HISTORY OF LIFE ON EARTH EARTH S HISTORY Earth s age: - about 4.6 billion years old (big bang) First life forms appeared ~3.5 billion years ago How did life arise? 1. Small organic molecules were synthesized (amino acids, N bases) 2. Small molecules macromolecules (proteins, nucleic acids) 3. Packaged into protocells (membrane-containing droplets with internal chemistry different from surroundings) 4. Origin of self-replicating molecules allow for inheritance RNA World : 1st genetic material most likely RNA First catalysts: ribozymes (RNA) Models of Formation of Life Reducing Atmosphere /Primordial Soup Model 1920 s: Oparin (Russian), Haldane (British) Atmosphere made of H2O vapor, NH3, CH4, and CO2 (no free O2- atmosphere couldn t sustain life ) Abiotic synthesis of organic molecules from inorganic molecules - Molecules pushed together by energy of sun and lightening - Molecules split, and formed new organic molecules (a.a., nucleic acids) Organic molecules formed under conditions of volcanic eruptions 1953: Miller, Urey Tested primordial soup model by placing same molecules in chamber with electric sparks. After few days found organic molecules were formed. Murchison Metiorite Hypothesis Murchison Metiorite (4.5 BYO) fell to Earth in 1969 Carbon 80+ amino acids Lipids Simple sugars N bases (uracil) Origin of Cells (Protobionts) Protocells Led to the current membrane bound system Bubbles - separate inside from outside - show metabolism & reproduction (self replicating) * Montmorillonite clay increases vesicle formation* thought to have been common on Earth 1
RNA is likely first genetic material Roles of RNA Ribozymes- type of RNA found in some unicellular eukarytoes - able to act as an enzyme and replicate itself (Thomas Cech- early 1980 s) Self replicating RNA- new studies indicate that life may have started this way - it would: a. have heredity: be able to provide hereditary information that cell like structures lack b. be able to respond to natural selection and evolve Fossil Record Documents History of Life Sedimentary rock (layers called strata) Mineralized (hard body structures) Organic rare in fossils but found in amber, frozen, tar pits Incomplete record many organisms not preserved, fossils destroyed, or not yet found Determining the Age of the Earth Relative Dating Uses order of rock strata to determine relative age of fossils Radiometric Dating Measure decay of radioactive isotopes present in layers where fossils are found -radioisotope: unstable isotopes of certain elements that break down (decay) and lose neutrons. As they break down, they release charged particles (electrons) in the form of radioactivity -decay: changing of one element into another as elec. are given off - half life: time period in which half the initial number of atoms decay into atoms of the element they change into (non radioactive) Radiometric Dating By knowing the time of the half life and how many have passed, number of years can be calculated by counting number of remaining parent isotope atoms left in sample and number of accumulating daughter isotopes formed. Limitation: 75,000 years. Scientists then use relative dating. 2
Key Events in Origin of Life Geologic Time Scsle Eon Era Period Epoch (longest to shortest) Archaean/Proterozoic Eons (4 b years duration) Pharaerozoic Era (.5 b years duration) Animals on Earth Life originated 3.5-4 bya Key Events in Origin of Life Key Events in Origin of Life First Unicellular Organisms (2.1 bya) Stromatolites : fossilized layered rocks of prokaryotes that resemble modern microbial colonies O2 accumulates in atmosphere (3.5-2.1 bya) Humans (200K) Development of Complex Organisms Prokaryotes archaebacteria: - thrived under harsh environmental conditions - most likely first organisms on earth - probably anaerobes (very little oxygen present) - chemiautotrophs: CO2 serves as carbon source to make organic molecules - evidence indicates eukaryotes evolved from these Evidence of Oxygen Atmosphere Oxygen begins to accumulate 2.7 bya ancient cyanobacteria and photosynthetic algae reducing oxidizing atmosphere - evidence in banded iron in rocks - rusting makes aerobic respiration possible - cyanobacteria: more modern photosynthetic bacteria that released oxygen into the atmosphere ( 3
First Eukaryotes (2.1 bya) Endosymbiont theory: mutually successful beneficial relationship between two organisms Mitochondria & plastids (chloroplasts) formed from small prokaryotes living in larger cells mitochondria- evolved from non-photosynthetic bacteria invading bacteria chloroplasts- evolved from photosynthetic bacteria invading bacteria (closely related to cyanobacteria) - both have own DNA (plasmids) - able to replicate on their own - replication by binary fission - ribosomes (similar to prokaryote ribo.) - enzymes similar to living prokaryotes - two membranes Multicellular Eukaryotes (1.5 bya) Snowball Earth hypothesis Most Earth landmasses covered with ice Most life near deep sea vents, hot springs, equator Cambrian Explosion (40 m years) Diversification of Animals - within 10 20 million years most of the major phyla of animals appear in fossil record Colonization of Land (500 mya) Pangaea: Supercontinent Formed 250 mya Continental drift explains many biogeographic puzzles Arrows show direction of movement Major periods in Earth s history end with mass extinctions and new ones begin with adaptive radiations. - at least 50% of marine species become extinct - mass extinctions increase diversity of life Permian mass extinction (96% of marine animal species ) - mass volcanic eruptions (Siberia) - global warming, ocean anoxia, destruction of ozone Cretaceous mass extinction (50% marine sp., dinosaurs) - asteroid or meteor (Mexico) 4
Major events during each Era Precambrian: microscopic fossils (stromatolites) Photosynthesis, atmospheric O 2 Eukaryotes (endosymbiont theory) Paleozoic: Cambrian Explosion Plants invade land, many animals appear Permian Extinction (-96% species) Mesozoic: Age of Reptiles, dinosaur, plants Formation of Pangaea supercontinent Cretaceous Extinction asteroid off Mexico s coast Cenozoic: Primates Consequences of Mass Extinctions Reduction of thriving ecological communities Long time periods for recovery of life Changes is type of remaining organisms Pave way for adaptive radiations - adaptations in new species fill different vacant niches Worldwide Adaptive Radiations * All end with major extinction & start with adaptive radiation* Effects of Developmental Genes Regional Adaptive Radiations - a few organisms make it to a new distant location - different island conditions increase evolutionary divergence by natural selection Evolution of new body forms results from changes in DNA or regulation of developmental genes Changes in Rate and Timing Heterochrony: evolutionary change in rate of developmental events Changes in Spatial Pattern and Genes Homeotic genes: master regulatory genes determine location and organization of body parts ex: Hox genes: tells cells to develop intro appropriate structures for body location Rapid reproductive organ development Salamander adult retains juvenile structures in ancestral specie limb development changes in insect body plan Ubx (suppresses leg development) expressed in abdomen vs main body 5
Changes in Gene Regulation Caused by mutations in regulation of developmental genes Evolutionary Trends Evolution is the result of the interactions between organisms and their current environments. If environmental conditions change, an evolutionary trend may cease or even reverse itself. What caused loss of spines in stickleback fish? PTX1 expression in ventral spine PTX1 expression in mouth region 6