The Origins of Life on Earth (or, a History of our planet in a week or less)

1. The Origins of Life on Earth(or, a History of our planet in a week or less) Biology 2, College of the Atlantic Spring 2002

2. How old is this planet anyway? • Theories of Origin • Geological and Biological timescales • Phylogeny (and an awful lot of it)

3. How old is this planet anyway? • The Universe is probably ~13 billion years old (Big Bang Theory/Doppler Shift) • Earth is ~4.5 billion years old (begins with cooling of crust/solidification) • Earliest records of life ~3.5 billion years ago • First humans (Australopithecus), 0.005 billion years ago • Discovery of Australopithecus fossils , 0.0000000002 billion years ago

4. The Fragility of Life - Coincidence #1 • Life can only exist within temperatures corresponding to the boiling and freezing point of water • This range is a fraction of the range between absolute zero (-273°C) and the temperature of the sun (106°C)

5. How did life evolve? • Three theories • Creationism • Extraterrestrial origin (Panspermia) • Spontaneous Origin (Coincidence #2)

6. Physical conditions of early Earth - Coincidence #3 • Temperatures in correct range (in general, water in fluid state, carbon compounds non-brittle) • Size of planet retains an atmosphere • Early atmosphere lacked oxygen, therefore highly reductive • High energy bombardment from sun promotes generation of organics

7. Spontaneous origins of life - 4 steps • Abiotic synthesis and accumulation of organic compounds • Polymerization • Aggregation of polymers into nonliving structures (Protobionts) • Origin of heredity

8. Experimental evidence of Spontaneous Origin • Theories of Oparin and Haldane—tested by Miller and Urey—demonstrate formation of organics under conditions typical of early Earth • Polymerization can occur with appropriate substrate • Abiotically produced proteins (proteinoids) self-assemble into Protobionts (selectively permeable membrane)

9. The final key - Heredity • First passage of genetic information probably occurred through short strands of RNA (also autocatalyst, e.g ribozymes) • Mutations cause variation • “Natural selection” of molecular combinations • Origin of DNA

10. Biological time scales • Biological timescales by necessity follow geological timescales • Often, geological events marked by key biological events (mass extinctions/diversifications) • First fossil record of life 3.5 billion years ago (prokaryote), in the Precambrian • Earliest eukaryote ~1.5 billion years ago (endosymbiotic theory)

11. Earth - The Early Years • Late Precambrian saw the first eukaryotic multicellular life • Boundary between Precambrian and Cambrian (580 mya) marked by a rapid adaptive radiation/diversification of marine life (Cambrian explosion) • By the middle of the Cambrian, all of the animal phyla existing today had evolved

12. The drive behind Macro-Evolution • Biological forces: natural selection working in general, but particularly effectively on genes controlling • allometric growth • paedomorphosis • Physical forces • Plate tectonics, leading to formation and splitting of supercontinents

13. The study of evolutionary history: Phylogeny • Modern Darwinian synthesis suggests adaptive radiation from a common ancestor • Concept of phylogeny supported through studies of homology • Traditional classification systems (Linnaeus) are monophyletic, based on homology  parallel or divergent evolution • Some groupings are polyphyletic, with analogous structure  convergent evolution

14. The Kingdom System • Scientists follow various taxonomic systems: Campbell uses the 5 kingdom classification scheme • Monera • Protista • Plantae • Fungi • Animalia

15. Phylogeny recounts the “natural selection” of species (Earth: the Middle Years) • First major extinction at end of the Paleozoic era (the Permian Extinction), probably caused by collision of tectonic plates to form the supercontinent, Pangaea • Pangaea marks the birth of a new era, the Mesozoic (Triassic, Jurassic, Cretaceous) • Mesozoic ends with second mass extinction—the Cretaceous Extinction (impact hypothesis)

16. And now... • Currently in the Recent epoch of the Quarternary period of the Cenozoic era • History may tell of a third mass extinction? • Radically changing planet will continue to apply selective pressure to species

17. Monera : the Pioneers of Life on Earth • The most ‘successful’ group of organisms on the planet • 3.5 billion year history • Although only 4000 species known, the number of extant species is thought to be ~4,000 – 4 x106 • Found in all ecological niches, including some where other forms of life cannot exist

18. The current importance of Monera • In some cases at base of food chain • Vital roles in various elemental cycles • Carbon cycle • Nitrogen cycle • Interactions with human life • Symbiosis (E.coli) • Pathogenic bacteria (physical, exo/endotoxins) • Commercial/Industrial/Scientific uses

