Unformatted text preview: The History of Life on Earth
25 Jan 08 BS 110 Accounting for the origin of life Problems: How were the organic molecules of life created?
e.g. amino acids, nucleotides How were they assembled into macromolecules?
e.g. proteins and nucleic acids requires catalysts How could they reproduce themselves? How were they assembled into a discrete system (i.e., a cell)? Abiotic Synthesis of Organic Molecules Three scenarios proposed for synthesis of organic molecules: 1. In surface water from inorganic compounds in the atmosphere 2. rained down on earth from outer space ("Panspermia") 3. In hydrothermal vents on the ocean floor 1. Synthesis in surface water from inorganic compounds in atmosphere Conditions on early Earth very different from now very little atmospheric O2 molecules lightning, volcanoes, UV more intense than today to attack complex Life on Earth probably developed from nonliving materials became ordered into self-replicating, metabolizing aggregates All life today arises only by reproduction of preexisting life principle of biogenesis Replaces "spontaneous generation" In 1953, Miller & Urey recreated "early Earth" conditions in the laboratory discharged sparks in "atmosphere" of gases & water vapor produced amino acids, other organic molecules Variants have produced nearly all small molecules associated with life 17 of the 20 amino acids used in protein synthesis all purines and pyrimidines abiotic synthesis of ribose much more difficult Problem: early earth atmosphere assumed by Miller probably wrong 2. Molecules from outer space?
Organic molecules in meteorites Organic molecules in interstellar space Can synthesize organic molecules, e.g. amino acids, in conditions mimicking space Organic matter rains down on earth continuously (30 tons/day) 3. Deep-sea hydrothermal vents These discharge hydrogen, hydrogen sulfide, and carbon dioxide at c. 100C gases bubble up through chambers rich in iron sulfides (FeS) life today depends on enzymes with Fe and S atoms can catalyze the formation of simple organic molecules Temperature gradient favors different steps can catalyze formation of ammonia (important precursor to organic compounds) from inert nitrogen gas Tiny chambers may have functioned like cells Abiotic Origin Hypothesis 4 likely stages of production of first simple cells: (1) abiotic synthesis of small organic molecules (2) joining these small molecules into polymers (3) origin of self-replicating molecules (4) packaging of these molecules into "protobionts" Next problem---how were first polymers formed? Abiotic origin hypothesis: monomers should link to form polymers without enzymes, other cell processes Doesn't work in solution Polymers, even polypeptides and RNA, can be produced from monomers on hot mineral surfaces conditions like those on early Earth amino acids and nucleic acids synthesized abiotically So, doesn't work in surface water but could on solid surfaces An RNA beginning?
Metabolism depends on proteins (enzymes) Proteins synthesized from DNA code translated by RNA, mediated by enzymes So, DNA can't be made without enzymes But, first hereditary material may have been RNA, not DNA Some kinds of RNA (ribozymes) can both store information and catalyze replication helps resolve paradox of which came first, genes or enzymes DNA may have come later But---ribose hard to synthesize! Machinery of life must be in fluid separated from environment by membrane Living cells may have been preceded by protobionts aggregates of abiotically produced molecules "reproduce" by simple splitting maintain an internal chemical environment differing from their surroundings show some properties associated with living cells: protobiont metabolism excitability membrane potential osmosis Liposomes are protobionts with membrane structure similar to that in cells Last Universal Common Ancestor (LUCA) All 3 domains (Bacteria, Archaea, Eukarya) share many features that must have been present in LUCA DNA, RNA, etc. Bacterial and Archaean cell membranes very different, may have evolved after split from LUCA Archaea and Eukaryotes sister taxa Eukaryote organelles (mitochondria and chloroplasts) from Bacteria only Problems with reconstruction of LUCA: loss of genes, gene swapping Fossils: What are they and why study them?
