History of Life. How do We Know How Old Rocks and Fossils Are?

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Lucas Brown
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1 History of Life Once the first living cells arose, 3.9 BY ago, Natural Selection easily explains the appearance of all other species Chemical evolution became biological evolution 3.8 BY indirect evidence that life may have existed 3.5 BY 1 st true fossils: very simple cells = procaryotes 11 different species have been found In 1800 s, Challenger Expedition deep sea collections thought they had found the first life forms: Bathybius haekelii primordial slime turned out to be precipitate formed when sediment was mixed with alcohol How do We Know How Old Rocks and Fossils Are? Relative Ages There are several kinds of rock found on earth the easiest rocks to date are Sedimentary Rocks rocks formed in the ocean of thousands or millions of years from the slow deposition of successive layers of sediment and dead shells and organisms we have 350,000 ft (75 miles) of sedimentary rock deposits preserving an extensive record of past life but still many gaps; not a continuous series at first was only a relative dating method later became more accurate as we got a better understanding of how sedimentary rock is produced Absolute Dating much more accurate dating technique Life, Biodiversity, History: History of Life; Ziser,

2 based on the very precise rate of radioactive decay for various isotopes some systems currently used: Decay System Half Life Useful Timespan Rubidium Strontium 1.42X10-11 yr BY Lutetium Hafnium 1.94x10-11 yr ±1.2BY Uranium Lead 1.55 X10-10 yr BY Potassium Argon X10-10 yr BY 14 C 5568±30 yr 3 Once life arose, what were some of the major events that occurred from then until now? Major events in History of Life (fossils): 1. Photosynthesis (3.5 by) 2. Aerobic Respiration (1.5 by) 3. Eucaryotes (1.8 by) 4. Multicellular Forms (~0.8 by) first animals first fungi first plants 3.5 BY ago Photosynthesis Once life appeared the organic molecules that had been accumulating for a billion years were quickly consumed as food supply dwindled any adaptation that would have enabled an organism to manufacture its own food would be favored some of the earliest fossils we have are blue green bacteria (= autotrophs) use sunlight as energy to make large organic molecules so photosynthesis arose fairly quickly photosynthesis uses carbon dioxide and some source of hydrogen as building blocks to make sugars these molecules could then be broken down when energy was needed Life, Biodiversity, History: History of Life; Ziser,

3 eg. plant/algae photosynthesis: CO 2 + H 2 O sunlight sugar + O 2 first autotrophs probably did not produce oxygen hydrogen source was H 2 or H 2 S (produced S 2 ) CO 2 + H 2 S sunlight sugar + S 2 only later ~2.8BY [BG bacteria] was water used as H source (far more abundant as other sources dwindled) probably the most important innovation in the history of life on earth: life was freed from energy scarcity forever from then on life was limited primarily by the scarcity of just a few specific nutrients, eg P or N, but not an energy source; had limitless energy when the new form of photosynthesis originated the waste product of the process was O2 this increased autotroph efficiency and global productivity by 100 s or 1000 s of times but: O2 is lethal to most cells O2 was probably fatal to most of the earth s early life forms (an unrecorded mass extinction?) cells exposed to oxygen had to develop protections against it eventually oxygen began to accumulate in the atmosphere oxygen gas didn t show up in the atmosphere for ~500MY after it was first produced [2.8BY BG bacteria 2.3 BY atm oxygen] abundant metals and volcanic gasses of early earth reacted with free O 2 indications of oxygen accumulation are found in rocks up to 2.3 BY old produced banded iron formations (oldest evidence for bacterial Life, Biodiversity, History: History of Life; Ziser,

4 oxygen production greatest pollution crisis the world has ever known it took over 1.5 BY for O 2 to accumulate to near current levels ALL the free O 2 in our atm was produced by living cells ozone began to accumulate in upper atmosphere blocking UV rays UV O2---- O + O O2 + O O3 UV was main energy source in origin of life ~2BY ago ( BY) Life could probably never again arise spontaneously on earth Aerobic Respiration new forms of generating energy probably arose that were more efficient aerobic respiration original heterotrophs probably used fermentation-like processes which produce very little energy Only partially breaks down organic molecules: CCCCCC CCC + CCC aerobic respiration extracts much more energy from organic food breaks sugars down into inorganic molecules; carbon dioxide and water eg fermentation of sugar 2 units of energy aerobic respiration of sugar units of energy (19x s more) Life, Biodiversity, History: History of Life; Ziser,

