Bio1151 Chapter 25 The History of Life on Earth
  1. A major tool biologists use to study early life on earth is the fossil record buried in layers of                rock.

    • Sedimentary strata.

      Sedimentary rocks are deposited into layers of strata, with the older fossils buried in deeper layers.

      The earliest fossils (3.5 billion years old) are stromatolites formed when layers of sediment were trapped by bacteria.

      Dimetrodon was an early tetrapod adapted for terrestrial life.

  2. The 4.6 billion years of earth's history is divided into three geologic        , which are further divided into         and           .

    • Geological eons.

      Multicellular eukaryotes dominate the Phanerozoic eon.

      The Phanerozoic is divided into three eras: the Paleozoic, Mesozoic, and the current Cenozoic.

      The scale of geologic time can be interpreted with a clock analogy.

    • Geologic time: clock analogy.

      This clock scales Earth's history of 4.6 billion years to one hour.

      Humans appeared about one second ago when the earliest hominin appeared within the last 6 to 7 million years.

      The Phanerozoic eon of over 500 million years occupies about 11 minutes on this clock.

  3. Early photosynthetic                released oxygen, allowing the evolution of              .

    • Origin of oxygen.

      Ancient photosynthetic prokaryotes released oxygen into the ocean, reacting with dissolved iron that left banded iron formations.

      The buildup of oxygen allowed the evolution of bigger eukaryotes, and then multicellular organisms.

  4. The first eukaryotes probably evolved from                  of prokaryotes within larger host cells and developing into organelles.

    • Endosymbiosis.

      An ancient prokaryote developed an endomembrane system by infolding of its plasma membrane.

      Infolding also allowed it to engulf smaller cells by endocytosis.

      The smaller cell may escape digestion and develop a symbiotic relationship:

    • An aerobic endosymbiont may evolve into a mitochondrion and allow its anaerobic host to use oxygen as an energy source.
    • Another endosymbiotic event may allow a photosynthetic prokaryotes to become a plastid (a pigment-bearing organelle such as a chloroplast ).

  5. Most of the major animal phyla appeared during the early             period in what is known as the             Explosion.

    • Cambrian Explosion.

      Some animal phyla, such as Cnidaria, date back to the Proterozoic eon.

      Most other animal phyla had their origins in the Cambrian period (535 to 525 million years ago), the beginning of the Paleozoic era.

      Humans and other mammals belong in phylum Chordata.

  6. Mammals are synapsids that belong to the group of chordates called             .

    • Mammals belong to the group of animals called tetrapods that have 4 limbs. Synapsids possess one temporal fenestra (opening) behind the eye socket. Bones around the jaw (articular and quadrate) of the early tetrapod evolved into the hammer and anvil of the inner ear.
  7. The movement of the Earth's               due to continental          has shaped the diversity of life on Earth.

    • Earth's major crustal plates. Earth's continents drift on tectonic plates of crust that float on the underlying mantle. The red dots indicate zones of tectonic events such as earthquakes and volcanic eruptions.

    • The upper surface of the Earth is a thin layer (5 to 70 km) of crust that floats on the hot, solid/plastic mantle.

    • Continental drift.

      Near the end of the Paleozoic era (250 MYA), the major landmasses moved together into the supercontinent Pangaea.

      Some effects of Pangaea on life include:

    • a reduction in shallow water habitat.
    • a drier and colder climate inland.
    • changes in climate as continents through latitudes.
    • changes in ocean currents leading to global cooling.


  8. There have been several episodes of mass               , though the trend has been an             in diversity over time.

    • Mass extinctions.

      The fossil record reveals a general increase in the diversity of organisms over time, interrupted by periodic mass extinctions.

      Five mass extinctions occurred at the end of the Ordovician, Devonian, Permian, Triassic, and Cretaceous periods.

      The Permian mass extinction was the most severe.

    • The               (KT) extinction is famous for the disappearance of              (except birds).

    • Cretaceous (KT) extinction. The extinction of dinosaurs was probably caused by the impact of a large meteorite. The 65-million -year-old Chicxulub impact crater is located in the Caribbean Sea near Mexico's Yucatan Peninsula, and deposited a layer of the mineral iridium around the world. KT extinction video:

    • Diplodocus Pterodactyl Tyrannosaurus rex Triceratops Parasaurolophus Stegosaurus Megazostrodon
  9. The extinction of the dinosaurs may have led to the             radiation of other species, such as mammals, which diversified to adapt to new           now made available.

    • Adaptive radiation. After the KT extinction, new environmental opportunities allowed one group of synapsids, the mammals, to diversify into several thousand species in a process called adaptive radiation.

    • Adaptive radiation. These varied Hawaiian plants, known as the "silversword alliance", are all descended from a common ancestral tarweed that arrived on the islands 5 million years ago from North America, and adapted to new niches in the archipelago.
    A Scrolling Geologic Record: