Exactly what counts as the oldest fossil evidence of life is somewhat controversial. However, many accept that the very earliest evidence includes structures known as stromatolites, which are made of layers of bacteria-like organisms of various types (Schopf 2000). Details are controversial with such old and often ambiguous material, but the evidence suggests that at least some stromatolites radiometrically date to about 3.85 billion years ago. Because the Earth is only around 5 billion years old, these fossils suggest that cells were already quite elaborate relatively early in the history of life. Furthermore, they attest to the robust nature of the early cellular "designs" because some stromatolites are indistinguishable from microbes alive today. This fact is sometimes used to treat time as if it were compressed, and the cell as we know it as an inevitable or even automatic consequence of biochemistry. Nonetheless, we need to keep things in perspective and not assume that reaching this stage was very early in the history of life on earth: it has been guesstimated that perhaps up to 0.5 billion years were required from the time of the first self-replicating molecules that we might call "life" to the appearance of the cells fossilized in these stromatolites, suggesting that life began on Earth around 4-4.3 billion years ago.
The most striking thing (from the perspective of biologists) that happened in the subsequent 3.85 billion years was the evolution of the much larger and more complex eukaryotic cell and the proliferation of multicellular plant and animal life. Of course, it is difficult to judge by just how much unicellular (or more simple viral) life has changed since its external shapes were first fossilized. As will be seen throughout this book, the elaboration of multicellular life was extensive, but it again and again employed basic mechanisms, and very similar if not identical biochemistry, that presumably were present in early eukaryotic cells, in whatever form, or kind of organism, that first arose.
The classical idea of a universal Tree of Life is based on a single origin of cells and generally divides all the forms of life found today into three main domains: Eucarya, Archaea, and Bacteria. The latter two branches comprise unicellular prokaryotes, and the former, the Eucarya, includes organisms from tiny protists (unicellular nucleated eukaryotes) to the most complex organisms on Earth. Because prokaryotes are less complex than eukaryotes, it had long been thought that the universal ancestor of all life must have been a simple prokaryotic cell, from which eukaryotes descended by adding structures over time.
As discussed in Chapters 1 and 2, current phylogenetic evidence built on DNA sequences from a diversity of genomes suggests that this unitary view of life is at least partially wrong. Primordial life may have consisted of a kind of primitive bacterial biofilm, not a single cell, with genomic material encased in permeable membranes that were not very efficient at sequestering the cell's contents from the external world. Lateral exchange of genetic information between cells and the acquisition of characteristics from anywhere in the biotic mass probably occurred much more readily than is possible between cells with the enclosed genetic systems that subsequently evolved. The primordial cells probably lacked sophisticated translation mechanisms, and it is likely that proteins were simple and short and that replication was inaccurate. This type of simple beginning could account for the evolution of the codon system, of genes by exon shuffling, and of the transition from RNA to
DNA discussed in earlier chapters. Woese calls these primitive cells progenotes (e.g., Woese 1998). According to some views, as we have noted, there was so much horizontal transfer of information among progenotes that the idea of a single ancestral life form cannot be correct. See Figure 2-1 for a tree supporting this view (Doolittle 1999).
Stromatolites are prokaryotic organisms that most closely resemble modern cyanobacteria (Schopf 2000), single-celled, photosynthetic organisms that contain a blue pigment in addition to chlorophyll and that live singly or in colonies. Their descendants or closely related organisms still thrive; cyanobacteria are commonly found as the green scum on a stagnant pond. Interestingly, if indeed these earliest life forms were cyanobacteria, this would mean that photosynthesis had already evolved more than 3.5 billion years ago. Green plants use photosynthesis to synthesize carbohydrates from carbon dioxide and water, using light as the source of energy, thus suggesting that water and carbon dioxide were present very early in Earth's history (Schopf 1992,1994, 2000).
Oxygen is the byproduct of most forms of photosynthesis; various geological formations from around 3.75 billion years ago show evidence that oxygen was already in the atmosphere, probably being produced by cells capable of photosynthesis (Schopf 1992, 2000). This implies that life already existed long before 3.75 billion years ago, perhaps at least another 0.5 billion years before (Schopf 1992). If the Earth was formed roughly 5 billion years ago, this would mean that life began relatively soon after the environment cooled from its extremely hostile early state. Fossil evidence shows that eukaryotic cells had arisen around 1 billion years ago. This is a minimum time estimate, based on the size of the cells in microfossils; the organelles that comprise eukaryotes, which would be definitively diagnostic, generally do not preserve as fossils. This represents a huge gap in time between the appearance of prokaryotes and eukaryotes, but the evidence shows that this evolution happened before, rather than as part of, the evolution of complex differentiated organisms: the first eukaryotic fossils are of apparently single-celled organisms. Given the successful persistence of prokaryotes to the present day, it is curious both that single-celled eukaryotes would evolve and that they would take so long in doing so.
Modern genomic techniques show that eukaryotes are not simply prokaryotes with more complex cellular machinery added on. Indeed, as we have seen in Chapter 2, comparisons of the DNA sequences of different components of eukaryotic organisms, compared with bacteria and archaea, suggest that eukaryotes are chimeric, the result of a fusion between (or at least sharing the intrinsic characteristics of) archae-bacteria, eubacteria, and a cytoskeleton-bearing prokaryote (Doolittle and Logsdon 1998; Katz 1999; Vellai and Vida 1999). Similarities between genes involved in transcription, DNA replication, and translation in eukaryotes and archaea suggest that the genetic mechanisms for these functions in eukaryotes came from archaea; other genes, however, are closer to those of bacteria, and still others are too divergent from either lineage for their ancestry to be determined. Phylogenetic analysis of mitochondria and chloroplasts suggests that these organelles were once free bacteria that entered the cell as the result of an endosymbiosis event, the phagocytosis of a bacterium by perhaps an archaebacterium. An endosymbiotic origin for other eukaryotic organelles has also been suggested (Margulis 1970). There is also recent evidence of other levels of gene transfer, including between eukaryotes and prokaryotes and even between eukaryotes (Andersson et al. 2003; Archibald et al. 2003).
Multicellular organisms appear to have arisen first in the Vendian period of the late Precambrian, about 600 million years ago (Gerhart and Kirschner 1997), and 3-3.5 billion years after the development of the first single-celled organism. The reason for the immense time lag between the development of single and multicellular organisms is not specifically known, but we can make educated guesses, keeping in mind that (like Darwin) we are somewhat blinded by, among other things, the rather minimal fossil record. The standard default assumption is that multicellularity evolved because increased complexity was favored by natural selection for some reason(s). However, any such selective advantage did not mean that single-celled life became obsolete. The extent to which characteristics are shared between contemporary single and multicellular organisms suggests that single celled life was probably pretty much as sophisticated then as it is today. Single-celled organisms predominate on Earth today and are at least as diverse as any kind of multicellu-lar organism.
However, if no inherent advantage is associated with multicellular life, at some time(s) and at some place(s), opportunity(ies) must have arisen at which cells that aggregated gained some sort of local advantage—and we should keep open the possibility that the first instance(s) were due to chance and became assimilated or committed over time, subsequently ramified by selection and drift. Because this did not put single-celled life out of business, a default presumption is that this represented a new kind of niche rather than obsolescence of the old one.
Initially, multicellular life probably occurred by little if anything more than the failure of dividing cells to fully separate or by the adhesion of similar cells. Some primitive multicellular life today is little more than this; bacterial biofilms are an example: they consist of sheets of bacteria that can survive as single cells but that under some natural circumstances can mass together to form a film with different characteristics and different genes expressed compared with their single-celled planktonic counterparts (Prigent-Combaret et al. 1999).
COMMON CELLULAR FEATURES OF MULTICELLULAR LIFE Once cellular differentiation became possible (whether it happened many times or only once), an advantage or some resource was eventually gained by the use of both intercellular signaling and signaling between different kinds of cells. As a general guide to cellular diversity as an index of complexity, prokaryotes manifest a very small number of different cell types. Plants have about 30-40 cell types, and higher animals like ourselves have about 150-200. To achieve whatever advantage there is to becoming complex in this way, cells had to become able to differentiate and, at the same time, to communicate with different cell types. This may be why multicel-lular life took so long after the origins of single-celled organisms to arise. Today, several classes of activity are important across the spectrum of multicellular life. Most of these clearly have their origin in unicellular times.
Cells in multicellular organisms communicate via hundreds of kinds of signaling molecules, using many pathways (not all of which would necessarily be found in the same cell type). Figure 6-5 schematically shows some of these pathways. Most of these molecules are ligands secreted by a signaling cell and received by a specific receptor protein on a target cell; the receptor-ligand binding then activates a response inside the cell. Signal molecules can be secreted by active transport mechanisms through the cell membrane into the extracellular space to be picked up at either short or long range by target cells, or the signal can be tightly bound to the surface of the producing cell and transmitted only to target cells that come into contact with the signal.
Some small signaling molecules, like retinoic acid, thyroid hormone, vitamin D3, and steroid hormones, are hydrophobic and diffuse through cell membranes but then are bound by specific binding proteins, at which time their information is used to activate or repress specific genes. However, most signaling factors (SFs) cannot directly enter a cell because the lipid membrane, being uncharged, is largely impermeable to water-soluble molecules or because the molecules are too large; therefore, the information they carry is transmitted by the binding of the SF to a receptor on the cell surface, which communicates the occurrence of the event to the cell.
Signaling can occur at any distance from a cell, from intracellularly, to a few to thousands of cell diameters distant, or indeed even among distant organisms. A variety of terms are used to indicate the distance relationships, although as usual
Protein filaments cytosol
Epithelial tissue cytosol
■ Protein channels
Figure 6-5. Some types of cell junctions.
■ Protein channels
they are not unambiguous and can overlap. Short-range signaling between adjacent cells is called paracrine signaling. The transmission of signals between specialized sites, called synapses, in the cell for long-range signaling, such as along nerve cells, is synaptic signaling (the role of ion channels in this process was referred to briefly above). Endocrine signaling in the form of a hormone molecule secreted from endocrine cells into an animal's bloodstream or the sap of a plant or from one cell to its neighboring cells or even for intracellular use targets cells anywhere in the organism. Hormonal signaling from one organism to others works through pheromones, specific molecules released into the air or water for which conspecifics have receptors.
These kinds of signaling mechanisms pass messages between different types of cells, but cells can also send signals that are bound by their own receptors or those of the same type of cell. This is called autocrine signaling and is important in development, as it allows all cells of a single type to respond identically to the same differentiation signals. Other signaling is transmitted through shared "pores" among adjacent cells (see below). In this case, whatever the signaling molecule is, and whether it simply diffuses through the pore to the adjacent cell or is actively transported, when inside, it works as it would within the cell in which it was produced. Signaling to neighbors directly via their surface constituents is known as juxtacrine signaling.
A number of different cell-cell and cell-extracellular matrix pores form the foundation of the network of interactions between cells or between a cell and the extracellular matrix. They are of three main classes: occluding junctions, which seal together cells in an epithelial sheet to prevent leakage through the sheet; anchoring junctions; which attach adjacent cells to one another or attach cells to the extracellular matrix; and communication junctions, which regulate the passage of chemicals or electrical impulses between the cytoplasm of two cells. These are illustrated schematically in Figure 6-5.
Perhaps the most important tissue to develop in the evolution of complex animals was the epithelial sheet of cells. Tight sheets of epithelial cells form the skin and line the digestive system, body cavities, organs and glands, and one of their important functions is to serve as a selective sequestering barrier, preventing the leakage of fluids from one side to the other. Occluding or tight junctions help create that barrier by sealing together the cells in the sheet.
Anchoring junctions connect filaments of the cytoskeleton of one cell to that of another, allowing a large number of cells to function as a structural group. They are especially abundant in tissues that undergo stress, such as skin and muscle. Three forms of anchoring junctions have been described: adherens junctions, desmosomes, and hemidesmosomes. A number of genes that code for these structures are known, in part because they are associated with skin diseases, such as skin fragility and carcinomas. Adherens junctions and desmosomes both play roles in cell-to-cell connections, whereas hemidesmosomes help to connect the basal surface of a cell to the adjacent connective tissue.
Gap junctions, chemical synapses, and plasmodesmata are communicating junctions. Gap junctions are found in most cells in most tissues and in almost all animals, both vertebrates and invertebrates. Gap junctions are channels in cell membranes formed by two neighboring cells that allow the two cells to communicate, to share cytoplasmic ions, small regulatory molecules, and macromolecular substrates, selected principally by size. Cells can readily share small molecules, passing them from cytoplasm to cytoplasm via gap junctions, but they cannot share larger molecules, nucleic acids, or proteins in this way.
Gap junctions are gated, that is, sometimes open and sometimes closed (like ion channels), and transient; they do not necessarily exist for the life of the cell. They are important for the normal functioning of organs such as the heart or the intestine, which requires constant calibration of ion concentrations. Gap junctions seem to be important in development as well; the existence of these communication channels allows a group of cells to function as a whole (Kumar and Gilula 1996; Wilson et al. 2000). In very early embryos, cells are electrically coupled to each other, and this is maintained by gap junctions. Figure 6-6 illustrates several signaling mechanisms.
Higher plant cells have a communication mechanism similar to gap junctions called plasmodesmata. These, in fact, are the only form of intercellular communica-
G-protein-linked receptor signaling effector cell membrane
G-protein-linked receptor signaling effector cell membrane
. second messenger
. second messenger
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