Chapter 21: The Origin and Evolutionary History of Life

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Chemical Evolution on Early Earth

• Biologists generally agree that life originated from nonliving matter by chemical evolution.
  • Hypotheses about chemical evolution are testable.
• Four requirements for chemical evolution are:
  • (1) the absence of oxygen, which would have reacted with and oxidized abiotically produced organic molecules
  • (2) energy to form organic molecules
  • (3) chemical building blocks, including water, minerals, and gases present in the atmosphere
  • (4) sufficient time for molecules to accumulate and react.
• During chemical evolution, small organic molecules formed spontaneously and accumulated.
  • The prebiotic soup hypothesis proposes that organic molecules formed near Earth’s surface in a “sea of organic soup” or on rock or clay surfaces.
  • The iron–sulfur world hypothesis suggests that organic molecules were produced at hydrothermal vents, cracks in the deep-ocean floor.

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The First Cells

• After small organic molecules formed and accumulated, macromolecules assembled from the small organic molecules.
  • Macromolecular assemblages called protobionts formed from macromolecules.
  • Cells arose from the protobionts.
• According to a model known as the RNA world, RNA was the first informational molecule to evolve in the progression toward a self-sustaining, self-reproducing cell.
  • Natural selection at the molecular level eventually resulted in the information sequence DNA → RNA → protein.
• The first cells were prokaryotic heterotrophs that obtained organic molecules from the environment.
  • They were almost certainly anaerobes.
  • Later, autotrophs— organisms that produce their own organic molecules by photosynthesis—evolved.
• The evolution of oxygen-generating photosynthesis ultimately changed early life.
  • The accumulation of molecular oxygen in the atmosphere permitted the evolution of aerobes, organisms that could use oxygen for a more efficient type of cellular respiration.
• Eukaryotic cells arose from prokaryotic cells.
  • According to the hypothesis of serial endosymbiosis, certain eukaryotic organelles (mitochondria and chloroplasts) evolved from prokaryotic endosymbionts incorporated within larger prokaryotic hosts.

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The History of Life

• Life at the beginning of the Proterozoic eon (2500 mya to 541 mya) consisted of prokaryotes.
  • About 2.2 bya, the first eukaryotic cells appeared.
  • The Ediacaran period, from 635 mya to 541 mya, is the last period of the Proterozoic eon.
  • Ediacaran fossils are the oldest known fossils of mul- ticellular animals.
  • Ediacaran fauna were small, soft-bodied invertebrates.
• During the Paleozoic era, which began about 541 mya and lasted approximately 289 million years, all major groups of plants, except flowering plants, and all animal phyla appeared.
  • Fishes and amphibians flourished, and reptiles appeared.
  • The greatest mass extinction of all time occurred at the end of the Paleozoic era, 252 mya.
  • More than 90% of marine species and 70% of land-dwelling vertebrate genera as well as many plant species became extinct.
• The Mesozoic era began about 252 mya and lasted some 186 million years.
  • Flowering plants appeared, and reptiles diversified.
  • Dinosaurs, which descended from early reptiles, dominated.
  • Insects flourished, and birds and early mammals appeared.
  • At the end of the Cretaceous period, 66 mya, many species abruptly became extinct.
  • A collision of a large extraterrestrial body with Earth may have resulted in dramatic climate changes that played a role in this mass extinction.
• In the Cenozoic era, which extends from 66 mya to the present, flowering plants, birds, insects, and mammals diversified greatly.
  • Human ancestors appeared in Africa during the Late Miocene and Early Pliocene epochs.

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