lesson 6

• Life on Earth arose 3.8 billion years ago
• Changes in body structures and molecules have slowly accumulated through that time, producing the variety of organisms we see today
• Geologic timescale


  A system used by scientists to divide the history of the Earth into eons and eras, defined by major geological or biological events
• Even though the events that led to today's diversity of life occurred in the past, many clues suggest that all organisms derived from a common ancestor
• Paleontology


  • The study of fossil remains or other clues to past life
  • Fossils provided the original evidence for evolution
• Fossils form in many ways and preserve evidence of many types of organisms
• Even though fossil evidence is diverse, it is often challenging—or impossible—to find fossils of transitional forms between groups
• The fossil record is incomplete, partly because some organisms (such as those with soft bodies) fail to fossilize. Also, erosion and movement of Earth's plates might destroy fossils
• Fossils help researchers piece together Earth's history
• Relative dating


  A simpler, less precise method of dating fossils that assumes lower rock layers have older fossils than newer layers
• Absolute dating


  A method that uses chemistry to determine how long ago a fossil formed
• Radiometric dating


  A type of absolute dating that uses radioactive isotopes
• Carbon-14 dating


  1. Living organisms accumulate carbon-14, a radioactive isotope, along with stable carbon-12

  2. After the organism dies, no more carbon-12 or carbon-14 is added

  3. Carbon-14 decays at a constant rate, leaving the organism as nitrogen

  4. During any 5730-year period, the amount of carbon-14 in the organism divides in half

  5. By determining the amount of carbon-14 in a fossil, scientists can estimate when the organism lived
• Earth's geography has changed drastically over the last 200 million years
• Plate tectonics


  The theory that Earth's surface consists of several rigid layers, called tectonic plates, that move in response to forces acting deep within the planet
• Earthquakes and volcanoes are evidence that Earth's plates continue to move today
• Fossils help geographers piece together Earth's continents into Pangaea
• Biogeographical studies shed light on evolutionary events
• Animals on either side of Wallace's line have been separated for millions of years, evolving independently, resulting in a unique variety of organisms on each side of the line
• Homologous structures


  Two structures that have similarities reflecting common ancestry
• Homologous structures need not have the same function or look exactly alike, as different selective pressures in each animal's evolutionary line have led to small changes from their ancestor's bone structure
• Vestigial structures


  Structures that have lost their function but are homologous to a functional structure in another species
• Homologous structures


  • Have shared evolutionary origin
  • Need not have the same function or look exactly alike
• Different selective pressures in each animal's evolutionary line


  Led to small changes from their ancestor's bone structure
• Vestigial structure


  Structure that has lost its function but is homologous to a functional structure in another species
• Vestigial structures


  • Hind limbs in some snake species
  • Pelvises in whales
• Analogous structures


  Anatomical structures that are superficially similar but did not derive from a common ancestor
• Lack of pigment and eyes arose independently in cave animals through convergent evolution
• Streamlined shapes of dolphins and sharks

  Evolved independently through convergent evolution
• Homeotic genes


  • Control an organism's development
  • Small differences in gene expression might make the difference between a limbed and limbless organism
• Comparing DNA and protein sequences determines evolutionary relationships in unprecedented detail
• It is highly unlikely that two unrelated species would evolve precisely the same DNA and protein sequences by chance
• Similarities in DNA and proteins were likely inherited from a common ancestor and differences arose by mutation after species diverged
• Molecular clock


  Assigns dates to evolutionary events based on mutation rates
• If a gene mutates once every 25 million years, two differences from an ancestor might arise in 50 million years
• Two species derived from the same common ancestor 50 million years ago might have four differences in the nucleotide sequence of a gene
• Evolution does not always lead to greater complexity, sometimes features are lost
• Fossil evidence shows snakes descended from lizards with four legs and lost their forelimbs, then hindlimbs
• Discovery of a terrestrial snake species with functional hindlimbs supports the loss of limbs occurring on land