genbio2

• Animals


  • Multicellular, heterotrophic eukaryotes with tissues that develop from embryonic layers
  • Most are mobile and use traits like strength, speed, toxins, or camouflage to detect, capture, and eat other organisms
  • Lack cell walls, held together by structural proteins like collagen
  • Nervous tissue and muscle tissue are unique, defining characteristics
• Nutritional mode


  Heterotrophs that ingest their food
• Animal cells


  • Lack cell walls
  • Held together by structural proteins like collagen
• Animal reproduction and development

  1. Sexual reproduction with diploid stage dominating

  2. Zygote undergoes cleavage to form multicellular, hollow blastula

  3. Blastula undergoes gastrulation to form gastrula with different embryonic tissue layers

  4. Most have at least one larval stage that undergoes metamorphosis to become juvenile
• Hox genes


  • Regulate development of animal body form
  • Highly conserved but can produce diverse animal morphology
• History of animals spans more than half a billion years
• More than 1.3 million animal species have been named to date, far more estimated to exist
• Common ancestor of all living animals likely lived between 700 and 770 million years ago
• Fossil record


  • Reveals changes in history of life on Earth
  • Sedimentary rocks deposited in layers called strata are richest source of fossils
  • Absolute ages of fossils can be determined by radiometric dating
• Key events in life's history include origins of unicellular and multicellular organisms and colonization of land
• Photosynthesis and the oxygen revolution

  1. O2 produced by oxygenic photosynthesis reacted with dissolved iron and precipitated out to form banded iron formations

  2. By 2.7 billion years ago, O2 began accumulating in atmosphere and rusting iron-rich rocks

  3. This "oxygen revolution" caused extinction of many prokaryotic groups, but some adapted using cellular respiration
• First eukaryotes


  • Oldest fossils date back 1.8 billion years
  • Have nuclear envelope, mitochondria, endoplasmic reticulum, cytoskeleton
  • Endosymbiont theory - prokaryotic ancestors of mitochondria and plastids gained entry as undigested prey or internal parasites
• Second wave of diversification occurred when multicellularity evolved and gave rise to algae, plants, fungi, and animals
• Choanoflagellates


  Protists that are closest living relatives to animals
• Origin of multicellularity


  • Requires evolution of new ways for cells to adhere and signal to each other
  • Molecular analysis reveals similarities between genes coding for adherence and attachment proteins in choanoflagellates and animals
• Ediacaran biota were an assemblage of larger and more diverse soft-bodied organisms that lived about 560 million years ago
• Early animal embryos and evidence of predation have also been found in Neoproterozoic rocks
• Cambrian explosion (535 to 525 million years ago)

  • Marks earliest fossil appearance of many major groups of living animals
  • Most fossils are of bilaterians - organisms with bilateral symmetry, complete digestive tract, one-way digestive system
• A few animal phyla appear even earlier: sponges, cnidarians, and molluscs
• Hypotheses regarding Cambrian explosion and decline of Ediacaran biota

  • New predator-prey relationships
  • Rise in atmospheric oxygen
  • Evolution of Hox gene complex
• Animal diversity continued to increase through Paleozoic, but was punctuated by mass extinctions
• Fungi, plants, and animals began to colonize land about 500 million years ago
• Terrestrial animals


  • Arthropods and tetrapods are most widespread and diverse
  • Tetrapods evolved from lobe-finned fishes around 365 million years ago
• Coral reefs emerged, becoming important marine ecological niches for other organisms
• Ancestors of plesiosaurs were reptiles that returned to the water
• Dinosaurs were the dominant terrestrial vertebrates
• The first mammals emerged
• Beginning of Cenozoic era followed mass extinctions of both terrestrial and marine animals, including large, nonflying dinosaurs and marine reptiles
• Mammals increased in size and exploited vacated ecological niches
• Global climate cooled
• The "Big Five" Mass Extinction Events

  • In each, 50% or more of marine species became extinct
• Factors that might have contributed to Mass Extinctions


  • Intense volcanism in Siberia
  • Global warming and ocean acidification from CO2 emissions
  • Anoxic conditions from nutrient enrichment
  • Meteorite impact 65 mya that blocked sunlight and disturbed global climate
• Scientists estimate current extinction rate is 100 to 1,000 times the typical background rate
• Extinction rates tend to increase when global temperatures increase
• Data suggest a sixth, human-caused mass extinction is likely to occur unless dramatic action is taken
• Body plan


  A set of morphological and developmental characteristics that define an animal
• Intense volcanism in what is now Siberia
• Global warming and ocean acidification resulting from the emission of large amounts of CO2 from the volcanoes
• Anoxic conditions resulting from nutrient enrichment of ecosystems
• The presence of iridium in sedimentary rocks suggests a meteorite impact about 65 mya