in their physiology. It’s generally agreed that the earliest life, earlier than that represented by the oldest fossils, must have been much simpler and less sophisticated biochemically The clear implication is that life must have evolved much earlier than 3.5 Ga —although it is doubtful if fossils much older than that will ever be found. Why not? Perhaps largely because the sedimentary rocks in which they would have been preserved are no longer around, in pristine, unmetamorphosed condition, for us to scrutinize.
1.2 This section purports to tell you something about the nature and evolution of life in the Precambrian (more specifically, up to the latter part of the Proterozoic; for the rise of eukaryotic organisms in general, and multicellular eukaryotes in particular, late in the Proterozoic, see the following section). For the sake of full disclosure here, I should point out that I am a novice in biology! I feel myself to be especially inadequate when it comes to the biochemistry of metabolic process and the molecular biology of replication and inheritance. With that disclaimer, however, I have attempted here to present to you the basics of the fossil record of the Precambrian and some of its implications for the early evolution of life. If you have a special interest in paleontology, and in particular the earliest life you might consider going into the literature. I have provided a fairly long list of materials, largely review papers by specialists in the field.
1.3 Just to set the stage at this early point in the chapter, Figure 9-1 is a diagram that shows, in a very generalized way, the known distribution of stromatolites and microbial microfossils through geologic time. The clear message from Figure 9-1 is that the Archean fossil record is scanty but real, and the Proterozoic fossil record is far more abundant. The terms used in Figure 9-1, as well as some of the place names, will be elaborated later in this section.
Figure by MIT OCW.
Figure 9-1: Diagram showing the known distribution of stromatolites and microbial rnicrofossils through geologic time
2. THE KINGDOM OF LIFE
2.1 Back when I was a child, the standard idea was that life was divided into the plant kingdom and the animal kingdom. (In fairness to biologists, I should point out that the experts knew then that the situation was more complicated: what to do about fungi, for example?). Progress on deciphering the “tree of life” (that is, the evolutionary development of the various life forms known today, through geologic time, from a presumably common original ancestor) has been truly spectacular in recent years. With the development of techniques for
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PHANERO-ZOIC
2.6 Photosynthesis is a complex set of biochemical processes. The basic nature of all photosynthesis is to start with some relatively reduced compound that’s stable in the natural environment, and oxidize it using solar radiation as the source of the needed energy to drive the reaction, and using certain compounds, like chlorophyll, as catalysts to enable the reaction to happen. The general reaction can be written as
CO 2 + 2H 2 X + light energy → (CH 2 O) + H (^) 2O + 2X
The representative of element “X” in this reaction that is most familiar to you is, of course, oxygen: by photosynthesis, the organism splits the water molecule (which takes a lot of energy) and combines the hydrogen released with carbon dioxide to manufacture carbohydrate cell material (represented here by the simplest form of carbohydrate; the carbohydrate sucrose, heavier but still simple, has the molecular formula C (^) 6H 12 O (^) 6) and free molecular oxygen. That’s called oxygenic photosynthesis (and the organisms that do it are called oxygenic photoautotrophs ). Many organisms do not have chlorophyll and so can’t use oxygen for the “X” in photosynthesis. Many bacteria contain compounds catalytically similar to chlorophyll that enable them to use H (^) 2S, hydrogen sulfide, as the raw material. That kind of photosynthesis, called anoxygenic photosynthesis , produces free sulfur rather than oxygen as the by-product. The organisms that do this are called anoxygenic photoautotrophs.
2.7 Now for the main show of this section. Figure 9-2 is a diagram that shows, schematically, the “tree of life” (which is more properly called a phylogenetic tree —that is, how the major taxa of organisms have developed through time) as it was understood in the early 1990s. (I have not discovered a more recent version.)
Figure by MIT OCW.
