Lecture 2 Life Origins: Lecture Notes 3

A BRIEF HISTORY OF LIFE!

I. Key Points

  • There is a tree of life and we can’t truly understand ourselves unless we understand our evolutionary history
  • Underlying philosophical understanding is necessary as well as scientific
  • If it is important to conserve nature, we should know how it came to be in the first place
  • fThe links between biology and geology show how careful we should be with the planet
  • Topic of evolution is a hot topic because it is politically charged and extremely controversial (many people in the United States still feel that intelligent design is the proper theory).

III. Early life

  • No planned sequence, unpredictable, and often due to crises
  • Earth was formed 4.6 Billion years ago from condensation of dust and rock around the sun
  • As earth cooled water vapor condensed into oceans
                      • In other words, no water and therefore no life until earth cooled
                      • The make-up of the atmosphere consists of: NH3, h2S, CH4, H2, NOx, N2

Early life hypothesis

  • Hypothesis 1: Primodial Soup
                  • Atmosphere is strongly reducing (ammonia and methane)
                  • 1930s then tested in 50s
                  • Presence of energy (lightning or UV light)
                  • Supported by Haldane and Oparin
  • Hypothesis 2: Deep Sea Vents
                  • The whole atmosphere isn’t as reducing as in the primordial soup hypothesis, but there are highly reducing areas like in deep sea vents where volcanic activity takes place
  • Hypothesis 3: Seeding from Elsewhere
                  • Meteorite comes caring organic compounds
                  • Chondrites are carbon-rich meteorites with amino acids in them
                  • When stromatolites show cyanobacteria’s photosynthesis, it is proof that oxygen was accumulating in the air. Once all the iron was oxidized, the oxygen started to build in the atmosphere, it was traumatic for organisms and some were forced to evolve or where forced into anaerobic areas

IV. Best Answers

  • Miller and Urey Experiment
                    • Flask was the source of water vapor and keeps out atmosphere contaminations
                    • “early earth” molecules were attached and shocked with an electrode to simulate the primordial soup hypothesis
                    • The experiment was successful and produced organic compounds
                    • Since then many experiments have refined the results: UV light has substituted for a spark, etc
                    • In a reducing atmosphere, energy input cause abiotic synthesis of organic molecules
  • Polymers develop
                    • Chains of aminos create proteins from clays and evaporation
  • Molecular clues
              • Molecules of living organisms are rich in carbon compounds containing hydrogen: Suggests little or no free oxygen on primitive earth
              • Only 20 amino acids of the left-handed variety are used by living things in proteins: Suggests a single origin of life
              • DNA and RNA are the universal basis of all life forms: Suggests great advantage of this molecular machinery for reproduction and growth
              • ATP is the universal energy currency of all living things: Suggests a common origin for metabolism
              • In all cells, the first steps of carbohydrate metabolism involve fermentation, and the last steps in aerobic organisms use oxygen in respiration: Suggests that aerobic respiration evolved from anaerobic

V. rRNA is important!

  • Life’s origin requires a molecule that can both store information and catalyze the synthesis of other molecules. RNA can catalyze simple reactions and can help as a template for protein synthesis and for more RNA synthesis. This suggests that RNA was probably the first genetic molecule to start life. Later we suspect that DNA evolved to be a more stable molecule, and proteins evolved to be more efficient enzymes. RNA with catalytic activity is referred to as ribozyme.
  • The first genetic material was probably RNA, not DNA
  • Some RNA molecules act as catalysts (called ribozymes) and can make complimentary copies of short RNA strands
  • Natural selection may have operated on RNA strands of different conformation (phenotype), favoring those with stability and precise replication
  • We might imagine a step from this to the catalysis of polypeptide formation (like rRNA today) inside “protobionts”
  • RNA can also provide a template for DNA synthesis (reverse transcription) – DNA is much more stable and might “out compete” RNA in the early world of replication

VI. Crisis in Innovation

  • Heterotroph (use other organic compounds for energy) versus Autotroph (use inorganic compounds for energy
  • Heterotrophy (consuming organic compounds) almost certainly evolved before autotrophy (producing organic compounds from inorganic materials)
                • Innovation: autotrophy. The earliest autotrophs likely derived their H from H2 or H2S (akin to chemosynthesis by bacteria of deep sea vents)
                • Crisis: the H source became exhausted
                • Innovation: Photosynthesis (using energy of sunlight to cleave H from H20)
                • Crisis: the resulting O2 poisoned the atmosphere (after more than one billion years of earth ‘rusting”)
                • Innovation: aerobic respiration
  • Autotrophy is attractive because if you can feed yourself, you don’t have to compete for food
  • Once hydrogen was no longer available in the atmosphere, the organisms had to find an alternate source of energy in the sun or other compound. This resulting in the advent of photosynthesis and the release of oxygen in the atmosphere

VII. Eukaryote vs Prokaryote

  • Eukaryotes are more organized and have a membrane within a membrane (nucleus) and division of labor (organelles like mitochondria and chlorophyll); they are more efficient and larger than prokaryotes
  • Prokaryotic cell
            • lacks internal membranes
            • little internal organization
            • bacteria, blue-green algae
  • Eukaryotic cell
            • nucleus (internal membrane)
            • sub-cellular organelles: chromosomes, mitochondria and chloroplasts
            • Examples are plants, animals, protozoans, fungi
  • How did it evolve?
            • Engulfing of prokaryotic cells allowed of endosymbiosis (cell living within a cell and working together)
            • Modern symbiosis between prokaryotes still exists: The ancestors of the chloroplasts in today's plant cells may have resembled Chlorella, the green, photosynthetic, single-celled algae living symbiotically within the cytoplasm of the Paramecium
            • Prokaryotes are still vital! They help with nutrient cycles (ex: cyanobacteria, have the ability to “fix” inorganic atmospheric nitrogen (N2) into ammonia (NH3), and so are critical to the nitrogen cycle) and have huge numbers in biomass (ten times that of eukaryotes)

