Bio Lecture 5: History of Life on Earth PDF

Bio Lecture 5 History of Life on Earth Chapter 4, page 61; Chapter 25, pages 563 - 574 Origin of Life How Did Life on Earth Begin? Direct evidence of life on early earth comes from fossils of microorganisms that are about 3.5 billion years old. But when and how did the first liv...

Bio Lecture 5 History of Life on Earth Chapter 4, page 61; Chapter 25, pages 563 - 574 Origin of Life How Did Life on Earth Begin? Direct evidence of life on early earth comes from fossils of microorganisms that are about 3.5 billion years old. But when and how did the first living cells appear? The truth is that we don’t really know for certain! But observations and experiments in chemistry, geology, physics and biology have led scientist to make several important hypotheses. Origin of Life How Did Life on Earth Begin? Scientists hypothesize that the chemical and physical processes on early Earth (aided by natural selection of course) could have produced very simple cells through a sequence of four main stages: 1. The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nitrogenous bases (these are the small chemical building blocks found in the molecules that make up cells,). 2. The joining of these small molecules into macromolecules, such as proteins and nucleic acids (these are the larger molecules that make up cells, i.e. Proteins & RNA). 3. The packing of these molecules into protocells, droplets with membranes that maintained an internal chemistry different form that of their surroundings. 4. The origin of self-replicating molecules that eventually made inheritance possible Science: Let's examine evidence for each stage….. Earth: history Earth formed about 4.6 billion years ago, condensing from a vast cloud of dust and rocks that surrounded our young sun. Earth: history The Hadean: The First Geological Eon The name "Hadean" comes from Hades, the ancient Greek god of the underworld, in reference to the "hellish" conditions on Earth at the time. The planet had just formed and was still very hot due to high volcanism, a partially molten surface and frequent collisions with other Solar System bodies Earth: history The Hadean: The First Geological Eon Life originated under very different conditions than those experienced today. Early earth provided the conditions for chemical evolution The atmosphere of the early earth may have been a *reducing atmosphere thick with water vapor, nitrogen and its oxides, carbon dioxide, methane, ammonia, hydrogen, and hydrogen sulfide. *NB – lack of O2 → promotes chemical reduction of molecules. “Reducing” refers to the addition of electrons → allows formation of organic compounds from simple molecules (e.g., formation of macromolecules) Synthesis of Organic Molecules Abiotic Synthesis of Monomers A. I. Oparin During the 1920s, both these scientists independently hypothesized that: Earth’s early atmosphere was a reducing (electron- adding) environment in which organic molecules could have formed. The energy for this organic synthesis could have come from lightning and intense UV radiation J.B.S. Haldane “Early oceans were a solution of organic molecules, a “primordial soup” from which life arose” - Haldane  Oparin-Haldane “primordial soup” hypothesis (1920s) Stated 4 requirements necessary for the creation of organic chemicals from simple molecules: 1. No free oxygen 2. Source of energy (volcanism, thunderstorms, lighting, and UV radiation) 3. Chemical building blocks (water, ions, and dissolved gases) 4. Lots of time! (earth is approximately 4.6 billion years old) In 1953, Stanley Miller and Harold Urey tested the Oparin- Haldane hypothesis by creating laboratory conditions comparable to those that had been postulated for early Earth. Can organic molecules be formed under conditions believed to simulate those on early earth? 1) The Abiotic synthesis of small organic molecules Synthesis of Monomers Simple organic molecules formed! Amino acids (used to make proteins), sugars, lipids, nucleotide bases (used to make DNA/ RNA) On display at the Denver Museum of Nature & Science Residues collected from Miller’s lab reanalyzed in 2007 (many more organic molecules discovered!!) Reference: Science (2008), vol. 322, page 404  Where did life originate? Near volcanoes, deep sea vents or meteorites? Reducing Atmosphere (Miller & Urey and more) Deep Sea Vents https://www.youtube.com/ watch?v=C0YzFLV_fVs https://www.youtube.com/ watch?v=8W_ywzhkR90 Start at 1:40 – 7:25 Extraterrestrial - 1969, a meteorite fell in Australia was found to contain amino acids (survived impact). Based on this observation some scientist postulate that meteorites could have transported organic molecules to ancestral Earth. - Expt’s have shown organic compounds and bacteria can withstand impact. 