2 Biology 2016-4-11 chapter 25 history of life on earth notes PDF

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CHAPTER 25: THE HISTORY OF LIFE ON EARTH NOTES Lost worlds  Past organisms were very different from those now alive  Fossil record shows macroevolutionary changes over large time scales, for example: o Emergence of terrestrial vertebrates o Impact of mass extinctions o Origin of flight in birds Origin of life  Conditions on early Earth made origin of life possible  Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages: o Abiotic synthesis of small organic molecules o Joining of these small molecules into macromolecules o Packaging of molecules into protocells o Origin of self-replicating molecules 1 – synthesis of organic compounds on early Earth  Earth formed about 4.6 billion years ago  Seas formed about 4 billion years ago  Earth’s early atmosphere likely contained water vapor and chemicals released by volcanic eruptions o Nitrogen, nitrogen oxides, carbon dioxide, methane, ammonia, hydrogen o Experiments replicated by scientists  1920s—A.I. Oparin and J.B.S. Haldane  1953—Stanley Miller and Harold Urey  Evidence is not yet convincing that early atmosphere was in fact reducing  Instead of forming in atmosphere, first organic compounds may have been synthesized near volcanoes or deep-sea vents  Amino acids have also been found in meteorites 2 – abiotic synthesis of macromolecules  RNA monomers have been produced spontaneously from simple molecules 3 – protocells  Replication and metabolism are key properties of life and may have appeared together in protocells  Protocells may have formed from fluid-filled vesicles with a membrane-like structure  In water, lipids and other organic molecules can spontaneously form vesicles with a lipid bilayer  Adding clay can increase the rate of vesicle formation  Vesicles exhibit simple reproduction and metabolism and maintain an internal chemical environment 4 – self-replicating RNA  First genetic material was probably RNA, not DNA  RNA molecules called ribozymes have been found to catalyze many different reactions o I.e. ribozymes can make complementary copies of short stretches of RNA  Early genetic material might have formed an “RNA world” o Early life forms might have consisted of protobionts with RNA as the genetic code 1

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o Early probionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection Vesicles containing RNA capable of replication would have been protocells RNA could have provided the template for DNA, a more stable genetic material

Fossil record  Documents the history of life  Reveals changes in history of life on Earth  Shows changes in kinds of organisms on Earth over time  Sedimentary rocks are deposited into layers called strata and are the richest sources of fossils Problem with fossils  Few individuals have fossilized, and even fewer have been discovered  Fossil record is biased in favor of species that: o Existed for a long time o Were abundant and widespread o Had hard parts Features of objects that fossilize  Durable  Buried before or shortly after death (usually in water-saturated sediment)  Located in areas devoid of oxygen  Most fossils are hard materials left in areas of deposition such as river deltas, flood plains, marshes, beaches, ocean bottoms and river beds  There is an abundant fossil record of organisms that normally burrow in sediments, such as bivalves Strengths and weaknesses of the fossil record  Bias—potential weakness  3 types of sampling bias: o Geographic bias—most fossils come from lowland and marine habitats where the conditions for fossilization are most prevalent o Taxonomic bias—marine fossils dominate fossil record but only 10% of extant species are marine  2/3 of extant animal species have no hard parts which would lend themselves to being easily fossilized  Critical parts of plants, like flowers, are seldom fossilized o Temporal bias—old rocks are more rare than new rocks because when tectonic plates subduct or mountains erode they take their fossils with them  Therefore, sampling of ancient life forms is poor How rocks and fossils are dated  Radiometric dating—a radioactive “parent” isotope decays to a “daughter” isotope at a constant rate o Each isotope has a known half-life  Half-life—time required for half the parent isotope to decay Geologic record  Divided into Hadean, Archaean, Proterozoic, and Phanerozoic eons 2

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Phanerozoic eon includes the last half billion years o Divided into Paleozoic, Mesozoic, and Cenozoic eras

First single-celled organisms  Stromatolites—rocks formed by accumulation of sedimentary layers on bacterial mats o Oldest known fossils o Date back 3.5 billion years ago  Prokaryotes were Earth’s sole inhabitants for more than 1.5 billion years Photosynthesis and the oxygen revolution  Most atmospheric oxygen (O2) is of biological origin  O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations  About 2.7 billion years ago, O2 began accumulating in the atmosphere and rusting iron-rich terrestrial rocks  Oxygen revolution—caused extinction of many prokaryotic groups o Some groups survived and adapted using cellular respiration to harvest energy The first eukaryotes  Oldest fossils of eukaryotic cells date back 1.8 billion years  Prokaryotic ancestors of mitochondria and plastids probably gained entry to host cell as undigested prey or internal parasites  Serial endosymbiosis—supposes that mitochondria evolved before plastids through a sequence of endosymbiotic events Evidence supporting an endosymbiotic theory  Inner membranes are similar to plasma membranes of prokaryotes  Division and DNA structure is similar in these organelles and some prokaryotes  These organelles transcribe and translate their own DNA  Their ribosomes are more similar to prokaryotic than eukaryotic ribosomes Origin of multicellularity  Evolution of eukaryotic cells allowed for a greater range of unicellular forms  A second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals Cambrian explosion  Refers to the sudden appearance of fossils resembling modern animal phyla in the Cambrian period o 535 to 525 million years ago o A few animal phyla appear even earlier: sponges, cnidarians, and mollusks  Provides first evidence of predator-prey interactions The colonization of land  Fungi, plants, and animals began to colonize land about 500 million years ago  Vascular tissue in plants transports materials internally and appeared by about 420 million years ago  Plants and fungi likely colonized land together