VIII. Geological Record

  • Breaks down time
            • Two eons (Precambrian) comprise about 4 billion years of history
            • The third eon (0.5 billion years) represents most eukaryotic history
            • Eras in the Phanerozoic are separated by mass extinctions
  • Cambrian Explosion
            • TONS of speciation occurred after the melting of snowball earth (when we enter the Phanerozoic eon, and the Cambrian period)
            • There was a sudden and massive radiation of eukaryotic animal phyla in the oceans
            • Evidence from molecular clocks suggest that the radiation may have happened before the Cambrian but doesn’t appear in the fossil record
            • Notice that most eukaryotic animal phyla appear in the fossil record during the Cambrian
            • This includes the phylum Chordata which holds the vertebrates

IX. Colonization of Land

  • About a billion years ago, cyanobacteria probably inhabited damp terrestrial environments
  • However, colonization of land by eukaryotes did not occur until about 480 MYA (Ordovician)
  • Diversity of animals all starting after land formation
  • Evolved from marine, mostly stationary
  • More phyla in marine, more species on land
  • All terrestrial fungi, plants, and animals have evolved during the last 480 MY
  • Issues with drying out, mobility, reproduction, support system, water and nutrients up/down the stem made conversion difficult!
                • Desiccation – Solution: Cuticle epidermis that consists of waxes that acts as waterproofing, helps prevent water loss
                • Acquiring water and nutrients – Solution: Apical meristem; root system for below ground resources (water, nutrients)
                • Acquiring light and CO2 – Solution: Apical meristem; stem system and leaf system to support photosynthetic structures
                • Transport system – Solution: Vascular tissue: Xylem and phloem
                • Gamete dispersal and fertilization – Solution: Wind pollination (e.g., gymnosperms), animal pollination (e.g., many angiosperms)
                • Zygote Dispersal – Solution: Seeds

X. Key Points

  • All animal phyla evolved in marine environments and most animal phyla remain in marine environments – terrestrial and freshwater animal groups are a very small subset of phyletic diversity (though a large proportion of species diversity)
  • Some animal phyla are sessile for much of their lives and grow by the addition of repeated modules, a bit like plants

XI. Biodiversity is dynamic

  • Macroevolution is just a series of single microevolutions over time
  • Genetic change resulting from mutation and recombination
  • Evolution resulting from natural selection and genetic drift
  • Interactions among biota
  • Effects of a variable environment and effects of biota on the environment
  • On a Global Level, diversity reflects the relative rates of speciation and extinction – an input/output model!

XII. Speciation

  • Biological evolution may be defined as the gradual change of living things from one form into another over the course of time, the origin of species and lineages by descent of living forms from ancestral forms, and the generation of diversity
  • Morphological Species Concept
                  • “some of these things are not like the other”
                  • Based on physical attributes
                  • Can be flawed because things physically misleading
  • Biological Species Concept
                  • Great because you can validate the species concept through reproduction
                  • “a species is a group of actually or potentially interbreeding individuals who are reproductively isolated from other such groups”
                  • Flaws: useless with fossils, museum specimens, can not use with spatially disjunct populations, hybridization

XIII. Reproduction isolation arise

  • Barriers
                  • Prezygotic – issues before the two get together
                                        • Geographic isolation
                                                            • Intact population becomes separated by geographic barrier (note link to earth processes)
                                                            • Different geographic regions have different environments, competitors, predators
                                                            • Separated populations are genetically independent (different mutations)
                                                            • Given enough time, the two populations may diverge enough to become distinct species
                                                            • If reproductively isolated, then two species
                                    • Postzygotic – not fit offspring
                                                            • Non-viable offspring
                                                            • Offspring cannot reproduce
                                                            • Offspring is less fit

XIV. Speciation by Geographic Isolation

  • Gets back to geological issues — a precursor to diversity (continental shift, glacial recession)
  • Time apart is necessary to having changes gene pools causing different traits and eventually speciation
                    • Continental Drift
                    • Shows the drift because 1 species was on 2 continents that were once connected
                    • American Exchange
                                  • Fully connected 3 million years ago
                                  • Interchanges and colonization events
                                  • Climate and size were both factors to which species survived. This seemed to favor North American species more and therefore more of them survived.
                    • Climate Change/ Milankovitch cycles
                                    • Toucans, 5 species in once area… so how does that happen?
                                    • Rainforests shrank during glaciations cycles, causing small, geographically isolated pockets
                                    • Islands
                                    • Darwin’s finches @ Galapagos
                                    • Limited gene flow between islands
                      • Evolutionary Convergence
                                    • Ex: African and South American both have animals with similar body forms
                                    • Unrelated lineages living under similar ecological conditions may evolve to resemble one another more than their ancestors did.
                                    • This suggests that evolutionary adaptation reflects ecological conditions and opportunities that shape the structure and function of organisms.

XV. Speciation —> Tree of Life

  • Time allows for speciation to occur again and again
  • Example: Allopatric Speciation — A small, isolated population is more likely to change substantially enough to become a new species than a large one, the geographic isolation of a small population usually occurs at the fringe of the parent population’s range. Peripheral isolates are good candidates for speciation, although most peripheral isolates do not survive long enough to speciate
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