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Chapter 26

Chapter 26. The Tree of Life: An Introduction to Biological Diversity. Overview: Changing Life on a Changing Earth. Life is a continuum extending from the earliest organisms to the variety of species that exist today Geological events change the course of evolution

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Chapter 26

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  1. Chapter 26 The Tree of Life:An Introduction to Biological Diversity

  2. Overview: Changing Life on a Changing Earth • Life is a continuum extending from the earliest organisms to the variety of species that exist today • Geological events change the course of evolution • Conversely, life changes the planet that it inhabits

  3. Geologic history and biological history have been episodic, marked by revolutions that opened many new ways of life

  4. Concept 26.1: Conditions on early Earth made the origin of life possible • Chemical and physical processes on early Earth may have produced very simple cells through a sequence of stages: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into polymers 3. Packaging of molecules into “protobionts” 4. Origin of self-replicating molecules

  5. Synthesis of Organic Compounds on Early Earth • Earth formed about 4.6 billion years ago, along with the rest of the solar system • Earth’s early atmosphere contained water vapor and chemicals released by volcanic eruptions • Experiments simulating an early Earth atmosphere produced organic molecules from inorganic precursors, but such an atmosphere on early Earth is unlikely

  6. LE 26-2 CH4 Water vapor Electrode NH3 H2 Condenser Cold water Cooled water containing organic molecules H2O Sample for chemical analysis

  7. Instead of forming in the atmosphere, the first organic compounds may have been synthesized near submerged volcanoes and deep-sea vents Video: Hydrothermal Vent Video: Tubeworms

  8. Extraterrestrial Sources of Organic Compounds • Some organic compounds from which the first life on Earth arose may have come from space • Carbon compounds have been found in some meteorites that landed on Earth

  9. Looking Outside Earth for Clues About the Origin of Life • The possibility that life is not restricted to Earth is becoming more accessible to scientific testing

  10. Abiotic Synthesis of Polymers • Small organic molecules polymerize when they are concentrated on hot sand, clay, or rock

  11. Protobionts • Protobionts are aggregates of abiotically produced molecules surrounded by a membrane or membrane-like structure • Experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds • For example, small membrane-bounded droplets called liposomes can form when lipids or other organic molecules are added to water

  12. LE 26-4 Glucose-phosphate 20 mm Glucose-phosphate Phosphorylase Starch Amylase Phosphate Maltose Maltose Simple reproduction Simple metabolism

  13. The “RNA World” and the Dawn of Natural Selection • The first genetic material was probably RNA, not DNA • RNA molecules called ribozymes have been found to catalyze many different reactions, including: • Self-splicing • Making complementary copies of short stretches of their own sequence or other short pieces of RNA

  14. LE 26-5 Ribozyme (RNA molecule) 3¢ Template 3¢ Nucleotides Complementary RNA copy 5¢ 5¢

  15. Early protobionts with self-replicating, catalytic RNA would have been more effective at using resources and would have increased in number through natural selection

  16. Concept 26.2: The fossil record chronicles life on Earth • Fossil study opens a window into the evolution of life over billions of years

  17. How Rocks and Fossils Are Dated • Sedimentary strata reveal the relative ages of fossils

  18. Index fossils are similar fossils found in the same strata in different locations • They allow strata at one location to be correlated with strata at another location Video: Grand Canyon

  19. The absolute ages of fossils can be determined by radiometric dating • The magnetism of rocks can provide dating information • Magnetic reversals of the magnetic poles leave their record on rocks throughout the world

  20. LE 26-7 Accumulating “daughter” isotope 1 Ratio of parent isotope to daughter isotope 2 1 Remaining “parent” isotope 4 1 8 1 16 1 2 3 4 Time (half-lives)

  21. The Geologic Record • By studying rocks and fossils at many different sites, geologists have established a geologic record of Earth’s history

  22. The geologic record is divided into three eons: the Archaean, the Proterozoic, and the Phanerozoic • The Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic • Each era is a distinct age in the history of Earth and its life, with boundaries marked by mass extinctions seen in the fossil record • Lesser extinctions mark boundaries of many periods within each era