2.8 Biologists now agree that there are three fundamental domains of life: eubacteria (formal name: Bacteria); archaebacteria (formal name: Archaea); and eukaryotes (formal name: Eucarya). This was generally known well before the breakthrough in use of techniques of molecular biology to pin down the phylogenetic tree, but the structure of the tree was not well known. All of the “branch tips” shown in Figure 9-2 are represented by living organisms: that’s what makes it possible to trace the lineages back in time. In Figure 9-2, there we are, up along the top, having branched off one of the four major lines of eukaryotes, at about the same time as plants and fungi. Is it some consolation that we are more closely related to slime molds than to bacteria? 2.9 The big mystery in a diagram like that in Figure 9-2 is, of course: what was the common original ancestor, represented by the open line at the bottom of the diagram? It’s extremely unlikely that we will ever find any direct fossil evidence of it in the early geologic record. Another very interesting feature of the phylogenetic tree in Figure 9-2 is that all of the earliest life forms seem to have been hyperthermophilic. As the term implies, a hyperthermophilic organism is one that loves extremely high temperatures , even greater than 100°C. They are
Figure 9-2: The universal phylogenetic tree. The bold lines are hyperthermopluiles
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3.1 What is a Fossil? 3.1.1 A fossil can be defined, broadly, as any evidence of past plant or animal life contained in a sediment or a sedimentary rock. The word comes from the Latin fossilis , an adjective meaning “dug up”. In the early days of geology, the term was applied to any interesting natural object (minerals; pieces of ore; pieces of rock; traces of life) that were, literally, dug out of the ground. The term gradually came to be restricted to materials that give evidence of past life.
3.1.2 (One qualifying note at this point, though: the mere presence of disseminated organic matter in ancient rocks, which might technically fall under the definition of a fossil above, is excluded by tacit agreement. And the average concentration of such organic matter in sedimentary rocks is surprisingly high, something like one to two percent. More later in the course on the role of that organic matter in our energy-hungry modern society.)
3.1.3 There are two kinds of fossils: body fossils and trace fossils. Body fossils are the actual organism or some part of it, or the imprint of the organism or some part of it. Even more abundant than body fossils, however, are trace fossils , which are physical evidence of the life activities of now vanished organisms. Tracks, trails, burrows, feeding marks, and resting marks are all trace fossils. Trace fossils are useful for geologists and paleontologists because certain kinds of organisms, which live in specific environmental conditions, make distinctive traces. When you hear the word “fossil”, you might think of shells or dinosaur bones. These are indeed good examples of body fossils, but there are many other kinds of fossils, including both body fossils and trace fossils.
3.1.4 In relatively young sediments and rocks, the actual body parts of organisms are often preserved. In older rocks, however, the body parts are usually dissolved away, or recrystallized, or replaced by another kind of mineral. Even so, the imprints of the organisms are still preserved, and they can be studied if the rock splits apart in the right place and the right orientation to reveal the imprint. Paleontologists usually collect large numbers of rock pieces and then split them in the laboratory with special mechanical splitting devices to try to find at least a few fossils.
3.2 Fossilization
3.2.1 Two important things to remember about the fossil record are that it’s imperfect and it’s biased. Something like a quarter of a million fossil species have been discovered and described. This represents only a small percentage of the million and a half species known to be living today. There are certainly a lot more fossil species yet to be discovered, but by the same token probably only a third of the species in existence today have been recognized and described so far. But things are not as bad as they might seem, because almost half of today’s species
3. THE NATURE OF FOSSILS
are insects, and only a few thousand fossil insects species are known, because of the difficulty of preserving insects in the fossil record.
