Significant Events in Evol of Life

• Geologic Time Scale:

  • System of chronological dating related to geological strata
  • Describes timing and relationship of events in Earth's history
  • Different spans of time marked by changes in strata composition indicating major geological or paleontological events
• Law of Superposition:

  • Proposed by Nicholas Steno
  • States that any given stratum is probably older than those above it and younger than those below it
  • Sequences of strata can become eroded, distorted, tilted, or inverted after deposition
• Divisions of Geologic Time:

  • Supereon largest division of the geologic time
  • Eons span hundreds to thousands of millions of years
  • Era spans tens to hundreds of millions of years
  • Period spans no more than one hundred million years
  • Epoch is the shortest time scale characterized by distinct organisms
• Origin of Life:

  • Estimates vary on when life first emerged: from 3.77 BYa to as early as 4.41 BYa
  • First direct evidence of life: 3.465 BYa
  • Major life forms were prokaryotes found around hydrothermal vents or in surface water
• Possible Sources of Biological Molecules:

  • Methane, Ammonia, Water, Hydrogen sulfide, Carbon dioxide, Carbon monoxide, Phosphate
  • Between 3.9 BYa to 2.5 BYa, chemoautotrophic prokaryotes were dominant
• Last Universal Common Ancestor (LUCA):

  • Most common recent ancestor of all living things on Earth
  • LUCA could have existed around 3.5 BYa
  • Originally thought to be a group of different microbes, but studies suggest a single LUCA for the three domains
• Evolution of Photosynthesis:

  • Early forms did not produce oxygen
  • Retinal-based photosynthesis
  • Bacteriorhodopsin in Haloarchaea captures light to move protons
• The Great Oxygenation Event:

  • More efficient photosynthesis
  • Accumulation of oxygen in the atmosphere by cyanobacteria
  • Drove evolution of eukaryotes and multicellularity
• Plate Tectonics and Continental Drift:

  • Large-scale movement of Earth's plates
  • Occurred around 3.3 to 3.5 Billion years ago
  • Resulted in volcanic activities, earthquakes, mountain-building, trench formation, and changes in climate
• Emergence of Eukaryotes:

  • Occurred about 1.85 BYa
  • Could be the result of symbiogenesis (endosymbiosis)
• The Origin of Sex:

  • By about 1.2 BYa
  • Advantages of sexual reproduction include increasing genetic variation and spreading advantageous traits
• Multicellularity:

  • Evolved multiple times among different eukaryotic lineages
  • Early challenges included generating a whole organism from two cells
  • Advantages: symbiosis and predator avoidance, size increase with absorbing and transporting materials, differentiation and longer lifespan.
• Ediacaran Period and the Avalon Explosion:

  • Preceded the Cambrian Explosion by about 33 million years
  • First appearance of complex multicellular life forms
  • after Snowball Earth
• The Cambrian Explosion:

  • Occurred around 541 MYa
  • Almost all major animal phyla emerged during this time
  • First time multicellular, eukaryotic organisms became dominant life forms
• Possible causes of the Cambrian Explosion:

  • Changing environment - Increase in oxygen levels, Ozone formation, Increase in calcium concentration
  • Developmental - Modifications of developmental patterns, Gene transfer
  • Ecological factors - End-Ediacaran Extinction, Predator-prey dynamics
• Colonization of Land:

  • Prokaryotes colonized land about 2.6 BYa
  • Plants colonized land around 700 MYa
  • Fungi colonized land around 1000 MYa
  • Animals colonized land estimated at 642 MYa
• Adaptations of Early Land Colonizers:

  Plants:

  • Adaptations against water loss
  • Adaptations against UV radiation
  • Adaptations against gamete desiccation
  • Evolution of vascular tissues

  Animals:

  • Adaptations against water loss
  • Evolution of new organs for gas exchange
  • Evolution of internal fertilization
  • Changes in locomotion and senses
• Extinction Events:

  • Events characterized by rapid decline of Earth's biodiversity
  • Extinction rates exceed speciation events
  • Often based on the decline in the diversity and abundance of multicellular organisms, mostly marine species
  • 7 Major Extinction Events and 19 Minor Extinction Events
• Major Causes of Extinction Events:

  • Flood Basalt Event (extensive volcanic activities - 11x)
  • Sea-level Fall (decline in sea-water level - 12x)
  • Impact Events (asteroid impact - once)
  • Other causes like global cooling, global warming, anoxic events, oceanic overturn, and plate tectonics
• Significance of Extinction Events:

  • Opens new niches
  • Leads to adaptive radiations
  • Organisms surviving extinction events may still undergo long-term decline
  • Push of the Past concept
• The Ordovician-Silurian Extinction (444 million years ago):