5 The Age of Bacteria most of life s history (~2 BY of it) consisted of prokaryotes (bacteria) only by 1.5 BY most biochemical evolution had been accomplished all the unique and unusual metabolic pathways found in all other kingdoms today were already present in bacteria along with many other more exotic pathways many of the bacteria alive today seem to have changed very little from those earliest ones: eg. stromatolites are still formed today eg. bg bacteria seem to have changed little even today bacteria are the most tenaceous beings known Bacterial Accomplishments: 1. prokaryotes could assemble and disassemble all molecules of modern life virtually all metabolic pathways: photosynthesis fermentation aerobic respiration with the exception of a few exotic chemicals eg, essential oils and hallucinogens of flowering plants eg. snake venoms 2. the earth s modern atmosphere and surface were largely established by bacteria 3. bacteria invented asexual and a simple form of sexual reproduction they grow and divide quickly life cycle in minutes not days or years bacteria are essentially immortal they don t age as complex cells do. bacteria are able to exchange genes with each other and from the environment Life, Biodiversity, History: History of Life; Ziser,

6 we can only exchange genes between males and females essentially makes all bacteria a superorganism = one worldwide species Many people contend that even today the world is run mainly by bacteria the age of mammals ; age of dinosaurs ; age of human civilization are myths 1.8 BY ago eg. more bacteria inhabit your mouth than there are people in NYC eg. 9x s more bacterial cells in your body than human cells (90 trillion vs 10 trillion cells) eg. eucaryotic food webs form a crown intricate and unnecessary atop ecosystems fundamentally maintained by prokaryotic metabolism Knoll Life on a Young Planet we cannot survive without bacteria but they would do just fine without us! Origin of Eukaryotes a stable aerobic environment was now established no major changes for 1.5 BY organisms had begun finding ways to protect themselves from free oxygen and apparently promoted the development of the next major step in the history of life 1.8 BY ago first eukaryotic cells appear (Protists) molecular evidence indicates they may have existed a billion years before that (2.7BY) but no true fossils eukaryotes appeared suddenly, not due to gradual changes this was a drastic change from over 2 BY of previous evolution today all cells are either prokaryotes or eukaryotes Life, Biodiversity, History: History of Life; Ziser,

7 all cells either have a nucleus or not prokaryotes and eucaryotes are chemically similar; all eukaryotic metabolic pathways are found in procaryotes major structural differences: -generally larger cells -genetic material enclosed within nucleus -DNA is more complex, more genes, associated with proteins called histones -cell walls, when present are made of different chemicals -contain many membrane bound organelles -more complex kinds of cell division must be some tremendous advantages: 1. more compartments, more organization enzymes for specific metabolic pathways are grouped together in specific organelles 2. more efficient almost all eucaryotes (including single celled) are aerobes most efficient metabolism x s more DNA eukaryotes have 1000 s times more DNA than prokaryotes more genetic instructions Eukaryotes evolved by symbioses of various prokaryotes (Mutualism) = endosymbiont theory mutualism is common in prokaryotes: 1. amenable to rapid genetic changes 2. don t reject intruders 3. chemically diverse, often complementary colonial associations Evidence for endosymbiont origin 1. many modern organisms contain intracellular symbiotic bacteria Bacteria are the best endosymbionts have been doing it for Millions of years invasive spirochaetes are one of most successful lifeforms on earth found in unusual niches: Life, Biodiversity, History: History of Life; Ziser,

8 eg. crystalline style of oysters eg. protists in termite gut 2. Some eukaryotic organelles still contain bacterial chromosomes today in eukaryotes chloroplasts and mitochondria have given most of their genes to the nucleus of the cell but all have retained a functional genome of at least 5 genes 3. some eukaryotic organelles contain bacterial ribosomes 4. Some eukaryotic organelles divide asexually in a process very similar to bacterial reproduction eg. mitochondria: main energy producing organelle in all eukaryotes -most protists, all plants, fungi & animals -contain their own DNA resembling bacterial DNA -contain many enzymes present in bacteria -contain bacterial type ribosomes -produced only by other mitochondria; host cell cannot produce them alone -resemble purple nonsulfur bacteria eg. chloroplasts: main organelle for photosynthesis in eukaryotes -contain DNA -contain same pigment as BG bacteria -photosynthetic procaryote ingested by larger non-photosynthetic procaryote -probably happened independently in several lines -some protists, fungi, plants and animals contain symbiotic cyanobacteria they lack cell walls divide same time as host are functionally chloroplasts eg. cilia and flagella: -perhaps from motile spirochaete-like bacteria -similar symbiotic associations are seen today in termite symbionts Life, Biodiversity, History: History of Life; Ziser,

9 Sexual Reproduction eg. centrioles (mitosis): -related to cilia and flagella structurally -divide independently of cell -some report finding DNA within them one of the most significant shifts in going from prokaryote to eukaryote is in how genes are exchanged prokaryotes freely exchange genes with each other or with environment all bacteria can easily trade genes with other bacteria don t have to be same species even though their main method of reproduction is asexual fission their genes are modular freely traded this creates a powerful tool for evolutionary adaptation eg. antibiotic resistance eg. S. pneumonia pathogenicity in rats once genes were trapped in a nucleus an organism s ability to freely exchange them becomes very limited eukaryotes traded genetic flexibility for an increase in size and complexity more efficient use of energy more complex cells and bodies but now genetic change is only possible during reproduction (with a few exceptions) Eukaryotes and all their offspring (Fungi, Plants, Animals) now mainly trade genes vertically from generation to generation. Rather than horizontally directly with their neighbors, Life, Biodiversity, History: History of Life; Ziser,