3.2.1 It is clear that some organisms are readily preserved and others are seldom, if ever, preserved. Organisms with robust hard shells composed of difficultly soluble material, like calcite, are readily preserved if the shells are buried permanently soon after death of the organism. For the body of an organism to become preserved as a fossil, it must escape destruction, at least in part, both before and after it is buried with sediments. Destruction before burial might result from chemical and/or biological decomposition, or from mechanical effects like abrasion and/or breakage during transport by wind or water currents, or a combination of both. (Fragments produced by breakage and abrasion during transport are among the major constituents of limestones. Technically, such particles are fossils, and sedimentologists tend to call such stuff “fossil fragments”, but they are not beloved of paleontologists because they are not very suitable for figuring out details of body morphology.) Soft-bodied organisms like worms and jellyfish become preserved only under special circumstances, when their freshly dead bodies come to rest in soft, fine mud and are buried immediately. There is a very high probability that any organism on Earth will be either consumed by another organism or decomposed by microorganisms following death. For an organism or body part to become a fossil, it must either live within or be moved to a place where it can be buried and isolated from decay. The more rapid the burial, the less decay and the better the chance of preservation. Burial alone, however, does not guarantee that fossilization will occur, because conditions conducive to decomposition or dissolution often persist to great depths of burial. We have to assume that only a minuscule percentage of organisms become preserved in the sedimentary record.
3.2.2 During the latter part of geologic time, at least, most subaerial environments (those exposed to the open air rather than being underwater) have been fully oxygenated, so the soft tissues of dead organisms, whether plants or animals, are susceptible to decay. Microorganisms like bacteria are especially important in facilitating such decomposition. Many if not most subaqueous (underwater) environments are also oxygenated, owing to the ability of water to dissolve the oxygen of the atmosphere—although the geologic record tells us that that have been times in geologic history when the oceans were largely stagnant, and reducing environments, in which organic matter accumulated in abundance, were widespread.
3.2.3 For organisms to escape decay, burial must be extremely rapid, or the depositional environment must be anoxic (without the presence of oxygen). Some of the best-preserved soft body fossils have been found in deposits that are interpreted to have formed in marine basins in which there is little or no vertical exchange of water, so that the bottom waters are stagnant, but at the same time there is a rain of organic matter from the near-surface waters, the result being anoxic bottom waters. Free-floating organisms that fall to the bottom in such a
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aggregate of quartz crystals, there is a non-vanishing likelihood of at least partial preservation of the organism. The very oldest preserved fossils so far found are of this kind; see a later section.
3.3.4 Many shales, which are derived from freshly deposited mud, are fossiliferous as well, because certain organisms like to live on muddy sea floors. Shales are often rich in trace fossils, but are less so in body fossils except when the chemical conditions during deposition were conducive to preservation rather than decomposition. The best representatives of soft-bodied organisms are from shales, although, frustratingly for paleontologists, instances of such preservation are very uncommon. Many sandstones are fossiliferous as well, although the body fossils in sandstones are usually relatively robust shelly materials, which are not highly susceptible to chemical decomposition. Conglomerates are the least fossiliferous of sedimentary rocks.
4. THE EARLIEST FOSSILS
4.1 Introduction 4.1.1 Before the middle of the twentieth century, the conventional wisdom among geologists (and paleontologists) was that there were no Precambrian fossils. I can remember that era well, when I was an undergraduate geology major in the late 1950s. My best friend, a fellow geology major, once confided to me, when we were students, that he would like to be the first geologist to find a Precambrian fossil: what an advance in science that would be! In fact, however, it was during the 1950s that a few geologists and paleontologists began to discover Precambrian fossils. It was the start of a great reorientation in geological thinking.
4.1.2 There were good reasons why Precambrian fossils, now known to be plentiful in the Proterozoic and present, though still scarce, even as early as the early Archean, were virtually unknown. Geological work and thought were dominated by the geoscience “establishment” in Western Europe and the United States, where, basically just by geological accident, there was a major unconformity below the Lower Cambrian, and the rocks beneath the unconformity were mainly crystalline basement rocks—an unlikely venue for fossils. Moreover, no one until the 1950s seemed to have thought to train the advanced microscopic techniques of the time on unmetamorphosed or only mildly metamorphosed Precambrian sedimentary rocks. When they finally did, they found some spectacularly interesting microfossils.