Phylogenetic analyses allow inference on events of evolution and relationships of species Only the fossil record provides direct evidence on past forms of life Morphology Distribution Temporal occurrence How do fossils form? Most organisms decay quickly after death Rarely, a dead organism is rapidly buried in sediment Ash, sand, mud, etc. Decomposition slow Stagnant water acidic, oxygen-poor Hard parts (shells, bones, teeth, branches, pollen, seeds) usually all that's preserved How do fossils form? Possibilities: Can be preserved intact Can be cemented into sedimentary rocks Compressed into thin carbon-rich film Fills with dissolved minerals, creating cast slow replacement of organic matter by minerals Can decompose after burial, creating hole Can become permineralized How are fossils found? Exposed by Erosion Roadcuts Building sites Quarrying Paleontologists Professional fossil collectors Amateurs Workers, bystanders Who finds them? How are fossils studied? They may be: Plaster-jacketed Site details recorded Prepared from matrix Hardened Morphology compared with recent and other fossil species Age estimated based on dates of nearby rock layers Published Catalogued into museum collection where available for other researchers How common is fossilization? Vast majority of dead organisms decompose without trace Perhaps one in a few hundred million organisms fossilize Most fossils are destroyed by earth processes or never exposed Many not recognized Many in museums still unstudied Many in private/commercial hands What are the limitations of the fossil record? Biases: Habitat bias Biased toward where sediments being deposited Burrowing organisms favored Organisms with hard parts Recent fossils favored Common, widespread species with long temporal occurrence favored Taxonomic bias Temporal bias Abundance bias Nonrandom sample of scientific treasures Determining relative ages of fossils Fossils are frozen in time relative to other strata in a local sample Younger sediments lie above older ones Strata at a location can be correlated in time to those at another using index fossils Species that are Widely distributed Fossilize well Have narrow temporal ranges Readily identifiable Serial record of fossils in rocks provides relative ages
Not absolute ages, the actual time of organism's death But main strata from many areas are accurately dated By comparing different sites, geologists have established a geologic time scale Has consistent sequence of historical periods around world grouped into 4 eras:
Precambrian Paleozoic Mesozoic Cenozoic Boundaries between geologic eras & periods are times of great change, especially mass extinctions Eras and periods not same length The Precambrian Era Prokaryotes dominant from 3.5-2.0 bya Prokaryotes Eukaryotes Earliest organisms almost certainly prokaryotes Early on, prokaryotes diverged into 2 main branches
bacteria and archaea Both thrive today Early forms used very little oxygen Early prokaryote fossils mostly: Stromatolites fossilized layered microbial mats Still being made in places Bacteria from ancient warm water vents Oxygen began accumulating in the atmosphere about 2.7 bya O2 accumulation gradual until 2.2 bya, then shot up to 10% of current values "Oxygen revolution" Due partly to photosynthesis Oxygen a waste product "corrosive" O2 had enormous impact on life May have doomed many prokaryotes Some survived in anaerobic (oxygen-free) habitats Other species evolved to use O2 Eukaryotic life began by 2.1 bya Eukaryotic cells All other life forms than prokaryotes Mostly larger, more complex Incorporated prokaryotes into their cells These evolved into cell organelles 1st eukaryote known from slightly after O2 revolution (2.1 bya) 1st multicellular organisms around 1 bya The Paleozoic Era Cambrian explosion 2nd radiation of eukaryotes produced most major animal groups in early Cambrian corals and sponges existed earlier Plants, fungi, & animals colonized land c. 500 mya (Ordovician) Colonization of land a milestone in the history of life Photosynthetic bacteria on damp terrestrial surfaces > 1 bya Complex life did not colonize land until c. 500 mya Diversification of plants created niches for other life Plant-eating animals & their predators Greatly increased surface area for habitation Protection from predators Root fungi (mycorrhizae) colonized land along with their plant hosts Evolution of terrestrial vertebrates Fishes arose first (Ordovician, c.470 mya) Terrestrial vertebrates (amphibians) evolved from fishes (Devonian, c.370 mya) Reptiles evolved from amphibians Birds & mammals evolved from different reptile groups Biogeographical bases of phylogeny Earth's history helps explain the current geographical distribution of species E.g. emergence of volcanic islands opens new environments adaptive radiation fills available niches Continental drift is the major geographical factor determining spatial distribution of life & evolutionary episodes E.g. adaptive radiations, extinctions The continents drift about Earth's surface on plates of crust floating on the hot mantle. 1st major shock to life: C. 250 mya, all land masses were joined into 1 supercontinent, Pangaea Dramatic impacts on life on land and the sea Species that had evolved in isolation now competed Total shoreline reduced Shallow seas drained Interior of supercontinent drier, weather more severe Reshaped biological diversity by causing extinctions Provided new opportunities for taxonomic groups that survived Tremendous environmental impacts 2nd major shock to life started c. 180 mya Pangaea began to break up Laurasia Gondwanaland Each continent became a separate evolutionary arena Organisms in different biogeographic realms diverged E.g.: similar Triassic (c. 240 mya) reptiles in W Africa & Brazil contiguous during Mesozoic era Australia: marsupial radiation a product of 50 my of isolation Fill most ecological roles of eutherian (placental) mammals on other continents Processes affecting number and types of species on earth = biodiversity