2) The joining of monomers into POLYMERS Synthesis of Polymers Laboratory simulations of early- Earth conditions have produced organic polymers Polymers, made by dripping solutions of monomers onto hot sand, clay, or rock. Amino Acids Hot Sand Nucleotides Hot Clay Hot Rock RNA Small molecules that make up the larger molecules that make up cells Proteins (amino acids make up proteins, nucleotides make up DNA / RNA).  3) Formation of Protocells Protocells / Protobionts / Liposomes Aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure (similar to cells, but not cells): Experiments have shown that protocells exhibit certain properties of life: 1. Capable of self-replication 2. Metabolism 3. Homeostasis (maintenance of internal chemical environment) Surrounded by a membrane and were able to maintenance of an internal chemical environment different from surroundings 4) The Origin of Self-replicating Molecules that Eventually Made Inheritance Possible The first genetic material was most likely RNA. – RNA has the ability to store information in its nucleotide sequence Ribozymes (RNA enzymes) can make copies of RNA (provided that they are supplied their building blocks) RNA could *catalyze protein formation *catalyze means to make a chemical reaction happen RNA RNA molecules are “flexible”: Single stranded Can assume a variety of shapes In 2003 researchers were able to show that if RNA was on the surface of the clay, protocells were formed that enclosed RNA. These experiments are exciting because they show that the formation of membrane vesicles containing RNA molecules is a plausible route to the first living cells based on simple physical and chemical forces. Review: Protocells were replaced by cells that could produce all their needed compounds from molecules in their environment Abiotic synthesis – 4 steps: macromolecules Protocells Monomers: small organic molecules Polymers: macromolecules that make up cells Prokaryotes dominated from 3.5 to 2.0 billion years ago - A rich variety of autotrophs emerged: some of which could use light as their energy source (Photo-) Some which used energy form other molecules (Chemo-) Chemoautrophs (e.g. bacteria that create their food using inorganic sulfur compounds Autotrophs – organisms capable of converting CO 2 gushing out of the vents) (inorganic carbon source) into an organic carbon source (i.e. glucose) Photoautotrophs: use light energy (i.e. cyanobacteria) The Fossil Record documents the history of life Sedimentary Strata Many fossils are found in sedimentary rocks formed from the sand and mud that settle to the bottom of seas, lakes, swamps and other aquatic habitats. New layers of sediment cover older ones and compress them into superimposed layers of rocks called strata. The Fossil Record The fossil record shows changes in the kinds of organisms on Earth over time. The deeper you go through the strata, the older the organisms are. Some organism that were once common are now extinct. Some of these extinct organisms look very similar to organisms that are living today! As substantial and significant as the fossil record is, it is an incomplete chronicle of evolutionary change. How do scientists determine the age of fossils: https://www.youtube.com/watch?v=phZeE7Att_s The Fossil Record Incomplete Fossil Record Many of earth’s organisms did not die in the right place at the right time to be preserved as fossils. The fossil record is biased in favour of: 1. species that were abundant and widespread. 2. had hard parts.  Evidence of Early Life – Stromatolites (prokaryotes) – The oldest known fossils are 3.5- billion-year-old STROMATOLITES – Stromatolites: rocklike structures composed of cyanobacteria (photosynthetic bacteria) and sediment. - If bacterial communities existed 3.5 billion years ago, it seems reasonable that life originated much earlier, perhaps 3.9 billion years ago - Prokaryotes were Earth’s sole inhabitants from 3.5 to 2.1 billion years ago.  Prokaryotes dominated from 3.5 to 2.0 BYA Autotrophs produce organic molecules from carbon dioxide. This prevented the depletion of organic foodstuffs that would have been exhausted if only heterotrophs existed. Cyanobacteria produce OXYGEN (product of photosynthesis) PHOTOsynthesis: Energy from the SUN + 6CO2+6H2O → C6H12O6 (sugar) + 6O2 CHEMOsynthesis: 3H2S+ 6CO2+6H2O + → Photoautotroph C6H12O6 (sugar) + 3H2SO4 (e.g., cyanobacteria) Chemoautotroph (e.g., sulphur-reducing bacteria)  Photosynthetic Bacteria released O2 into atmosphere Cyanobacteria (photosynthetic bacteria), generated oxygen. As oxygen slowly saturated the oceans, the atmosphere also became oxygenated Most atmospheric oxygen is of biological