Chapter 25 ~ The History of Life on Earthsparked by just the right combination of physical events & chemical processes

The Origin of Life is HypothesisSpecial CreationWas life created by a supernatural or divine force?not testableExtra-terrestrial OriginWas the original source of organic (carbon) materials comets & meteorites striking early Earth? testableSpontaneous Abiotic OriginDid life evolve spontaneously from inorganic molecules?testable

Conditions on early EarthReducing atmospherewater vapor (H2O), CO2, N2, NOx, H2, NH3, CH4, H2S lots of available H & its electronno free oxygenEnergy sourcelightning, UV radiation, volcaniclow O2 = organic molecules do not breakdown as quicklyWhats missing from that atmosphere?

Origin of Organic MoleculesAbiotic synthesis1920 Oparin & Haldane propose reducing atmosphere hypothesis 1953 Miller & Urey test hypothesisformed organic compoundsamino acidsadenineCH4NH3H2

Stanley MillerUniversity of Chicagoproduced-amino acids-hydrocarbons-nitrogen bases-other organicsWhy was this experiment important??!

The Origin of Life

The 4-stage Origin of life Hypothesis:1- Abiotic synthesis of organic monomers2- Polymer formation3- Origin of Self-replicating molecules4- Molecule packaging (protobionts)

Key Events in Origin of LifeOrigin of Cells (Protobionts)lipid bubbles separate inside from outside metabolism & reproductionOrigin of GeneticsRNA is likely first genetic materialmultiple functions: encodes information (self-replicating), enzyme, regulatory molecule, transport molecule (tRNA, mRNA) makes inheritance possiblemakes natural selection & evolution possibleOrigin of Eukaryotesendosymbiosis

The fossil recordSedimentary rock: rock formed from sand and mud that once settled on the bottom of seas, lakes, and marshesDating:1- Relative~ geologic time scale; sequence of species2- Absolute~ radiometric dating; age using half-lives of radioactive isotopes

TimelineKey events in evolutionary history of life on Earth3.54.0 bya: life originated2.7 bya: free O2 = photosynthetic bacteria2 bya: first eukaryotes

First EukaryotesDevelopment of internal membranescreate internal micro-environmentsadvantage: specialization = increase efficiencynatural selection!infolding of the plasma membraneDNAcell wallplasmamembrane ProkaryoticcellProkaryotic ancestor of eukaryotic cellsEukaryoticcellendoplasmic reticulum (ER)nuclear envelopenucleusplasma membrane~2 bya

mitochondrionchloroplastEukaryotic cell withchloroplast & mitochondrionEndosymbiosisphotosynthetic bacterium2nd EndosymbiosisEvolution of eukaryotesorigin of chloroplasts engulfed photosynthetic bacteria, but did not digest themmutually beneficial relationshipnatural selection!Eukaryoticcell with mitochondrion

Evidencestructuralmitochondria & chloroplasts resemble bacterial structuregeneticmitochondria & chloroplasts have their own circular DNA, like bacteriafunctionalmitochondria & chloroplasts move freely within the cellmitochondria & chloroplasts reproduce independently from the cellTheory of Endosymbiosis

Cambrian explosionDiversification of Animalswithin 1020 million years most of the major phyla of animals appear in fossil record543 mya

Biogeography: the study of the past and present distribution of speciesPangaea-250 mya Permian extinctionGeographic isolation-180 mya African/South American reptile fossil similarities Australian marsupials

Mass extinctionPermian (250 million years ago): 90% of marine animals; Pangea mergeCretaceous (65 million years ago): death of dinosaurs, 50% of marine species; low angle comet

*It is unclear whether young Earths atmosphere contained enough methane and ammonia to be reducing. Growing evidence suggests that the early atmosphere was made up primarily of nitrogen and carbon dioxide and was neither reducing nor oxidizing (electronremoving). MillerUreytype experiments using such atmospheres have not produced organic molecules. Still, it is likely that small pockets of the early atmosphereperhaps near volcanic openingswere reducing.Instead of forming in the atmosphere, the first organic compounds on Earth may have been synthesized near submerged volcanoes and deepsea ventsweak points in Earths crust where hot water and minerals gush into the ocean.

***Life is defined partly by two properties: accurate replication and metabolism. Neither property can exist without the other. Selfreplicating molecules and a metabolismlike source of the building blocks must have appeared together. How did that happen?The necessary conditions for life may have been met by protobionts, aggregates of abiotically produced molecules surrounded by a membrane or membranelike structure. Protobionts exhibit some of the properties associated with life, including simple reproduction and metabolism, as well as the maintenance of an internal chemical environment different from that of their surroundings.Laboratory experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds. For example, small membranebounded droplets called liposomes can form when lipids or other organic molecules are added to water.****