Biodiversity = Speciation minus extinction Speciation Arisal and proliferation Extinction Disappearance History of life punctuated by mass extinction
Long quiescent periods Brief intervals of extensive species turnover Followed by extensive diversification of some surviving groups A species may become extinct because: 1) habitat destroyed 2) environment changed in an unfavorable direction 3) evolutionary changes by other species in community may impact it negatively E.g. late Precambrian/early Cambrian lack of predators Evolution of jawed, shelled organisms in Cambrian doomed defenseless earlier ones Extinction inevitable in a changing world Three major factors affecting earth's long-term patterns of speciation and extinction Large-scale movement of continents Continental drift Over millions of years Gradual climate changes Caused by continental drift Slight shifts in earth's orbit Rapid climate change caused by natural catastrophes Large volcanic eruptions Meteorite and asteroid showers Release of methane from ocean floor Extinction
The ultimate (long-term) fate of all species The vast majority of species (c.99.9%) that have existed since life arose 3.5 bya are extinct Extinction rates Background extinction: low, few Mass depletion: moderate; 3 in past 500 mya Mass extinction: high; 2 in past 500 mya Extinction and Biodiversity Mass depletions/extinctions can result in adaptive radiations takes c. 5 mya Can't replace what is lost Extinction natural humans are causing much higher rates than background extinctions Projected human population growth will cause mass depletions/extinction in next century Global crises caused when conditions changed so rapidly and disruptively that most species died out The fossil record shows 2 severe mass extinctions, several mass depletions Not a mass extinction because so few taxa existed Permian mass extinction (250 mya) Defines Paleozoic/Mesozoic boundary c.90% of marine species went extinct 30% of orders of Permian insects 75% of terrestrial species Occurred in less than 5 my Causes may include: disturbance to habitats due to the formation of Pangaea massive volcanic eruptions in Siberia released enough carbon dioxide to warm the global climate reduced the amount of O2 for marine organisms changes in ocean circulation The Mesozoic Era Age of Reptiles--Dinos Ushered in by Permian extinction First dinos (Triassic) all carnivores Herbivores by Jurassic Greatest diversity early Cretaceous Lystrosaurus `mammal-like reptile' One of the few survivors of end-Permian extinction Single species dominated Triassic terrestrial environments on all continents 240 mya unique in earth's history Lineage diversified, eventually gave rise to mammals The Cretaceous (K-T) mass extinction (65 mya) Defines Mesozoic/Cenozoic boundary Half of marine species went extinct Many families of terrestrial plants & animals went extinct
all land animals > 55 pounds nearly all dinosaur lineages All but one bird lineage But most dinosaurs already extinct by then pterosaurs marine reptiles ammonites Causes of Cretaceous extinction may include: Cooling climate Shallow seas receded from continental lowlands Large volcanic eruptions in India
contributed to global cooling by releasing material into the atmosphere occurred right at end of Cretaceous Asteroid impact theory Proposed by Walter & Luis Alvarez Asteroid 4-9 mi in diameter hit earth Produced huge dust/debris cloud Initially caused fires, severe storms, tsunamis, acid rain Blocked sunlight causing cooling of climate for months Photosynthesis disrupted Herbivores died, then carnivores Evidence for asteroid impact: Widespread, thin layer of iridium-rich clay Iridium rare on Earth, common in meteorites and other extraterrestrial debris Chicxulub crater, a 65-my-old scar beneath sediments on the Yucatan coast Another major asteroid hit same time in India Critical evaluation of impact hypothesis as the cause of the Cretaceous extinctions is ongoing Evidence in favor: Impact large enough to darken the Earth for years reducing photosynthesis so food chains collapsed more severe, temporally compacted extinctions in North America variable rates of global extinction Debris initially inundated North America Elsewhere, effect slower after impact Link between Chicxulub impact event & Cretaceous extinctions still unclear Changes in climate due to continental drift, increased vulcanism, etc. could have caused mass extinctions 65 mya Impact could have been final blow in an environmental assault on late Cretaceous life that included more gradual processes Some groups (e.g. dinos) already in decline Some groups survived, e.g. Plants Vertebrates: Turtles Crocs Amphibians Many reptiles Fishes Birds (at least one group) mammals The up side Mass extinctions create opportunities for survivors Survival may be due to adaptive qualities or luck Survivors become the stock for new radiations Fill biological roles vacated by the extinctions Cretaceous vs. Tertiary mammals Mammals have existed since Triassic All small, furtive Only 2-3 lineages survived K/T event One gave rise to almost all others Very similar pattern for birds The Cenozoic ...
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