  23. Mass Extinctions • The fossil record chronicles a number of occasions when global environmental changes were so rapid and disruptive that a majority of species were swept away Animation: The Geologic Record

  24. LE 26-8 Millions of years ago 600 500 400 300 200 100 0 100 2,500 Number of taxonomic families 80 2,000 Permian mass extinction ) Extinction rate 60 1,500 Number of families ( Extinction rate ( 40 1,000 Cretaceous mass extinction ) 20 500 0 0 Cambrian Ordovician Silurian Devonian Carboniferous Permian Triassic Jurassic Cretaceous Paleogene Neogene Proterozoic eon Ceno- zoic Paleozoic Mesozoic

  25. The Permian extinction killed about 96% of marine animal species and 8 out of 27 orders of insects • It may have been caused by volcanic eruptions • The Cretaceous extinction doomed many marine and terrestrial organisms, notably the dinosaurs • It may have been caused by a large meteor impact

  26. LE 26-9 NORTH AMERICA Chicxulub crater Yucatán Peninsula

  27. Mass extinctions provided life with unparalleled opportunities for adaptive radiations into newly vacated ecological niches

  28. A clock analogy can be used to place major events in the context of the geological record

  29. LE 26-10 Ceno- zoic Meso- zoic Humans Paleozoic Land plants Animals Origin of solar system and Earth 1 4 Proterozoic Eon Archaean Eon Billions of years ago 2 3 Multicellular eukaryotes Prokaryotes Single-celled eukaryotes Atmospheric oxygen

  30. Concept 26.3: As prokaryotes evolved, they exploited and changed young Earth • The oldest known fossils are stromatolites, rocklike structures composed of many layers of bacteria and sediment • Stromatolites date back 3.5 billion years ago

  31. The First Prokaryotes • Prokaryotes were Earth’s sole inhabitants from 3.5 to about 2 billion years ago

  32. Electron Transport Systems • Electron transport systems were essential to early life • Some of their aspects may precede life itself

  33. Photosynthesis and the Oxygen Revolution • The earliest types of photosynthesis did not produce oxygen • Oxygenic photosynthesis probably evolved about 3.5 billion years ago in cyanobacteria

  34. Effects of oxygen accumulation in the atmosphere about 2.7 billion years ago: • Posed a challenge for life • Provided opportunity to gain energy from light • Allowed organisms to exploit new ecosystems

  35. Concept 26.4: Eukaryotic cells arose from symbioses and genetic exchanges between prokaryotes • Among the most fundamental questions in biology is how complex eukaryotic cells evolved from much simpler prokaryotic cells

  36. The First Eukaryotes • The oldest fossils of eukaryotic cells date back 2.1 billion years

  37. Endosymbiotic Origin of Mitochondria and Plastids • The theory of endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living within larger host cells

  38. The prokaryotic ancestors of mitochondria and plastids probably gained entry to the host cell as undigested prey or internal parasites • In the process of becoming more interdependent, the host and endosymbionts would have become a single organism

  39. LE 26-13 Cytoplasm DNA Plasma membrane Ancestral prokaryote Infolding of plasma membrane Endoplasmic reticulum Nucleus Nuclear envelope Engulfing of aerobic heterotrophic prokaryote Cell with nucleus and endomembrane system Mitochondrion Mitochondrion Engulfing of photosynthetic prokaryote in some cells Ancestral heterotrophic eukaryote Plastid Ancestral photosynthetic eukaryote

  40. Key evidence supporting an endosymbiotic origin of mitochondria and plastids: • Similarities in inner membrane structures and functions • Both have their own circular DNA

  41. Eukaryotic Cells as Genetic Chimeras • Endosymbiotic events and horizontal gene transfers may have contributed to the large genomes and complex cellular structures of eukaryotic cells • Eukaryotic flagella and cilia may have evolved from symbiotic bacteria, based on symbiotic relationships between some bacteria and protozoans

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