Origin Of Life PDF

Topic 9: Origin of life Earth’s biodiversity is built on profound biochemical and genetic similarity. BIOL 108 Winter 2024 © 2024 Neil Harris What is ‘life’? …easy to recognize but hard to define! − The three main biological polymers, nucleic acids, proteins, and polysaccharides, are built from five...

Topic 9: Origin of life Earth’s biodiversity is built on profound biochemical and genetic similarity. BIOL 108 Winter 2024 © 2024 Neil Harris What is ‘life’? …easy to recognize but hard to define! − The three main biological polymers, nucleic acids, proteins, and polysaccharides, are built from five nucleotide bases, 20 amino acids, and a few sugars. 1 Definition of life Generally accepted that an entity needs to have these features to be considered alive: Organization: structurally composed of one or more cells (basic units of life). Metabolism: a system of management of energy and materials via chemical reactions. Response to stimuli via changes in growth, alteration of chemical reactions, or movement. BIOL 108 Winter 2024 © 2024 Neil Harris Homeostasis: maintenance of some internal chemical and/or thermal consistency relative to variation outside of the organism. Adaptation: the ability to change over time in response to the environment. Reproduction: the ability to produce new individual organisms. 2 Are viruses alive? Viruses are infectious parasitic entities that span the boundary between living and non-living. − The biological status of viruses is controversial. − Viruses have nucleic acids that can replicate, mutate, and respond to natural selection. − But viruses lack metabolism and homeostasis, and cannot reproduce outside of a host cell. − Many conclude viruses are not alive.  They “exist in the shady area between life forms and chemicals” (pg 420, Campbell Biology 3ce). BIOL 108 Winter 2024 © 2024 Neil Harris − The origins of viruses are unclear because they do not form fossils.  Phylogenetic placement of viruses in the Tree of Life would be problematic since viruses have probably arisen multiple times. SARS-CoV-2, the virus that causes COVID-19 (WC) T4 bacteriophages infecting a bacterial cell 3 Important events in the history of life BIOL 108 Winter 2024 © 2024 Neil Harris 4.6 bya: Formation of Earth ~3.9 bya: 1st life (replicating molecules) ~2.7 bya: O2 in atmosphere from photosynthesis (cyanobacteria) bya = billions of years ago; mya = millions of years ago 4 Geologic record The geologic record is divided into the Archaean, the Proterozoic, and the Phanerozoic eons. BIOL 108 Winter 2024 © 2024 Neil Harris Fig 25.8 Visualizing the scale of geologic time Precambrian = Archaean + Proterozoic Phanerozoic includes the last half billion years and encompasses multicellular eukaryotic life: − Phanerozoic is divided into three eras: Paleozoic, Mesozoic, and Cenozoic. − Major boundaries between geological divisions correspond to mass extinction events in the fossil record. Cambrian explosion Permian mass extinction Cretaceous mass extinction 5 Fossil record BIOL 108 Winter 2024 © 2024 Neil Harris The fossil record provides direct evidence of evolutionary history. − Fossils are preserved remains/evidence of organisms that lived in the past. − Associated with sedimentary rocks.  Rocks formed through the accumulation of mud, silt, or sand.  Distinct layers of rock are called strata. − Fossils are used to calibrate phylogenies, record extinct species (e.g. linkage between dinosaurs and modern birds), and link evolutionary events (e.g. mass extinctions) with geological and environmental changes on Earth. Microraptor (WC) WC 6 Fossilization The fossil record is biased and incomplete. − Fossilization requires burial in sediment, but sediments accumulate episodically and discontinuously, and fossils typically preserve only the hard parts of organisms. − Most organisms were never fossilized, and even those that were fossilized are rarely discovered by humans. BIOL 108 Winter 2024 © 2024 Neil Harris − Probability of an organism (or part of it) becoming fossilized increases if: Existed for a long time. Was abundant and widespread. Hard rather than soft-bodied. Aquatic rather than terrestrial. Inshore marine rather than offshore marine.  Decomposing organisms were absent.      WC Fossil of a hard shelled marine ammonite 7 Bog man, ~400 BC (National Geographic) Types of fossil records BIOL 108 Winter 2024 © 2024 Neil Harris A cast forms when minerals fill space in sediment where the organism decays after having been buried. Replacement (petrified) fossils have had their tissues replaced by minerals. Cast fossil Replacement fossil Trace fossils record evidence of behaviour (tracks, burrows, feces). Preserved fossils retain the original organic material (carbon films, amber, tar or peat, frozen). Trace fossil − Sub-fossils have a high % organic matter. Frozen mammoth Preserved fossil (mosquito in amber) 8 Determining the age of fossils Relative dating Sedimentary strata reveal the relative ages of fossils. − stratum = layer Relative dating does not indicate how long ago a fossil was created. − Does not provide the absolute age of a fossil, but can tell which fossil came 1st, 2nd, etc. BIOL 108 Winter 2024 © 2024 Neil Harris Challenges to relative dating: − Common to have gaps in a sedimentary sequence.  Inconsistent sediment deposition; erosion of uplifted strata. − Sediments can be tipped or even inverted by major land movements.  Overcome by use of widespread, common index (= indicator) fossils.  Index fossils help to “read” incomplete or scrambled layers. 