4.1.3 For what it’s worth, here’s a personal recollection from a now very senior geologist. Once, when I was a undergrad, I went to an evening talk at the Harvard geology department, given by a paleobotanist named Elso Barghoorn. He had been working with a colleague on microscopic examination of a Proterozoic chert unit, in the Canadian Shield, named the Gunflint Formation, an interbedding of chert and volcanics about 2000 Ma old. Together, they discovered a rich fauna
of microfossils, fossilized in the chert. (Apparently, their discovery wasn’t just a matter of serendipity: they reasoned that the most likely kind of rock in which to find fossilized Precambrian microfossils would be an unmetamorphosed old chert.) It hit the audience like a bombshell. I remember clearly that one of the audience, the distinguished chairman of the MIT geology department, and a paleontologist himself, stood up at the end of the talk and opined that this was one of the most seminal moments in his professional career, a milestone in the advancement of geological science.
4.2 Archean Fossils 4.2.1 The fossil record of life on Earth is now known to stretch all the way back to the early Archean. In recent decades there have been many reports of Archean fossils : by one count, as of 1992, 43 categories of supposed Archean fossils, from 30 sedimentary units, had been reported. Of these, almost all have at one time or another been questioned as true fossils. That brings up the question: how does one recognize a given fossil-like object in the sedimentary record as a genuine fossil?
4.2.2 In the case of more recent fossils, during the Phanerozoic, almost all fossils are to some extent at least similar to living descendants. Even when poorly preserved, such fossils are commonly recognized as true fossils. Added to this is the “complexity effect”: the more advanced and complex the body morphology of an organism, the more likely is a fossil representative recognized as organic, rather than as some kind of inorganic feature.
4.2.3 Various kinds of objects, with globular, tubular, or other fairly regular shapes, are known to be inorganic, produced by abiotic processes. Examples are mineralized bubble cavities, mineral dendrites (frond-like growths of mineral crystals on fracture surfaces), and non-biogenic aggregates of fossil organic matter. When a geometrically simple fossil-like feature is found in very old Precambrian rocks, particularly when it is not obviously related to some previously authenticated early fossil, there is a natural skepticism about whether it is really biotic rather than the product of some abiotic process. Added to that are problems associated with contamination by modern unicellular organisms during sample preparation.
4.2.3 Scarcity of early Archean fossils is understandable, even if it is assumed that potentially fossilizable life existed at that time. Probably you have already learned, in some previous course, that the early Archean continents were small nuclei, without broad and tectonically stable cratonal areas. There are not many sedimentary successions in which to search for fossils in the first place, and most of them have suffered some degree of metamorphism since deposition.
4.2.4 The most promising targets for the search for Archean fossils have been in the Archean terranes of southern Africa and western Australia. A
4.2.7 That the fossils are indigenous to the deposit is not an issue here, because the fossils are found within small chert clasts, of sand size, in an enclosing layer of chert. The interpretation is that the chert grains were derived from an even older chert unit and transported mechanically to the site of deposition of the enclosing chert.
4.2.8 What are the phylogenetic affinities of these microfossils? (In plainer English, which known organisms, if any, are they closely related to?) They are very similar in morphology to Proterozoic and modern cyanobacteria. One of the big questions about these oldest known organisms is whether they were oxygenic autotrophs or anoxygenic autotrophs. Given that modern cyanobacteria are universally oxygenic photoautotrophs, that’s strong but not definitive evidence that these Archean cyanobacteria-like microfossils produced oxygen as well. We’ll never know for sure, of course.
4.2.9 One of the uncertainties about early fossils that look much like much younger fossils, or like modern (“extant”) organisms is what the specialists whimsically called the “Volkswagen effect”: A similar exterior morphology, but, hidden inside, substantial improvements in mechanisms and function.
