Lecture V Eukaryotes and the Cambrian Explosion. I. Early Metabolisms. 1. Heterotrophs: Anaerobic; aerobic; 2. Autotrophs: Anoxic; oxygenic. Some Metabolic Pathways Heterotrophy Pathway Energy Source Initial Product Final Product Anaerobic Respiration Aerobic Respiration Pathway Anoxic Photosynthesis Oxygenic Photosynthesis GLUCOSE PYRUVATE ATP Equivalents Lactic Acid or EtOH + CO 2 2 GLUCOSE PYRUVATE H 2 O & CO 2 36 Carbon Source Autotrophy Hydrogen Source Products CO 2 H 2 S GLUCOSE + SULFUR CO 2 H 2 O GLUCOSE + O 2
II. Stromatolites algal / bacterial mats date to 3000 Mya. 1. Long believed extinct. 2. Survive today in highly saline waters that protect them against invertebrate grazers such as snails. III. Oxygenation of the earth s ocean s atmosphere. 1. Consequence of oxygenic bacterial photosynthesis; 2. Geological evidence is BIFs date to 3500 Mya. 3. One of the greatest ecological crises in earth s history. IV. Eukaryotes. 1. A new kind of cell. a. Chromosomes b. Membrane bound organelles c. Cytoskeleton d. Etc. 2. Lack bacterial cell wall. 3. Date to 1750 Mya. 2
4. Eukaryote Origins Hybrid Theory: a. 4 Steps. b. Loss of bacterial cell wall. c. Pinching off cell membrane to form ER & nucleus. d. Symbiogenesis (see below) all other organelles i. Mitochondria from aerobic bacteria ii. Chloroplasts from cyanobacteria. e. Re-evolution of cell wall in plants. Hybrid origin of eukaryotes imagines initial pinching off (nucleus formation) followed by symbiogenesis (chloroplast, mitochondrion, and possibly peroxisome formation). Chloroplasts and mitochondria contain bacteria-like DNA; peroxisomes do not. 3
5. Eukaryote Origins Symbiogenesis: a. Eukaryotes symbiogenic ab initio. b. Mutualistic, reversible consortium between sulfur-reducing archaebacterium (thermoplasma) & hydrogen sulfide-oxidzing eubacterium (spirochete) became obligate. c. Reversible consortia exist today e.g., Thiodendron. d. Subsequent evolution per Hybrid Theory. First step in the evolution of eukaryotes by symbiogenesis as imagined by Margulis, Dolan and Guerrero. Note nucleus formation and transfer of spirochete DNA (now a flagellum) to thermoplasma nucleus. Persistent karyomastigont is a remnant of initial coordination of spirochete-thermoplasma activity. 4
Serial endosymbiosis theory (SET) as imagined by Margulis. Three symbiogenic events are shown: thermoplasma-spirochete fusion (=> first eukaryote); aerobic bacterium incorporation (=> mitochondria); cyanobacterium incorporation (=> chloroplasts). 5
V. Evolution of the Metazoa. 1. Principal Groups: a. Sponges collar cells embedded in gelatinous matrix. b. Cnidaria two cell layers; blind gut (corals, hydras, etc.) Carnivorous (nematocysts) and symbiotic (corals). c. Ctenophores two cell layers complete gut. No nematocysts. d. Protostomes three cell layers; blastopore becomes the mouth Various worms including annelids, lophotrochozoans (brachiopods, bryozoans, mollusks), ecdysozoans (arthropods and relations). e. Deuterostomes three cell layers; blastopore becomes the anus (echinoderms, chordates) 2. Red dots are the characters (shared derived traits see below) that unite descendant taxa. 6
VI. Principal Body Plans of Contemporary Phyla defined by 1. Number of tissue layers. Germ Layers and Tissues Ectoderm Mesoderm Endoderm Epidermis; nervous System. Muscle, bone, blood; other connective tissues. Mucous membranes lining the digestive tract and respiratory system. 7
2. Type of body cavity. 3. Symmetry 4. Type of skeletal support. a. Internal (vertebrates) b. Exoskeleton (insects) c. Shell (mollusks) 8
VII. Notes on Contemporary Systematics. 1. Traits. a. Ancestral. b. Derived. c. A trait is ancestral or derived depending on the group in question. 2. Difficulties in distinguishing ancestral from derived traits. a. Convergent evolution. b. Evolutionary reversals. 3. Similar traits. a. Traits similar by virtue of common descent are called shared derived traits (synapomorphies). b. Traits similar for other reasons said to be homoplastic (homoplasies). 4. Clades. a. Groups composed of an ancestor and all of its descendents. b. Defined by the presence of one or more shared derived traits. c. To be distinguished from grades of organization. d. By definition, clades are monophyletic groups i. Most recent common ancestor within the group. ii. All descendants of that ancestor also within the group. 5. Other types of groups. a. Polyphyletic derived from two or more groups. b. Paraphyletic some descendants of their common ancestor placed in another group. 6. Many traditional groups are paraphyletic e.g., reptiles. 9
Pink is a paraphyletic group because it includes some, but not all of the descendants of a single ancestor. Yellow is polyphyletic because it includes organisms that do not share a common ancestor. Green is monophyletic because it includes all descendants of a single common ancestor and no others. 10
VIII. Cambrian Explosion 1. Modern phyla appear more or less abruptly near the base of the Cambrian. 2. Precambrian animals known but many of doubtful affinity. 3. Molecular evidence suggests the modern taxa pre-date the Cambrian but fossil evidence has yet to be discovered. 4. One explanation the evolution of hard parts. 5. Causes. IX. Garden of Ediacara. 1. Giant protozoa with internal partitions? 2. No development per living metazoa? 3. Failed experiment? Above. Estimated divergence times for selected metazoan phyla, based on seven genes, with standard errors indicated by shaded bars. From Wray et al. 1996. Left. Tribrachidium heraldicum a bizarre Ediacaran fossil with tri-radial symmetry. 11