9 Determining the age of fossils Absolute dating (radiometric dating) Radioactive decay of isotopes of various elements provides a means of determining the age of fossils or rocks. − Radioactive isotopes decay from one form to another at a known constant rate. BIOL 108 Winter 2024 © 2024 Neil Harris Example: carbon-12 (12C) and carbon-14 (14C). − Cosmic radiation creates atmospheric neutrons that combine with nitrogen14 (14N) to create 14C. − Plants take up 12C and 14C as CO2 during photosynthesis → animals eat plants. − Stable isotope 12C is very common: accumulates while living, unchanged after death. − 14C rare: begins to decay (14C → 14N) at the time of death.  https://c14.arch.ox.ac.uk/calibration.html 14C half-life is 5,730 years. 10 Determining the age of fossils Absolute dating (radiometric dating) Half-life is the time required for 50% of atoms in a given amount of isotope to decay. − The decreasing ratio of 14C:12C allows specimens to be dated. − 14C is only suitable for archaeological specimens (up to ~62,000 yr old). BIOL 108 Winter 2024 © 2024 Neil Harris Other isotopes are suitable for rock and fossil specimens older than several thousand years. − e.g. potassium-argon dating (40K halflife 1.25 billion years) and uraniumlead dating (235U half-life 710 million years). Fig 25.6 Radiometric dating 11 Plate tectonics: continental drift Determining the location and time of origin of fossils contributes to understanding the history of Earth. At three points in time, the landmasses of Earth have formed a supercontinent: 1.1 billion, 600 million, and 250 million years ago. BIOL 108 Winter 2024 © 2024 Neil Harris Plate tectonics theory considers that Earth’s crust is composed of large plates that have been slowly moving since about 3.4 billion years ago. − Tectonic plates move slowly through the process of continental drift. The mantle is predominantly solid but behaves as a very viscous fluid on a geological timescale. Tectonic plates ‘float’ on the mantle. Fig 25.14 Cutaway view of Earth 12 Continental drift Continental drift causes tectonic plates to collide, separate, or slide past each other. Interactions between plates cause the formation of mountains, islands, and earthquakes. BIOL 108 Winter 2024 © 2024 Neil Harris Figure 25.15 Earth’s major tectonic plates − Tectonic boundaries are sites of earthquakes and volcanoes. The relative locations of continental landmasses have changed over time. 13 Continental drift during the Phanerozoic Eon BIOL 108 Winter 2024 © 2024 Neil Harris Consequences of continental drift: 1. Formation of supercontinent Pangaea ~250 million years ago had many effects: − Deepening of ocean basins. − Reduction in shallow water habitats. − Colder and drier climate inland. Pangaea (pan = everywhere, everything; gaia = Earth) Fig 25.16 The history of continental drift during the Phanerozoic eon 14 Continental drift during the Phanerozoic Eon BIOL 108 Winter 2024 © 2024 Neil Harris Consequences of continental drift: 2. Continental drift has strongly influenced Earth’s biodiversity. − Changes to the environment and climate as continents move north or south. − Opportunities for diversification.  e.g. when land masses become isolated, each can develop unique species via allopatric speciation. − Mass extinctions. Antarctic dinosaur, Cryolophosaurus (‘cold crested lizard’)  e.g. when Antarctica drifted to the south pole and froze solid. Fig 25.16 The history of continental drift during the Phanerozoic eon 15 Continental drift during the Phanerozoic Eon BIOL 108 Winter 2024 © 2024 Neil Harris Consequences of continental drift: 3. The distribution of fossils reflects the geologic movement of continents. − e.g. the same genus of fossil plants found in Australia, Antarctica, and South America provides evidence for palaeo-continent Gondwana. Glossopteris spp. (extinct seed-bearing fern trees and shrubs) Fig 25.16 The history of continental drift during the Phanerozoic eon 16 Extinctions The fossil record shows that most species that have ever lived are now extinct. − >99% of all species that ever lived are now extinct. − The causes of extinction are varied, but it generally occurs when a species cannot adapt to changes in the species’ environment. BIOL 108 Winter 2024 © 2024 Neil Harris At times, the rate of extinction increased dramatically and caused mass extinctions. − Mass extinctions are the result of disruptive global environmental changes. − The history of life is characterized by five mass extinctions that changed the evolution of Earth’s biota. 17 “Big five” mass extinction events BIOL 108 Winter 2024 © 2024 Neil Harris In each of the five mass extinctions, more than 50% of Earth’s species became extinct. − Two of the notable mass extinctions are the Permian extinction and the Cretaceous extinction. Fig 25.17 Mass extinction and the diversity of life 18 Permian mass extinction The Permian mass extinction defines the boundary between the Paleozoic and Mesozoic eras 252 million years ago. BIOL 108 Winter 2024 © 2024 Neil Harris WC Highly diverse marine arthropods, trilobites, did not survive the Permian mass extinction − The Permian mass extinction was rapid, occurring over 10× Earth’s human population!  ~1010 strains of prokaryotes on Earth, 1000s of times more than the number of eukaryote species. Prokaryotes thrive almost everywhere, from your gut to deep in the Earth’s crust, including habitats too acidic, salty, cold, or hot for most eukaryotes. − e.g. Alberta oil sands tailings ponds are toxic to flora and fauna but are an energy source for many microbes.  Prokaryotes that live in tailings ponds emit large quantities of methane, a potent greenhouse gas. Figure 27.1 44