4.2.10 How about other Archean fossils? Does it seem depressing to you to learn that there are only a few other undoubted examples? Slightly younger microfossils similar to those described above, although not as clearly preserved, are also known from cherts of a unit of chert known as the Onverwacht Group, in southern Africa, dated at 3.540 ± 0.030 Ma, and from younger cherty carbonates (2.768 ± 0.014 Ma) from another unit in western Australia. There are several reported microfossils that are currently considered “dubiofossils” (is that term sufficiently self-explanatory?); some of these are probably also fossils. It seems reasonable to conclude that, as the search for Archean fossils continues, more undoubted cases will be discovered—but probably never more than a handful.
4.2.11 In addition to the Archean fossils described or mentioned above, which are true body fossils, several examples of Archean stromatolites are known. Modern stromatolites are thought to be, at least in part, sedimentary features built by microorganisms, and if that is true of the Archean examples, then that adds to the short list of Archean fossils. The problem is that the fossil nature of ancient stromatolites is inferential. The following section tells much more about stromatolites.
“Stromatolites are organogenic, laminated calcareous rock structure, the origin of which is clearly related to microscopic life, which in itself must not be fossilized” (the original definition; Kalkowsky, 1908, translated from the original German) “Stromatolites are organosedimentary structures produced by sediment trapping, binding, and/or precipitation as a result of growth and metabolic activity of micro-organisms, principally cyanophytes” (Walter, 1976) “A stromatolite is an attached, laminated, lithified sedimentary growth structure, accretionary away from a point or limited surface of initiation” (Semikhatov et al., 1979)
Make careful note that the first two definitions are genetic and the third is purely descriptive.
4.3.2 Stromatolites are distinctive sedimentary features that are present in rocks as old as Archean. They can be observed forming in subaqueous environments today. Their origin, their significance for paleoenvironmental interpretation, and their significance for the evolution of life through geologic time has been controversial for a hundred years, and continues to be.
4.3.3 The principal features of morphology of stromatolites are easy to describe, in a general way at least. Stromatolites range widely in shape, from domes and cones with rather regular shape, to fairly regular individual cylinders, to irregularly branched columns. The basic motif is that they grew upward from a number of points or small areas to form an array of convex-upward features of positive relief, separated by low areas. Figure 9-5 (Walter, M.R., Grotzinger, J.P., and Schopf, J.W., 1992, Proterozoic stromatolites, in Schopf, J.W., and Klein, C, eds., The Proterozoic Biosphere; A Multidisciplinary Study: Cambridge University Press, 1348 p. (Figure 6.2.1, p. 254)) should give you an idea of the morphological diversity of stromatolites. (Figure 9-5 contains much more morphological information than we can deal with here; it’s intended only to give you the “flavor” of stromatolite morphology.) The morphological elements of stromatolites range in size from smaller than a decimeter to many tens of meters. Stromatolites are characteristically laminated , typically on a submillimeter to millimeter scale.
4.3.4 The earliest students of stromatolites believed them to be actual organisms, and they gave the various shape categories of stromatolites names in the same way that all organisms, fossil and modern, are given binomial names according to the formal Linnaean naming system. It is now universally recognized, however, that stromatolites are not individual organisms.
4.3 Stromatolites
4.3.1 Here are three partly conflicting definitions of stromatolites:
Figure by MIT OCW.
4.3.8 Where does this leave us? I don’t know. Just because modern stromatolites involve trapping and binding by microbial mats doesn’t automatically mean that ancient stromatolites developed in just the same way, although that’s the reigning paradigm. On the other hand, it’s widely believed that most, if not all, ancient stromatolites give evidence of the existence of microbial biota. Does the clear evolution of the nature of stromatolites reflect evolutionary changes in microbial populations, or gradual changes in such nonbiotic (or only indirectly biotic) things as ocean chemistry? That’s still an unresolved question. 4.3.9 Now to get back to Proterozoic microfossils. The record of body fossils of microbes is far more abundant in the Proterozoic than in the Archean.