origin, from the process of photosynthesis. This OXYGEN REVOLUTION had an enormous impact on life. O2  Cyanobacteria, Photosynthesis & the Oxygen Revolution Early Earth received high doses of ultraviolet (UV) light radiation from the sun, because the atmosphere had no mechanism to reduce it. Intense UV light can kill organisms, so life was inhibited. However, as oxygen increased in the atmosphere it interacted with UV light to form ozone. A layer of ozone in the upper atmosphere reduced the amount UV reaching the Earth’s surface, making land more hospitable. Sometime between 2.8 and 2.4 billion years ago, oxygen levels reached a point that allowed the evolution of aerobic organisms (including all eukaryotes, including us) to occur.  Eukaryotic life began 2.1 billion years ago Oldest fossils with eukaryotic cells are 1.8 byo Eukaryotic cells arose from *symbioses and genetic exchanges between prokaryotes *A symbiotic relationship is one in which two different species live in direct contact with each other.  Origin of Eukaryotes All eukaryotic cells have a nucleus and an endomembrane system Formation of nuclear membrane and endoplasmic reticulum by infoldings of plasma membrane Support for this hypothesis: 1) infoldings of plasma membrane, analogous to those shown here, have been observed in bacteria living today = ER 2) Nuclear envelope and ER are continuous (ER is an organelle found in eukaryotic cells)  Endosymbiont Theory Eukaryotes arose from prokaryotes All eukaryotic cells have MITOCHONDRIA (Organelle where cellular respiration takes place in eukaryotes) Mitochondria arose from heterotrophic aerobic bacteria: Ingested but not digested Reproduced along with host cell Relationship between host and engulfed cell presumed to be stable because a mutual advantage existed between them: o the host supplied bacterium with protection and C compounds from its other prey, o while the bacterium produced much more ATP than the host cell could synthesize on its own Endosymbiont Theory Some eukaryotes have *chloroplasts… (*organelle within the cells of plants and certain algae that is the site of photosynthesis) Formation of Chloroplasts: Endosymbiotic theory: chloroplasts and mitochondria were prokaryotic cells that began living within larger cells & became mutually beneficial All eukaryotic cells have mitochondria But not all eukaryotes have chloroplasts Therefore: Mitochondria thought to evolve before chloroplasts Evidence supporting Endosymbiont Theory: Chloroplasts and mitochondria: are about the size of bacteria have a double membrane (inner membrane has prokaryotic proteins) have their own cell structures to make their own proteins have DNA that is closely related to bacteria DNA Evidence supporting Endosymbiont Theory:  Endosymbiotic Theory: support - Many species of alpha-proteobacteria are only found living within eukaryotic cells – scientists hypothesize that mitochondria evolved from aerobic alpha-proteobacteria through endosymbiosis - If you look inside a termite, you will find the gut is crammed with protists. These protists help the termite to digest wood. The actual digestion of the wood is done by bacteria living inside the protists. Green sea slug : - feeds on algae but instead of digesting the entre cell contents, it retains the algal chloroplasts. Within the slug's own cells, the chloroplasts can survive and function, happily photosynthesizing despite the absence of an algal nucleus. - Amazingly, these slugs have somehow incorporated some of the genes necessary for chloroplast upkeep from algae into their own genomes.  Learning Objectives: List and be able to describe the four stages needed for the abiotic origin of cells Understand the differences between our present and Hadean atmosphere Understand Oparin and Haldane Primordial Soup hypothesis. What did Miller & Urey do? Know the approximate age of the earth and origin of the first cells (prokaryotic and eukaryotic). Be able to support your answer How did oxygen accumulate in atmosphere? Why was this important? Be able to describe the sequence of events that lead to origin of eukaryotes; Understand Endosymbiotic Theory Vocabulary: autotroph, heterotroph, chemoautotroph, photoautotroph, protocell, stromatolite  More videos: Hydrothermal Vents: http://www.youtube.com/watch?v=D69h GvCsWgA https://www.youtube.com/watch?v=BLO UFrncG7E https://www.youtube.com/watch?v=JtV- FP212Uc Wonders of the universe - part 2: early universe / big bang https://www.youtube.com/watch?v=DEw 6X2BhIy8 Full episode: https://vimeo.com/200519769 Endosymbiotic Theory: https://www.youtube.com/watch?v=4LhB Z2H5SwM how fossils are dated: https://www.youtube.com/watch?v=Kcuz 1JiMk9k https://www.youtube.com/watch?v=XRW- ATOUJus