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The History of Life on Earth Earth = 4.6 billion years oldFirst life forms appeared ~3.8 billion years ago How did life arise?1. Small organic molecules were synthesized2. Small molecules à macromolecules (proteins, nucleic acids)3. Packaged into protocells (membrane-containing droplets)4. Self-replicating molecules allow for inheritance -first genetic material was likely RNA-first catalysts – ribozymes (RNA) Synthesis of Organic Compounds on Early Earth-Oparin and Haldane: -early atmosphere – H2O vapor, N2, CO2, H2, H2S, methane, ammonia -energy = lightening and UV radiation -conditions favored synthesis of organic compounds – a primitive soup-Miller and Urey: -tested Oparin-Haldane hypothesis -simulated conditions in the lab -produced amino acids Fossil Record: used to reconstruct history-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 Dating Fossils to Determine AgeRelative Dating – uses order of rock strata to determine relative age of fossilsRadiometric Dating – measure decay of radioactive isotopes present in layers where fossils are found -half-life: # of years 50% of original sample to decay Geologic time scale (longest to shortest)Eon à era à period à epoch Present Day: Phanerozic eon, Cenozoic era, quaternary period, halocene epoch Key events in Life’s history:-3.5 billion years ago (bya) – first prokaryotes (single-celled)-2.7 bya – O2 accumulates in atmosphere-2.1 bya – first eukaryotes (single-celled)-1.2 bya – first multicellular eukaryotes-535-525 mya – Cambrian explosion (great increase in diversity of animal forms)-500 mya – colonization of land by fungi, plants, and animals-200,000 ya – humans Endosymbiont Theory:-mitochondria and plastids (chloroplasts formed from small prokaryotes living in larger cells-evidence: --replication by binary fission --singular, circular DNA (no histones) --ribosomes to make proteins --enzymes similar to living prokaryotes --two membranes Pangaea = supercontinent formed 250 mya-continental drift explains many biogeographic puzzles Movement of continental plates change geography and climate of earth à extinctions and speciation Major periods in Earth’s history end with mass extinctions and new ones begin with adaptive radiations Major Events During Each Era:-Precambrian – microscopic fossils (stromatolites). Photosynthesis, atmospheric O2. Eukaryotes (endosymbiont theory)-Paleozoic – Cambrian explosion. Plants invade land, many animals appear. Permian extinction (~96% species)-Mesozoic – age of reptiles, dinosaurs, plants. Formation of Pangaea supercontinent. Cretaceous extinction – asteroid off mexico coast-Cenozoic – primatesNOTE: all end with major extinction and start with adaptive radiation Evo-Devo: evolutionary + developmental biology-evolution of new forms results from changes in DNA or regulation of developmental genes-Heterochrony – evolutionary change in rate of developmental events-Paedomorphosis: adult retains juvenile structures in ancestral species-Homeotic genes: master regulatory genes determine location and organization of body parts (ex: hox genes – evolution of hox genes changes the insect body plan) Exaptations: structures that evolve but become co-opted for another function (ex: bird feathers = thermoregulation à filght)
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