5. THE FOSSIL RECORD OF PROTEROZOIC PROKARYOTES
5.1 The fossil record of Proterozoic prokaryotes is very abundant, relative to that of the Archean: about three hundred species have been recognized, from a large number of localities. They are found in both shales and cherts. The fossil record of these prokaryotes becomes abundant and widespread by about 2100 Ma. The great majority of these fossil organisms “are of cyanobacterial affinity” (meaning that they look like modern cyanobacteria, in a general way). In particular, a lot of them look like a particular group of modern cyanobacteria called chroococcaleans, which come in ellipsoidal to coccoidal (i.e., spheroidal) shapes, and are both solitary and colonial; others look like a modern group called 197
Figure 9-6: The distribution of known Archean stromatolite localities
(^0) KILOMETERS 5000 Homolosine Equal Area Projection
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nostocaleans, which are colonial in long, filamentous chains. Figure 9-7; Schopf, J.W., 1992, Proterozoic prokaryotes: affinities, geologic distribution, and evolutionary trends, in Schopf, J.W., and Klein, C, eds., The Proterozoic Biosphere; A Multidisciplinary Study: Cambridge University Press, 1348 p. A) Figure 5.4.1, p. 195; B) Figure 5.4.2, p. 196, shows some representative modern cyanobacteria. (No scale was given in the original figures, but I think that these features are of the order of ten micrometers to a few tens of micrometers in size.)
5.2 Just to calibrate you, slightly, to the nature of these Proterozoic cyanobacteria-like fossils, here are a few comments about modern cyanobacteria. Modern cyanobacteria are unicellular oxygenic photoautotrophs (i.e., they are photosynthesizing plants, with chlorophyll, that produce oxygen during photosynthesis). (Incidentally, cyanobacteria used to be called blue-green algae, but they are not true algae. Algae are eukaryotic plants, only very distantly related to cyanobacteria; see Figure 9-2.) They resemble bacteria, but they differ in that bacteria are not photosynthesizers. (If you go back to Figure 9-2, you can see that both bacteria and cyanobacteria belong to the domain of Bacteria, so they are relatively closely related.) Most are colonial, forming long filaments. They are extremely widespread in their distribution: they are found in hot springs, soils, freshwater bodies, and the oceans.
5.3 Not all Proterozoic prokaryotes are cyanobacteria-like. A small minority of them are more like modern bacteria, and some are classed as “problematica” (meaning that paleontologists don’t know what to do with them). Whatever their taxonomic affinities, however, it seems clear that they underwent very little morphological evolution over extremely long periods of geologic time. It’s possible that their internal anatomy and physiology advanced more than their external morphology during that long time (the “Volkswagen effect” mentioned above): there is absolutely no fossil evidence of their innards. Given the relative simplicity of the internal anatomy and physiology of modern cyanobacteria, however, that seems unlikely. This matter has relevance to the important question of whether these Proterozoic prokaryotes engaged in oxygenic photosynthesis. There is, of course, no direct evidence of that, but there is good circumstantial evidence: the atmosphere became oxygenated in the course of the Proterozoic, and if these prokaryotes were the dominant organisms until late in the Proterozoic, it’s reasonable to assume that they were photosynthesizers. Or were other organisms, which never became fossilized, responsible for the oxygenation? There’s a lesson in all of this: you can see how speculative much of the thought about the Precambrian has to be.
2. THE RISE OF THE EUKARYOTES
2.1 Eukaryotes and Prokaryotes
2.1.1 The appearance of organisms with eukaryotic cells was a major advance in evolution. Eukaryotic cells are much larger than prokaryotic cells, and they are more advanced in structure and physiology. Perhaps the most striking difference between the two kinds of cells (see background section below for more detail than is included in this paragraph) is that in the eukaryotic cell the genetic material is contained within a cell nucleus, a structure contained within the cytoplasm (the term used for the fluid content of the cell) but isolated from it by its own wall. In prokaryotes, on the other hand, the genetic material is dispersed within the cytoplasm. Moreover, eukaryotic cells contain a variety of special structures and bodies called organelles. (The nucleus itself is one such organelle.) The most characteristic of such organelles, besides the nucleus, are mitochondria , in which much of the metabolic activity of cell is carried out, and, in the case of plants, chloroplasts , in which photosynthesis occurs.
