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CHAPTER 25THE HISTORY OF LIFE ON EARTH11Artists Conception of Earth 3 billion years ago2

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I. Concept 25.1: The Origin of LifeA. Introduction1. Conditions at this time on Earth were such that life could have arisen by spontaneous generation or abiogenesis (life developing from nonliving materials)2. Louis Pasteur disproved spontaneous generation in 1862.3. Today we accept the theory of biogenesis (life comes only from preexisting life).4. It is possible that chemical and physical processes on early Earth produced the first cells.B. Earth and other planets in the solar system formed about 4.6 billion years ago.

44Louis Pasteur

55C. Current theory about how life arose consists of four main stages:1. Abiotic synthesis of small organic molecules (monomers)2. Joining of monomers to form macromolecules3. Packaging of macromolecules into protobionts, droplets with membranes that maintained a distinct internal chemistry4. Origin of self-replicating molecules that eventually made inheritance possible.66D. Closer Look at these Four Main Stages 1. Abiotic synthesis of small organic molecules (monomers)In the 1920s, A. I Oparin (Russian) and J. B. S. Haldane (British) independently postulated that the conditions on early Earth (reducing environment) favored the synthesis of organic compounds from simple molecules.Energy possibly from lightning and UV radiationIn 1953, Stanley Miller and Harold Urey tested the Oparin-Haldane hypothesis 77They conducted lab experiments that showed that the abiotic synthesis of organic molecules in a reducing atmosphere is possibleAtmosphere of primitive Earth:water vaporhydrogen gasmethaneammoniaUsed electrical sparks as source of energy

88Miller and Ureys Experiment9

92. Joining of monomers to form macromoleculesMonomers could link to form macromolecules without enzymes and other cellular equipment.Macromolecules, including polypeptides, have been produced in the lab by dripping solutions of monomers onto hot sand, clay, or rock. These conditions are thought to be similar to early Earth at deep-sea vents or when dilute solutions of monomers splashed onto fresh lava.10103. Packaging of macromolecules into protobionts, droplets with membranes that maintained a distinct internal chemistryTwo properties that defined life: accurate replication and metabolismProtobiontsaggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure--they exhibit simple reproduction and metabolism and maintain an internal chemical environmentCan be formed spontaneously from abiotically produced organic compounds (liposomes)

1111Protobionts (Liposome)12

124. Origin of self-replicating molecules that eventually made inheritance possible. First genetic material was probably RNA, not DNA RNA molecules called ribozymes can catalyze many different reactionsHas been demonstrated that RNA sequences can evolve under abiotic condition1313Clock Analogy of Earths History14

14II. Concept 25.2: Fossil Record of Life on EarthA. Sedimentary rocks are deposited into layers called strata and are the riches source of fossils. B. Methods used to date fossils and rocks:1. Radiometric datingCan determine absolute ages of fossilsInvolves half-life of isotopes (time required for half the parent isotope to decay)2. Radiocarbon dating (carbon-14)Used to date fossils up to 75, 000 years old

1515III. Concept 25.3: Major Lineages of LifeA. Oldest know fossils are stromatolites, rock-like structures composed of many layers of bacteria and sedimentDate back 3.5 billion years agoB. Prokaryotes were Earths sole inhabitants from 3.5 to about 2.1 billion years agoC. About 2.7 billion years ago, O2 (probably from bacteria similar to modern cyanobacteria) began accumulating in the atmosphereMany prokaryotes were doomed by the increase in atmospheric oxygen

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17D. Oldest fossils of eukaryotic cells date back to 2.1 billion yearsE. First Eukaryotes1. EndosymbiosisProposes that mitochondria and plastids (chloroplasts and related organelles) were formerly small prokaryotes living within larger host cellsEndosymbionta cell that lives within a host cellProkaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasitesEventually host and endosymbiots would have become a single organism1818Serial endosymbiosis supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events19

192. Evidence to support an endosymbiotic origin of mitochondria and plastids:a. Similarities in inner membrane structures and functionsb. Division is similar in those organelles and some prokaryotesc. Their ribosomes are more similar to prokaryotic than eukaryotic ribosomesd. Circular DNA3. Earliest Multicellular EukaryotesOldest known fossils of multicellular eukaryotes are of small algae that lived about 1.2 billion years ago

2020IV. Concept 25.4: Continental DriftA. Defined as the process of the Earths continents moving slowly over the underlying hot mantleB. Interactions between plates cause the formation of mountains and islands, and earthquakesC. Pangaeasupercontinent Break-up lead to allopatric speciationCurrent distribution of fossils reflects the movement of continental drift-Ex: the similarity of fossils in parts of South America and Africa is consistent with the idea that these continents were formerly attached

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2323V. Concept 25.5: Evolutionary Effects of Development Genes A. Genes that program development control the rate, timing, and spatial pattern of changes in an organisms form as it develops into an adultB. Changes in Rate and Timing1. HeterochronyDefined as an evolutionary change in the rate or timing of developmental eventsCan have a significant impact on body shapeContrasting shapes of human and chimpanzee skulls are the result of small changes in relative growth rates2424

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2626Can alter the timing of reproductive development relative to the development of nonreproductive organsIn paedomorphosis, the rate of reproductive development accelerates compared with somatic development-Sexually mature species may retain body features that were juvenile structures in an ancestral species2727Paedomorphosis

2828C. Changes in Spatial PatternSubstantial evolutionary change can result from alterations in genes that control the placement and organization of body partsHomeotic genes determine such basic features as where wings and leg will develop on a bird or how a flowers parts are arrangedHox genes are a class of homeotic genes that provide positional information during development-If expressed in the wrong location, body parts can be produced in the wrong location 2929VI. Concept 25.6: Evolution is Not Goal Oriented (not on exam)A. Evolutionary NoveltiesEvolve in many stages from previously existing structuresComplex eyes evolved from simple photosensitive cells independently many timesExaptations are structures that evolve in one context but become co-opted for a different functionNatural selection can only improve a structure in the context of its current utility3030You should now be able to:Define radiometric dating, serial endosymbiosis, Pangaea, snowball Earth, exaptation, heterochrony, and paedomorphosis Describe the contributions made by Oparin, Haldane, Miller, and Urey toward understanding the origin of organic moleculesList and discuss the 4 main stages of how life aroseExplain why RNA, not DNA, was likely the first genetic material

3131Describe and suggest evidence for the major events in the history of life on Earth from Earths origin to 2 billion years agoExplain the function of Hox genes

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