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Fossilized plants show evidence of mutually beneficial associations with fungi (mycorrhizae) that are still seen today Arthropods and tetrapods are the most widespread and diverse land animals Tetrapods evolved from lobe-finned fishes around 365 million years ago The huan lineage of tetrapods evolved around 6-7 million years ago o Modern humans originated only 195,000 years ago

Differences in speciation and extinction rates  History of life on Earth has seen rise and fall of many groups of organisms o Depends on speciation and extinction rates within group  Tectonic plates move slowly through process of continental drift Consequences of continental drift  Formation of supercontinent Pangaea about 250 million years ago had many effects o Deepening of ocean basins o Reduction in shallow water habitat o Colder and drier climate inland  Continental drift has many effects on living organisms o Continent’s climate can change as it moves north or south o Separation of land masses can lead to allopatric speciation  Distribution of fossils and living groups reflects historic movement of continents o I.e. similarity of fossils in parts of South America and Africa is consistent with idea that continents were formerly attached Mass extinctions  Result of disruptive global environmental changes  Fossil record shows that most species that have ever lived are now extinct  Extinction can be caused by changes to species’ environment  At times, rate of extinction has increased dramatically and caused mass extinction  Number of factors might have contributed to these extinctions o Intense volcanism in what is now Siberia o Global warming and ocean acidification resulting from emission of large amounts of CO 2 from volcanoes o Anoxic conditions resulting from nutrient enrichment of ecosystems  Presence of iridium in sedimentary rocks suggests a meteorite impact about 65 million years ago Is a sixth mass extinction under way?  Scientists estimate that current rate of extinction is 100 to 1,000 times the typical background rate  Threatened—species is declining so much it is likely to become endangered if not protected  Endangered—species that is likely to become extinct if not protected  Extinct—last members of a species die o Local extinction—species is extinct in local area/region o Global extinction—species is extinct on entire planet Consequences of mass extinctions  Mass extinction can alter ecological communities and niches available to organisms  It can take from 5 to 100 million years for diversity to recover following a mass extinction 4

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Lineages with novel and advantageous features can be lost during mass extinctions By eliminating so many species, mass extinctions can pave the way for adaptive radiations

Adaptive radiations  Rapid evolution of diversely adapted species from a common ancestor  May follow: o Mass extinctions o Evolution of novel characteristics o Colonization of new regions Examples of adaptive radiations  Mammals underwent adaptive radiation after extinction of terrestrial dinosaurs  Adaptive radiation of: o Photosynthetic prokaryotes o Land plants o Insects Changes in body form and sequences and regulation of developmental genes  Genes that program development control rate, timing, and spatial pattern of changes in an organism’s form as it develops into an adult  Studying genetic mechanisms of change can provide insight into large-scale evolutionary change Changes in rate and timing: heterochrony  Evolutionary change in rate or timing of developmental events  Can have a significant impact on body shape o I.e. skull shape of chimps  How this changes from infant to adult and how it is different between human and chimp  Contrasting shapes of human and chimpanzee skulls are result of small changes in relative growth rates  Can alter timing of reproductive development relative to development of nonreproductive organs  Paedomorphosis—rate of reproductive development accelerates compared with somatic development o Sexually mature species may retain body features that were juvenile structures in an ancestral species Changes in spatial pattern: homeotic genes  Determine basic features of body plan  Hox genes—class of homeotic genes that provide positional information during development o If Hox genes are expressed in wrong location, body parts can be produced in wrong location  I.e. in crustaceans, a swimming appendage can be produced instead of a feeding appendage  Substantial evolutionary change can also result from alterations in genes that control placement and organization of body parts o Determine such basic features such as where wings and legs will develop on a bird or how a flower’s parts are arranged Evolutionary novelties  Most novel biological structures evolve in many stages from previously existing structures 5

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Complex eyes have evolved from simple photosensitive cells many times Exaptations—structures that evolve in one context but become co-opted for a different function Natural selection can only improve a structure in the context of its current utility

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