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Chapter 25 Notes.pdf

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BIOL 226
Robert Weladji

• CHAPTER 25. THE HISTORY OF LIFE ON EARTH Overview • Past organisms were very different from those now alive • Macroevolution is the pattern of evolution over large time scales. • The fossil record shows macroevolutionary changes over large time scales including – The emergence of terrestrial vertebrates – The origin of photosynthesis – Long-term impacts of mass extinctions Origin of life • 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 4 main stages: 1. Abiotic synthesis of small organic molecules 2. Joining of these small molecules into macromolecules 3. Packaging of molecules into “protocells” 4. Origin of self-replicating molecules, which eventually made inheritance possible (The first genetic material was probably RNA, not DNA) The Fossil Record documents the history of life • There is however no record on these ancient events, hence the use of the fossil record • The fossil record is based on the sequence in which fossils have accumulated in such strata • Fossils reveal ancestral characteristics that may have been lost over time • The fossil record reveals changes in the history of life on earth • Fig 25-4 How Rocks and Fossils Are Dated • Sedimentary strata reveal the relative ages of fossils • The absolute ages of fossils can be determined by radiometric dating • A “parent” isotope decays to a “daughter” isotope at a constant rate • Each isotope has a known half-life, the time required for half the parent isotope to decay • However, we can go only up to 75000 years back in time for carbon 14!! Radiometric dating – Example Concept check – 25.2#1 • Fossilized skull • C14C 12= 1/16 (as compared to skull of present day animals) • C has half life = 5730 14 • Approximate age of the fossilized skull =? • ? 1 What can we learn from the fossils? The Origin of New Groups of Organisms • Mammals belong to the group of animals called tetrapods (four limbs) • The evolution of unique mammalian features through gradual modifications can be traced from ancestral synapsids through the present Key events in life’s history • The origins of single-celled and multicelled organisms • And the colonization of land First fossils Stromatolites of Cyanobacteria (single-celled organisms) Oxygen revolution • Most atmospheric oxygen (O ) is of biological origin 2 • Cyanobacteria perform oxygenic photosynthesis • Oxygen in water increases • Oxygen in atmosphere increases • Major extinction of anaerobes Evolution of eukaryotes • How did the complex organization of the eukaryotic cell evolve from the simpler prokaryotic condition? Endosymbiosis : The theory of endosymbiosis proposes that mitochondria and plastids were formerly small prokaryotes living within larger host cells (see Fig 25.9) The Origin of Multicellularity • The 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 The Cambrian Explosion • The Cambrian explosion refers to the sudden appearance of fossils resembling modern phyla in the Cambrian period (535 to 525 million years ago) • The Cambrian explosion provides the first evidence of predator-prey interactions • Cambrian explosion • The Colonization of Land • Fungi, plants, and animals began to colonize land about 500 million years ago • Plants and fungi likely colonized land together by 420 million years ago • Arthropods and tetrapods are the most widespread and diverse land animals • Tetrapods evolved from lobe-finned fishes around 365 million years ago 2 The rise and fall of dominant groups reflect continental drift, mass extinctions, and adaptive radiations • The history of life on Earth has seen the rise and fall of many groups of organisms • Continental Drift • At three points in time, th
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