  • Global cooling, sea-level drop, anoxic event
  • Wiped out ~85% of marine species, mostly corals, brachiopods, conodonts, and trilobites
  • Most endemic taxa were replaced by more cosmopolitan taxa
• The Late Devonian Extinction (383-359 million years ago):

  • Anoxic event triggered by a volcanic event
  • Plants causing global cooling
  • Possible asteroid impact
  • Wiped out ~75% of all marine species, about 40% of high-level vertebrate clades, Mostly shelled-marine species and reef-building organisms
  • Lead to the establishment of modern vertebrate taxa
• Tetrapods Conquering Land:

  • Proto-tetrapods likely originated in the Late Devonian period
  • Early tetrapods associated with deltaic, estuarine, or freshwater habitats
  • Reasons for conquering land: lesser competition, abundant food supply, drying of habitats
• Adaptations to Living on Land:

  • Modification of breathing mechanism
  • Movement
  • Feeding
  • Adaptations against desiccation
  • Shift from aquatic to aerial perception
  • Changes in reproductive strategies
• The Rise of Reptiles:

  • Early reptile ancestors emerged around 310-320 million years ago, during the Carboniferous Period, after Carboniferous Forest Collapse (~305 MYa
  • Evolution of amniotic egg and desiccation-resistant skin enabled early reptiles to dominate wider niches and habitats
• End Capitanian Extinction Event (260 million years ago):

  • Volcanic eruption leading to global cooling, ocean acidification, anoxic event
  • High extinction among shallow-water marine species and Dinocephalian therapsids
  • About 56% of plant species were affected
  • Rebuilding of complex trophic guilds
• Permian-Triassic Extinction (252 million years ago) The Great Dying:

  • Volcanic eruption (Siberian Traps), global warming, ocean acidification, anoxic event
  • Wiped out ~96% of marine species, 70% terrestrial species did not survive
  • Only extinction event significantly affecting insects
• The Triassic-Jurassic Extinction (201 million years ago):

  • Volcanic eruption (Central Atlantic Magmatic Province), global warming, ocean acidification, euxinic event
  • Wiped out ~70-75% of marine and terrestrial species, non-dinosaurian archosaurs, most therapsids, and large reptiles
  • Dinosaurian archosaurs are left with no competition on land.
  • Marine reptiles dominated the oceans.
• Cretaceous-Paleogene Extinction (66 million years ago):

  • Asteroid impact followed by global cooling, volcanic eruption
  • Wiped out ~76% of all species, all non-avian dinosaurs, mosasaurs, plesiosaurs, and ammonites
• The Rise of Angiosperms:

  • Ancestors emerged around the Triassic or Jurassic
  • Angiosperms became dominant plants in the Cretaceous
  • Genome downsizing likely explanation for diversification
• The Holocene Extinction:

  • Ongoing extinction event due to human activities
  • Affects almost all plant and animal groups
  • Extinction rates 100 to 1,000 times greater than background rates
  • Ecologically sensitive species are driven to extinction
  • Rate of speciation slows down since species are not able to adapt to sudden and oftentimes, permanent changes.
• International Commission on Stratigraphy to consolidate information about the geological time scale.
• Leading hypotheses on where prebiotic molecules could have come from:

  • Panspermia
  • Primordial soup hypothesis

  
• Results of continental drift:

  • Vicariance – separation of population by physical barriers
  • Dispersal – movement to another location
  • Extinction – extirpation of population
  • Change in climatic conditions
• Endosymbiotic Theory

  • prokaryotes to eukaryotes
  • AUTOGENOUS MODEL - The mitochondria, chloroplasts, and other organelles are independent and later engulfed by the proto-eukaryotic cells
  • CHIMERIC MODEL - Two independent prokaryotes merge or engulfed one another
• What triggered eukaryogenesis?

  • STRESS
  • Oxidative stress
  • Environmental stress
  • Selective pressure favoring cells with DNA repair mechanisms
• Evidences of Eukaryogenesis

  • Similarity of mitochondria and chloroplast to bacteria:
  • Divides by binary fission
  • Uses the same transport proteins (porins)
  • Presence of single circular DNA
  • Presence of 70S ribosomes
  • Genome comparison showed close relation between mitochondrial and Rickettsial bacteria genome
  • Genome comparison showed close relation between chloroplast and cyanobacteria genome
• Adaptive radiations – “rapid” diversification brought about by changes in the environment
• Organisms which may survive initial extinction events may still undergo long-term decline (walking dead clades/ survival without recovery)
• Push of the Past concept – clades which survive for a considerable time after mass extinction have a greater chance to evolve and diversify