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Chapter 8

Chapter 8 - From Single-Celled Organisms to Kingdoms.docx

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Western University
Biology 1001A
Tom Haffie

Biology1001 From Single-Celled Organisms to Kingdoms September 18, 2012  First organisms may have been single-celled methane-producing bacteria that existed more than 3.5 billion years ago  Earliest known fossils = single-celled cyanobacteria that deposited stromatolite reefs 3.5 billion years ago  Methanogens & cyanobacteria modified Earth’s atmosphere, resulting in rising levels of oxygen and declining levels of carbon dioxide  Organisms, speciation & ecosystems are ancient, having arisen at least 3.5 billion years ago  Cells with nuclear membranes & organelles (eukaryotes) arose 2.5 billion years ago  The ability to form organelles with specific functions was a major factor in the origin & diversification of eukaryotes  Organisms were classified into 2 kingdoms (plants & animals) until 50 years ago  50 years ago, using cellular characters, life was classified into prokaryotic and eukaryote domains  About 30 years ago, life was reclassified using molecular evidence into 3 kingdoms: eubacteria, archaea & eukarya  Despite being classified into kingdoms, all organisms area connected by having shared a common universal ancestor and as branches of a Universal Tree of Life (UToL)  Evidence for the UToL: shared fundamental cellular organization, metabolic pathways & genes  Impossible to construct a tree of life for cells with prokaryotic organization because of the horizontal gene transfer throughout the history of life Overview  Prokaryotic cells existed 3.5-3.8 billion year ago  About 2 billion years later, some diversified into eukaryotic cells  Extant prokaryotes: eubacteria & archaebacteria (archaea)  Small, have no nuclear membrane, cytoskeleton or complex organelles, divide by binary fission  Three kingdoms of organisms: Eubacteria, Archaea & Eukaryotes  Hypothesized to have originated from a single common ancestor  Although Archaea are similar in organismal complexity to eubacteria, they share many similarities with eukaryotic cells  All organisms are bound together by 4 essential facts: 1. They share a common inheritance 2. Their past has been long enough for inherited changes to accumulate 3. The discoverable relationships among organisms are the result of evolution 4. Discoverable biological processes explain how organisms arose and how they were modified through time by the process of evolution Kingdoms of Organisms  Organisms are arranged into major groups (kingdoms) using various schemes and types of evidence:  Unicellular & multicellular  Bacteria, plants & animals  Prokaryotes & Eukaryotes Biology1001 From Single-Celled Organisms to Kingdoms September 18, 2012 Two  Over 2000 years ago, life was once classified into two kingdoms: Plantae and Animalia  Assignment to kingdoms was based on structure & function, type of metabolism (photosynthesis in plants), and movement (animals move from one place to another, plants don’t) th  Mid-20 century: life divided into 2 domains: single celled organisms (prokaryotes – 3.5-3.8 bya), and multicellular organisms (eukaryotes 1.5 bya)  Main differences between prokaryotic and eukaryotic cells are the absence of any internal membranous networks like nuclear membranes, organelles, or cytoskeleton in the small prokaryotic cells, and the presence of all three in the generally larger eukaryotic cells  Prokaryotes reproduce by binary fission, eukaryotes by mitotic cell division  Prokaryotes lack membranous structures & organelles found in eukaryotes, including endoplasmic reticulum associated with ribosomes in protein synthesis, a Golgi apparatus as secretory body and mitochondria for energy production  The membrane-enclosed compartments allow eukaryotic cells to isolate enzymes for specifc reactions, confine transcription of DNA to the nucleus, translation of DNA to RNA in the cytoplasm, aerobic metabolism to mitochondria, etc.  The evolution of compartmentalization was a major series of steps in the origin and diversification of eukaryotes  A deep structural homology underlies life as much as do deep genetic and molecular homologies Two Became Five  It is recognized that while these two domains of prokaryotic & eukaryotic life accurately reflect the two types of cellular organization, neither domains are monophyletic branches of the tree of life; they are polyphyletic  Monophyletic: a lineage of organisms that share a single common ancestor  Polyphyletic: groups that have more than one independent origins  Unicellular organisms aren’t always closely related to one another since they aren’t monophyletic  Some unicellular organisms are more closely related to multicellular organisms than they are to other unicellular organisms  Prokaryotic and eukaryotic domains do not reflect these evolutionary realities  1959: Robert Whittaker presented evidence that the two domains were more naturally represented as five kingdoms, one for prokaryotes, and four (protists, fungi, plants & animals) to cover what was previously known as eukaryotes  System lasted for about 25 years Five Became Three  1980s: Carl Woese found that rRNA sequences from archaebacterium were sufficiently differnet from those of other prokaryotes, and that archaebacteria should be placed into a separate domain  Both prokaryote and eukaryote kingdoms were found to be artificial constructs with further molecular analyses (sequencing of entire organismal genomes)  Proposed a classification of life into three kingdoms Biology1001 From Single-Celled Organisms to Kingdoms September 18, 2012  Eubacteria, Archaea and Eukarya  Eubacteria (Bacteria): includes major forms of bacteria & the cyanobacteria  Archaea (Archaebacteria): unicells with cell walls made of different molecules than those found in Eubacteria; often live under more rigorous environmental conditions like hot sulfer springs or extreme salt concentrations  Eukarya (Eukaryota): includes some unicellular organisms (slime molds, ciliates, etc.) and the three groups of multicellular organisms: fungi, plants & animals  The methane-producing marine “bacterium,” Methanocccus jannaschii, can be used to explain why prokaryotes were subdivided  The kingdom to which M. jannaschii was assigned, the Eubacteria, was erected partly for organisms that were united by the lack of histone proteins  However, it was fonud to contain genes coding for histones  Furthermore, 56% of the genome in this “bacterium” was not found in other gouprs of eubacteria, and because of this, organisms similar to M. jannaschii were placed in a separate domain of life: the Archaea, which are more similar to Eukaryotes than they are to Eubacteria  Eukarya includes photosyn
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