19. The phylogeny of Prokaryotes

20. Archaebacteria • Treated either as Domain, or subphylum • Cell plan similar to most primitive prokaryotic fossils • Tend to exist in extreme environments • Smaller group of species • Methanogens (mmm-mmmm!) • Extreme Halophiles • Extreme Thermophiles (Sulfolobus)

21. Eubacteria • More diverse group • Spirochetes (Treponema) • Clamydias (Clamydia trachomatis) • Gram-positive eubacteria (Bacillus) • Cyanobacteria (blue-green alga) • Proteobacteria (E.coli, Salmonella)

22. Structure • Small (1-5 µm) • Of three general shapes • Coccus (pl. cocci), e.g. Streptococcus • Bacillus (pl. bacilli) e.g. Bacillus • Spirillum (pl. spirilla) e.g. Treponema • Cell wall made of peptidoglycan • Leads to gram +/-ve distinction • Some have a capsule, and/or pili, and/or flagella

23. Physiology • Various forms of nutrition • Autotrophs (obtain carbon from inorganic CO2 • Photoautotrophs (energy from sunlight) • Chemoautotrophs (energy from inorganics) • Heterotrophs (carbon from organics) • Photoheterotrophs • Chemoheterotrophs** • Origins of glycolysis, chemiosmosis and cellular respiration

24. Reproduction • Single strand of DNA • No mitosis/meiosis • Only Binary fission • Some sexual recombination through • Transformation • Conjugation • Transduction • Some form endospores

25. Kingdom Animalia Invertebrata

26. What is an animal? • Anatomy, Embryology and Ontogeny • Parazoa • Eumetazoa • Radiata/Bilateria • Acoelomate/Pseudocoelomate/Eucoelomate • Protostomes/Deuterostomes

27. What is an animal? • Likely ancestor is a protist: a Precambrian choanoflagellate • Multicellular, heterotrophic eukaryotes (usually exhibit ingestion) • Storage of energy-rich reserves as glycogen • Lack of cell walls. Unique cell junctions • Unique tissues: muscle, nervous tissue • Unique embryology

28. Embryology • Diploid zygote divides by the mitotic process of cleavage • Formation of blastula followed by gastrulation (creation of gastrula) • Mode of embryological development provides a taxonomic key to invertebrates

29. First taxonomic dichotomy • Asymmetry versus Symmetry divides Kingdom Animalia into sub-kingdoms: • Parazoa (lacks tissues, asymmetrical, “like animals”). Represented by only one phylum: Porifera (sponges) • Eumetazoa (“true animals”, exhibit symmetry, represented by remainder of Kingdom Animalia

30. Porifera (Sponges) • General form: Two layered cup (separated by mesohyl), with porocytes entering into spongocoel, exiting through osculum • Outer layer (epidermis) reinforced by spicules (Si) • Filter feeding by Choanocytes that line endodermis • Transport of materials by Amoebocytes

31. Second taxonomic dichotomy • Radial versus bilateral symmetry • Super-phylum Radiata exhibits radial symmetry. Two phyla include: • Cnidaria (jellyfish, anemones, corals and hydra) • Ctenophora (combjellies) • Super-phylum Bilateria exhibits bilateral symmetry (rest of invertebrata)

32. Bilateral symmetry leads to... Cephalization • Bilateral symmetry implies a directionality to the animal • With movement in a specific direction comes development of sensory equipment at end that encounters environment first • Collection of sensory nervous tissue at anterior end of animal = cephalization

33. Ectoderm Mesoderm Endoderm • Type of symmetry also reflected by embryonic germ layers seen in blastula/gastrula: • Diploblastic • = Radiata • (endoderm/ectoderm) • Triploblastic • = Bilateria • (endoderm/mesoderm • /ectoderm)

34. Cnidaria (Jellyfish, etc.) • Diploblastic, radially symmetrical (i.e. no cephalization) • Phylum characterized by cnidocytes that eject nematocysts • Gastrovascular cavity (GVC), with one opening (mouth/anus simultaneously) • Tentacles to pull prey into GVC • Some species exhibit alternation of sexual and asexual forms (e.g. Obelia)

35. Classes within Cnidaria include: • Hydrozoa (e.g. Obelia, Hydra) • Scyphozoa (jellyfish, e.g. Sea wasp, Lionsmane, Portuguese Man-o-war) • Anthozoa: calcareous secretions build an exoskeleton (e.g. coral, anenomes, Metridia)

