The History of Life

Fossil Formation and Classification

• General Conditions for Fossilization:

  • Specific environmental conditions are necessary for fossils to form.
  • Only a tiny percentage of living things actually become fossils.

• Methods of Fossil Formation:

  • Permineralization: Occurs when minerals carried by water are deposited around a hard structure.
  • Natural Casts: Formed when flowing water removes all original tissue from an organism, leaving behind an impression.
  • Trace Fossils: These fossils record the actual activity of an organism (e.g., footprints or nests).
  • Amber-Preserved Fossils: Organisms that become trapped in tree resin. This resin hardens after the tree is buried, preserving the organism inside.
  • Preserved Remains: Occur when an organism becomes encased in a material such as ice.
    • Example: A near-perfect frozen mammoth resurfaced after $40,000\,\text{years}$, providing clues to the vanished species.

Determining the Age of Fossils and Geologic Materials

• Radiometric Dating:

  • Provides an accurate way to estimate the actual age of fossils by measuring the decay of unstable isotopes.
  • Isotopes: Atoms of an element that differ in their number of neutrons.
    • Carbon-12 Nucleus: Contains $6$ protons and $6$ neutrons.
    • Carbon-14 Nucleus: Contains $6$ protons and $8$ neutrons.
  • Half-Life: The amount of time it takes for half of an isotope in a sample to decay.
  • Carbon-14 ($^{14}C$) Decay Scale:
    • $0\,\text{half-lives}$: $100\%$ of $^{14}C$ remaining ($0\,\text{years}$).
    • $1\,\text{half-life}$: $50\%$ of $^{14}C$ remaining ($5730\,\text{years}$).
    • $2\,\text{half-lives}$: $25\%$ of $^{14}C$ remaining ($11,460\,\text{years}$).
    • $3\,\text{half-lives}$: $12.5\%$ of $^{14}C$ remaining ($17,190\,\text{years}$).
    • $4\,\text{half-lives}$: $6.25\%$ of $^{14}C$ remaining ($22,920\,\text{years}$).

• Relative Dating:

  • Estimates the time during which an organism lived by comparing the placement of fossils in rock layers (strata).
  • Scientists use this to infer the chronological order in which species existed.

• Index Fossils:

  • Tools used to determine the relative age of rock layers.
  • Criteria for Index Fossils:
    • Must have existed only during specific, limited spans of time.
    • Must have occurred over large geographic areas.
  • Examples: Fusulinids and trilobites.

The Geologic Time Scale

• Hierarchy of Time Units:

  • Eras: The largest units, lasting tens to hundreds of millions of years.
  • Periods: The most common unit of time; associated with specific rock systems; last tens of millions of years.
  • Epochs: The smallest units, lasting several million years.

• Major Eras and Their Characteristics:

  • Cenozoic Era ($65\,\text{mya}$ to present):
    • Evolution of primates.
    • Diversification of mammals and flowering plants.
    • Includes the Tertiary Period (Paleogene, $65\text{--}1.8\,\text{mya}$) and the Quaternary Period (Neogene, $1.8\,\text{mya}$ to present).
  • Mesozoic Era ($248\text{--}65\,\text{mya}$):
    • Known as the "Age of the Reptiles."
    • Evolution of reptiles, mammals, and ferns.
    • Ended with the mass extinction of dinosaurs.
    • Periods: Triassic ($248\text{--}213\,\text{mya}$), Jurassic ($213\text{--}145\,\text{mya}$), and Cretaceous ($145\text{--}65\,\text{mya}$).
  • Paleozoic Era ($544\text{--}248\,\text{mya}$):
    • All animal phyla developed during the "Cambrian Explosion."
    • Earliest land plants developed.
    • Ended with a mass extinction.
    • Periods: Cambrian ($544\text{--}505\,\text{mya}$), Ordovician ($505\text{--}440\,\text{mya}$), Silurian ($440\text{--}410\,\text{mya}$), Devonian ($410\text{--}360\,\text{mya}$), Carboniferous ($360\text{--}286\,\text{mya}$), and Permian ($286\text{--}248\,\text{mya}$).

Detailed Chronology of Earth's History

• Precambrian Time:

  • $4500\,\text{mya}$: Origin of Earth.
  • $~3800\text{--}4000\,\text{mya}$: Earth cools enough for the crust to solidify.
  • $3500\,\text{mya}$: Oldest prokaryotic fossils; origin of life; accumulation of atmospheric oxygen from photosynthetic cyanobacteria.
  • $1500\,\text{mya}$: Oldest eukaryotic fossils.
  • $~1000\,\text{mya}$: Origin of multicellular organisms (oldest animal fossils).

• Paleozoic Highlights:

  • Cambrian: All existing animal phyla developed.
  • Ordovician: Diverse marine invertebrates and earliest vertebrates appear; massive glaciers cause sea levels to drop and a mass extinction.
  • Silurian: Earliest land plants arise; melting glaciers form seas; jawless and freshwater fish evolve.
  • Devonian: Fish diversify; first sharks, amphibians, and insects appear; first trees and forests arise.
  • Carboniferous: Coal-forming sediments laid down in swamps; amphibians and winged insects present.
  • Permian: Modern pine trees appear; Pangaea supercontinent forms.

• Mesozoic Highlights:

  • Triassic: Dinosaurs evolve after the largest mass extinction; ferns and cycads present; mammals and flying reptiles (pterosaurs) arise.
  • Jurassic: Dinosaurs diversify; oceans full of fish and squid; first birds arise.
  • Cretaceous: Dinosaur populations peak, then go extinct; flowering plants (angiosperms) arise.

Theoretical Origins of Life

• Early Earth Conditions:

  • Earth began forming about $4.6\,\text{bya}$.
  • The atmosphere was hot and unsuitable for life, containing poisonous gases and very little $O_2$.
  • By $3.8\,\text{bya}$, Earth cooled enough for oceans to form.

• Organic Molecule Hypotheses:

  • Miller-Urey Experiment: This simulation applied an electrical current (simulating lightning) to a closed system containing gases thought to be in the early atmosphere: methane ($CH_4$), ammonia ($NH_3$), hydrogen ($H_2$), and water vapor ($H_2O$). The experiment successfully produced simple organic molecules like amino acids.
  • Meteorite Hypothesis: Suggests amino acids may have arrived via meteorite impacts. Analysis of a meteorite that fell near Murchison, Australia, in $1969$ revealed over $90$ different amino acids.

• Early Cell Structure Hypotheses:

  • Iron-Sulfide Bubbles Hypothesis: Proposes that biomolecules formed in tiny rocky compartments created by hydrothermal vents on the ocean floor.
  • Lipid Membrane Hypothesis: Suggests that lipid spheres (liposomes) could forms around organic molecules, acting as the first cell membranes ($1500\times$ magnification shows these membranes are similar to living cell membranes).

• Genetic Material:

  • RNA as the First Genetic Material: Ribozymes are RNA molecules that can self-replicate without enzymes. DNA, by contrast, requires enzymes to replicate.

Early Life Forms and Transitions

• Single-Celled Organisms:

  • Oldest known fossils are marine cyanobacteria ($3.8\,\text{bya}$).
  • Cyanobacteria are prokaryotic cells that added oxygen to the atmosphere and deposited minerals.
  • Stromatolites: Fossilized evidence of early prokaryotic colonies.
  • