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Indeed, eukaryotic cells are thought to be the descendants of ancient prokaryotic communities. When and where did cellular life begin? What were the conditions on Earth when life began? We now know that prokaryotes were likely the first forms of cellular life on Earth, and they existed for billions of years before plants and animals appeared.
The Earth and its moon are dated at about 4. This estimate is based on evidence from radiometric dating of meteorite material together with other substrate material from Earth and the moon. Early Earth had a very different atmosphere contained less molecular oxygen than it does today and was subjected to strong solar radiation; thus, the first organisms probably would have flourished where they were more protected, such as in the deep ocean or far beneath the surface of the Earth.
Strong volcanic activity was common on Earth at this time, so it is likely that these first organisms—the first prokaryotes—were adapted to very high temperatures. Because early Earth was prone to geological upheaval and volcanic eruption, and was subject to bombardment by mutagenic radiation from the sun, the first organisms were prokaryotes that must have withstood these harsh conditions.
Microbial mats or large biofilms may represent the earliest forms of prokaryotic life on Earth; there is fossil evidence of their presence starting about 3. A microbial mat is a multi-layered sheet of prokaryotes Figure 1 that includes mostly bacteria, but also archaeans. Microbial mats are only a few centimeters thick, and they typically grow where different types of materials interface, mostly on moist surfaces.
The various types of prokaryotes that comprise them carry out different metabolic pathways, and that is the reason for their various colors. The conserved nature of these molecules and their ubiquity makes them extremely useful for inferring evolutionary distances between organisms.
Comparing the sequence of nucleotides in a ribosomal RNA shows how much the molecule has changed in the course of evolution from one organism to another. Change is thought to be a function of time. If a great deal of time elapses between the emergence of organisms, there is more opportunity for the sequences of their nucleic acids and proteins to change.
If two organisms evolve in a shorter space of time, there are fewer opportunities for the sequences to change. By analyzing the nucleotide sequences of ribosomal RNAs from different organisms and determining the degree of similarity or difference between these RNAs, we can reach some conclusions about how closely or distantly the organisms are related.
With the aid of a computer, the sequences can be organized into a phylogenetic tree whose branching order indicates the likely order in which the organisms diverged from some common ancestor. Such phylogenetic trees can help clarify the order of branching among prokaryotes and eukaryotes and assess which organism represents the earliest line of descent among the eukaryotes.
The team in which one of us was a member constructed an evolutionary tree by comparing analogous segments of one type of ribosomal RNA from 54 organisms representing all levels of evolution. By this method we discovered that Giardia lamblia has a ribosomal-RNA sequence that is very close to that of prokaryotic cells.
Thus Giardia is evolutionarily closer to the prokaryotes than other eukaryotes are, and we can take this to mean that Giardia is a member of the earliest emerging eukaryotic lineage. Given that Giardia lacks mitochondria and is necessarily anaerobic, its phylogenetic position provides compelling evidence for Cavalier-Smith's hypothesis that the anaerobic eukaryotes preceded the aerobes.
At the same time, this evidence conflicts with Margulis's position that eukaryotes evolved in response to the buildup of oxygen on the primitive earth. The ability to use oxygen would appear to have arisen later, rather than earlier, in the progression from prokaryote to eukaryote. Valuable Lessons Having established that Giardia occupies an important position in the transition from prokaryotes to eukaryotes, we have been interested in exploring aspects of its life history to see what we can learn about the early eukaryotes.
But interest in this organism predates the discovery of its place on the evolutionary tree. Giardia , a unicellular organism, is a major intestinal parasite capable of infecting a variety of species, including human beings. The parasite can be found in both developed and underdeveloped countries, and causes diarrhea, abdominal cramps, malaise and weight loss. Because of its wide-spread distribution and the potential severity of infections, Giardia has been of interest to parasitologists and epidemiologists for some time.
There are two phases in the Giardia life cycle: trophozoites and cysts. Each cyst contains two trophozoites. Cysts are found in the feces of infected animals. Infected fecal matter can contaminate water supplies, where other animals can ingest the cysts. Inside the stomach, cysts are exposed to digestive acids, which cause the release of the trophozoites.
Once released, a trophozoite attaches to cells of the upper intestine by means of a disc on the parasite's ventral surface. The parasites are thought to remain for a time in the upper intestine, where they feed and replicate.
When they are forced further down the intestinal tract into the small bowel or colon, the trophozoites form cysts. These cysts are then excreted and the cycle is repeated.
Because Giardia has the potential to infect a number of species, the possibilities for transmission are considerable. The Giardia trophozoite lacks many of the subcellular organelles characteristic of higher aerobic eukaryotes. As mentioned before, it has no mitochondria. It also has no apparent Golgi apparatus.
Some investigators report seeing a primitive endoplasmic reticulum, but this claim has yet to receive biochemical support. Giardia has an internal cytoskeleton as well as lysosomes that contain digestive enzymes. Its genetic material is encased in membrane-enclosed compartments.
In our laboratory, studies are carried out using these trophozoites, graceful, tear-shaped cells with four pairs of flagella. Of all the cellular features within the trophozoite, the most puzzling and intriguing is that Giardia has not one nucleus, but two nuclei. Furthermore, these nuclei are equal in size. We have undertaken a study of the dual nuclei in the hope of contributing to a fuller understanding of the evolution of higher eukaryotic cells.
Several other single-celled eukaryotes also have more than one nucleus, but only Giardia and a few closely related organisms making up the group called diplomonads have exactly two nuclei of equal size.
In light of the evolutionarily significant position Giardia holds, we have been most eager to understand the importance of this unusual nuclear configuration. We have been trying to elucidate the structural and functional contributions made by the two nuclei.
Recently, we have shown that the two nuclei are not only equal in size, but also that they contain the same amount of DNA. In addition, using techniques that enable us to see the nucleic acids DNA and RNA under the microscope, we have demonstrated that each nucleus contains four major chromosomes. We were then curious to learn whether the DNA in each nucleus encodes the same information.
Since we know the sequence of one ribosomal RNA within the cell, and we know that the template for this RNA is archived in the DNA, we probed the DNA in each nucleus to see whether each contained this ribosomal sequence.
We found that each nucleus did indeed contain sequences specifying this ribosomal RNA. The mere presence of the sequence in each nucleus, however, does not necessarily mean that each one is used as a template for ribosomal RNA. Thus we sought to establish whether the ribosomal RNA in the cell could have been derived from the DNA in either nucleus. This suggests that the DNA in both nuclei is functionally equivalent and equally likely to serve as the template for the ribosomal RNA that makes its way out of the nucleus and into the cytoplasm.
Additional studies conducted on other multinucleated cells suggest that the DNA in these is not functionally equivalent. For example, in two well-studied single-celled organisms, Tetrahymena and Paramecium , there are unequal-size nuclei. Tetrahymena has one large nucleus and one small nucleus. The smaller nucleus contains two copies of the genome and is therefore diploid. The DNA in this micronucleus is not used as a template, but is passed on to progeny cells.
The small nucleus then develops into a large nucleus, and only then does its DNA assume a template role. Paramecium has a large nucleus and two small diploid nuclei, which are likewise only passed on to future generations. Since the arrangement of nuclei in Giardia is anomalous, even among multinucleated cells, the question arises: What is the possible evolutionary significance of the two equal-size nuclei of Giardia and their contents?
We have already noted that we detect four major chromosomes in each nucleus, and others have reported that there are between four and five major chromosomes in the organism. The amount of DNA in each cell, and its complexity, have been determined. Taken together, these data and other experiments suggest that each nucleus of Giardia is haploid; that is, each contains a single representation of the organism's genetic information.
The entire trophozoite, which contains two nuclei, would therefore be diploid. The diploid state can be greatly advantageous, and most highly evolved, complex organisms have adopted this arrangement.
If a cell has only a single copy of genetic information, any alteration or mutation of that information could result in nonfunctional proteins, with dire and possibly lethal consequences for the cell.
But if the cell has two sets of instructions and one of them becomes nonfunctional, the second set can serve as a backup, and may compensate for the loss of the first. Furthermore, if a segment of the first set undergoes a mutation that provides a new beneficial function, the other copy can still perform the original function. With only one set of instructions, the organism risks losing an existing function to gain a new one. Again, this could ultimately lead to the cell's demise. A diploid organism has the advantage of retaining the old while developing new, advantageous functions.
The putative haploidy of each of the Giardia nuclei is intriguing with respect to the organism's evolutionary importance. It is possible that a single haploid nucleus gave rise to a second identical nucleus, thus giving the entire organism the various advantages of the diploid state. Later in evolution, the two haploid nuclei could have fused to produce the sole diploid nucleus characteristic of most higher eukaryotes.
This hypothetical scenario explains the transition of a haploid prokaryote to a diploid eukaryote. It also predicts that the higher eukaryotes that contain a single diploid nucleus are the evolutionary descendants of a binucleated eukaryote. Bibliography Alberts, B. Bray, J. Lewis, M. Raff, K. Roberts and J. Molecular Biology of the Cell.
Second Edition. Garland Publishing, Inc. New York. Bockman, D. Electron microscopic localization of exogenous ferritin within vacuoles of Giardia muris. Journal of Protozoology Boothroyd, J. Wang, D. Campbell and C. An extracellular matrix composed primarily of polysaccharides holds the biofilm together. During stage 4, maturation II, the biofilm continues to grow and takes on a more complex shape. During stage 5, dispersal, the biofilm matrix is partly broken down, allowing some bacteria to escape and colonize another surface.
Micrographs of a Pseudomonas aeruginosa biofilm in each of the stages of development are shown. Biofilms are present almost everywhere: they can cause the clogging of pipes and readily colonize surfaces in industrial settings. In recent, large-scale outbreaks of bacterial contamination of food, biofilms have played a major role.
They also colonize household surfaces, such as kitchen counters, cutting boards, sinks, and toilets, as well as places on the human body, such as the surfaces of our teeth.
Interactions among the organisms that populate a biofilm, together with their protective exopolysaccharidic EPS environment, make these communities more robust than free-living, or planktonic, prokaryotes. The sticky substance that holds bacteria together also excludes most antibiotics and disinfectants, making biofilm bacteria hardier than their planktonic counterparts. Overall, biofilms are very difficult to destroy because they are resistant to many common forms of sterilization.
Privacy Policy. Skip to main content. Prokaryotes: Bacteria and Archaea. Search for:. Prokaryotic Diversity. Classification of Prokaryotes Prokaryotic organisms were the first living things on earth and still inhabit every environment, no matter how extreme.
Learning Objectives Discuss the origins of prokaryotic organisms in terms of the geologic timeline. Key Takeaways Key Points All living things can be classified into three main groups called domains; these include the Archaea, the Bacteria, and the Eukarya. Prokaryotes arose during the Precambrian Period 3. Prokaryotic organisms can live in every type of environment on Earth, from very hot, to very cold, to super haline, to very acidic. The domains Bacteria and Archaea are the ones containing prokaryotic organisms.
The Archaea are prokaryotes that inhabit extreme environments, such as inside of volcanoes, while Bacteria are more common organisms, such as E. Key Terms prokaryote : an organism whose cell or cells are characterized by the absence of a nucleus or any other membrane-bound organelles domain : in the three-domain system, the highest rank in the classification of organisms, above kingdom: Bacteria, Archaea, and Eukarya archaea : a taxonomic domain of single-celled organisms lacking nuclei, formerly called archaebacteria, but now known to differ fundamentally from bacteria.
The Origins of Archaea and Bacteria Archaea are believed to have evolved from gram-positive bacteria and can occupy more extreme environments. Learning Objectives Distinguish bacteria from archaea in terms of their origins. Key Takeaways Key Points The first prokaryotes were adapted to the extreme conditions of early earth. It has been proposed that archaea evolved from gram-positive bacteria as a response to antibiotic selection pressures.
Microbial mats and stromatolites represent some of the earliest prokaryotic formations that have been found. Extremophiles and Biofilms Prokaryotes are well adapted to living in all types of conditions, including extreme ones, and prefer to live in colonies called biofilms.
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