BACKGROUND: PROKARYOTIC AND EUKARYOTIC CELLS
1. There are great differences between prokaryotic and eukaryotic cells. Within each kind, of course, there is considerable variety of structure and biochemistry, but a number of fundamental differences reflect the accepted idea that eukaryotic cells are a late (in the context of geologic time, that is) evolutionary advance over prokaryotic cells. don’t interpret that to mean that eukaryotic cells are more successful than eukaryotic ells, though: prokaryotic organisms are highly successful in their diverse ecological niches—swarming all over us and within us throughout our lives! 2. Prokaryotic cells are both smaller and simpler than eukaryotic cells. The typical size range of prokaryotic cells is 0.2 to 2 μm (micrometers, or microns) in diameter, whereas eukaryotic cells are much larger, typically 10–100 μm in diameter. All cells, of course, have a cell wall or membrane of some kind, in order to isolate the contents of the cell from the outer world. Cell walls vary greatly in their composition and structure. Prokaryotic cells actually are more complicated than eukaryotic cells. They consist of a chemically complex macromolecular network, and they are semi-rigid. 3. Both kinds of cell are filled with a material called the cytoplasm. It is the internal matrix of the cell, in which all of the other internal constituents are embedded. It consists mostly of water but contains proteins, carbohydrates, lipids, inorganic ions, and various low-molecular-weight organic molecules. In eukaryotic cells, in contrast to prokaryotic cells, the cytoplasm is characterized by
a complex internal structure that consists of an assemblage of very small rods and tubules which together constitute what is called the cytoskeleton.
4. The fundamental differences between the two kinds of cells lie most distinctively in internal composition and structure. Eukaryotic cells contain several kinds of specialized structures called organelles. The most characteristic eukaryotic organelle is, of course, the nucleus, which contains the genetic information of the cell. In prokaryotic cells, the genetic material is distributed throughout the cytoplasm rather than being encapsulated in the nucleus. 5. Among several other kinds of eukaryotic organelles, two of the characteristic are mitochondria and chloroplasts. Mitochondria are spherical or rod-shaped bodies distributed throughout the cytoplasm. Much of the metabolic activity of the cell occurs in the mitochondria, including synthesis of ATP (adenosine triphosphate), the compound that powers metabolic processes. Chloroplasts, contained in the photosynthesizing eukaryotes (algae and plants) are membrane-bounded bodies that contain the chlorophyll and the enzymes that are required for photosynthesis. Chloroplasts are able to multiply on their own within the cell, by increasing in size and then dividing into two. (There’s a theory, rather generally accepted nowadays, that chloroplasts were once symbiotic photoautotrophic prokaryotes that eventually became an actual internal part of the host cell!)
2.2 How are Eukaryotes Recognized in the Fossil Record?
2.2.1 There has been a longstanding problem—which continues today— about when eukaryotic cells first evolved. The problem is that the interior content of the cells of organisms are extremely evanescent, and are not preserved in fossils. How, then, are we to tell whether a given Precambrian unicellular organism was a prokaryote or a eukaryote? One basis is size: eukaryotic cells are typically much larger than prokaryotic cells. The problem is that this is not definitive, because there is an overlap in the two size ranges. In the view of most paleontologists, this problem is always going to be with us.
2.2.2 The problem is not quite as hopeless as it seems, however., inasmuch as the details of composition of the cell walls of prokaryotes and eukaryotes is significantly different. The cell membranes of eukaryotes contain a class of compounds called sterols , which are complex lipids (a term used in organic chemistry for a wide variety of fats, oils, and waxes) not found in the cell membranes of the prokaryotes. Under the right conditions of preservation (that is, no more than moderate diagenesis, and no metamorphism) the sterols can survive (as what were called organic fossils or biomarkers in the preceding chapter).