36. Third taxonomic dichotomy • Design of body cavity (or lack thereof) characterizes Bilateria • Acoelomates lack a body cavity, e.g. Platyhelminthes (flatworms) • Body cavities • Pseudocoelomates have a body cavity lined by mesoderm-derived tissue on one side only, e.g. Nematoda (roundworms), Rotifera • Eucoelomates (coelomates) body cavity is lined on both sides by mesodermally-derived tissue (everything else upwards)

37. Tissue derived from: Ectoderm Mesoderm Endoderm GVC = Gastrovascular cavity DT = Digestive tract PC = Pseudocoelom euC = Coelom GVC DT DT PC euC Acoelomate Pseudocoelomate Eucoelomate note: true digestive tract first seen in pseudocoloemates

38. Platyhelminthes (flatworms) • Triploblastic, bilateral, cephalized acoelomates that have been flattened dorso-ventrally • No internal transport system • (Use of GVC, with muscular pharynx) • Some species exclusively parasitic

39. Classes of Platyhelminthes include • Turbellaria (planarians, e.g. Dugesia, Planaria). Free-living, aquatic. Movement by cilia, eased by secretion of mucus • Trematoda (parasitic flukes, e.g. Schistosoma) • Monogenea (parasitic flukes) • Cestoda (parasitic tapeworms) n.b. Parasitic forms do not possess GVCs

40. Body cavities have various functions • Cushions internal organs • Independence of movement • Primitive circulatory system • Transport of nutrients and metabolic wastes, gaseous exchange • Basis of hydrostatic skeleton • Helped development of a true digestive tract (phylum Nemertea)

41. Nematoda (Roundworms) • Cylindrical triploblastic pseudo-coelomates • Some are freeliving saprobes - important role in decomposition of dead organic matter • Other are parasitic (e.g. hookworm, Trichinella*) *hmmm, pork chop

42. Protostomes Spiral/Determinate cleavage Blastoporemouth Schizocoelous development e.g. Mollusca thru’ to Arthropods Deuterostomes Radial/Indeterminate cleavage Blastoporeanus Enterocoelous development e.g. Echinodermata and Chordata Fourth taxonomic dichotomy

43. Mollusca • Triploblastic coelomates: body plan divided into three (foot, visceralmass, and mantle) • Mantle may secrete calcareous shell for protection against dessication and predators • Gaseous exchange by gills. In some cases, gills are modified to filter feed • Open circulatory system

44. Classes of Mollusca include • Polyplacophora (Chitons, herbivorous grazers) • Gastropoda (snails and slugs: mostly grazers, but some predators—e.g. cone shells) • Bivalvia (Bivalves: clams, oysters, mussels—filter feeders) • Cephalopoda (shelled—nautilus, or unshelled—squid, octopus)

45. Segmentation • Defined as a system of similar units • Allows specialization of regions along body length • Evolved separately in protostomes (Annelida, Arthropoda) and deuterostomes (all Phyla)

46. Annelida (segmented worms) • Triploblastic segmented eucoelomates • Specialization of body regions (e.g. see digestive tract) • Closedcirculatorysystem • Three classes include: • Oligochaeta (earthworm—Lumbricus) • Polychaeta (marine segmented worms) • Hirudinea (leeches)

47. Arthropoda • The most successful animal phylum ever • Characterized by highly developed cephalization, exoskeleton (made from armor-tough chitin), division of body into head, thorax and abdomen • Open circulatory system, including haemocoel as well as coelom • Modified appendages per segment: first evolutionary development of flight

48. Classification of arthropods is complex, but subphylums include: • Trilobitomorpha (extinct trilobites) • Cheliceriformes (spiders, mites and ticks, scorpions) • Uniramia (insects) • Crustacea (crabs, lobsters, shrimp, copepods)

49. Chelicerates includes the class Arachnida • Simple eyes • Modified appendages into • walking legs (4 pairs) • feeding mouthparts, including pedipalps and fangs (chelicerae), not mandibles • Spinnerets

50. Uniramians • Insects (Insecta), Millipedes (Diplopoda) and Centipedes (Chilopoda). Have compound eyes, mandibles, and sensory antennae • Most insects have developed flight and occupy most ecological niches • Gaseous exchange through spiracles and tracheae • Waxy cuticle to prevent dessication • Other dessication defenses through reabsorption of water from faeces • Some species undergo metamorphosis