10 and in the same generation Result: genetically fluid bacteria are immortal but in eukaryotes, sex becomes linked with death from ~2BY to ~1BY before today, even after the origin of eukaryotes, bacteria reigned supreme eukaryotes existed for a billion years without going anywhere in terms of evolution why? if they were so much more efficient; such a good design did they not take off and diversify the answer might be due to low level of O 2 in atmosphere and in ocean O 2 1 st appeared in atmosphere ~ 2.5BY ago still, all but the uppermost layers of the ocean remained anoxic low levels of O 2 were not enough to free up critical nutrients such as Fe, Cu, Zn, etc from the minerals and molecules that contained them these minerals are required to make the enzymes needed to extract Nitrogen from seawater (N fixing requires Fe and other ions as cofactors) lack of excess O 2 made nutrients extremely hard to get for eucaryotes Only after oxygen levels increased were the micronutrients freed up for use by algae Multicellular Life 4/5ths of all geologic time was dominated by primitive, slowly evolving microbes prokaryotes and eukaryotic single celled organisms virtually the entire basic organization of the biological world dates from this time: aerobes/anaerobes Life, Biodiversity, History: History of Life; Ziser,

11 fermenters photosynthesis aerobic respiration producers/consumers bacterial fission sexual and asexual reproduction mitosis and meiosis shortly after eucaryotes appeared, multicellular organisms appear and diversify in the fossil record compared to the ~2BY that only bacteria were on the earth, Eukaryotes appeared and diversified into all major kingdoms in a relatively short time: a few 100 MY Unlike the appearance of eukaryotes: multicellularity was not a major step but a natural progression toward increased competitive interaction and specialization while we tend to think of bacteria and protists as single celled organisms they more typically form groupings, films, filaments, etc in other words they were functionally multicellular poor fossil record during most of this time mostly soft bodied forms leave few fossils there is molecular evidence that they may have existed much earlier in fossil record once multicellularity evolved the more complex lifeforms evolved very quickly all kingdoms, even bacteria, have multicellular members even bacteria don t really function as single cells in nature: in any ecological niche teams of several kinds of Life, Biodiversity, History: History of Life; Ziser,

12 bacteria live together responding to and creating each others environment wastes of one becomes another s food complementary enzymes eg. stromatolites layers of different kinds of bacteria in most multicellular forms the organism begins life as a single cell and this cell is cloned to produce the adult organism eg. human zygote 10 Trillion cells of adult not all multicellular forms begin as a single cell: eg. this is not the case in a few slime molds and fungi whose multicellular bodies form from an aggregation of several genetically distinct individuals there are 3 main kingdoms of mostly multicellular forms: animals fungi plants Life, Biodiversity, History: History of Life; Ziser,

13 History of the Earth in a Single Year days yrs b. p. (1) Jan 1, 12:00am - earth formed (4.6 BY) (63) May 5: - 1 st Cells (3.8 BY) (135) May 16: - O 2 producing photosynthesis (2.9 BY) (206) July 26: - aerobic respiration (~2.0 BY) (222) Aug 11: - origin of eucaryotes (1.8 BY) (301) Oct 28: - multicellularity-1 st animal fossils (800 MY) (320) Nov 16: - O 2 levels near 20%; Cambrian Explosion (570 MY) (326) Nov 21: - 1 st fossils of land animal (490 MY) (329) Nov 24: - 1 st plant fossils (460 MY) (335) Nov 30: - 1 st land vertebrate fossils (380 MY) (345) Dec 10: - Earth s greatest mass extinction (250 MY) (353) Dec 18: - 1 st fossil bird (150 MY) (355) Dec 21: - 1 st fossils of flowering plants (125 MY) (359) Dec 25: - end of dinosaurs; beginning of age of mammals;1 st primate fossils (65 MY) (363) Dec 28: - spread of grasses (364) Dec 30, 9:00pm: - 1 st human fossils Dec 30, 11:24pm: - 1 st Homo sapiens fossils Life, Biodiversity, History: History of Life; Ziser,

14 What is life? Life is the representation, the presencing of past chemistries, a past environment of the early Earth that, because of life, remains on the modern Earth. It is the water, membrane-bound encapsulation of spacetime. Life is a nexus of increasing sensitivity and complexity in a universe of parent matter that seems stupid and unfeeling in comparison. Preserving the past, making a difference between past and present, life binds time, expanding complexity and creating new problems for itself. L. Margulis & D. Sagan What is Life? Life, Biodiversity, History: History of Life; Ziser,

Chapter 19. History of Life on Earth

Chapter 19. History of Life on Earth Chapter 19 History of Life on Earth Adapted from Holt Biology 2008 Chapter 19 Section 3: Evolution of Life Key Vocabulary Terms Adapted from Holt Biology 2008 Cyanobacteria Photosynthetic prokaryotes Adapted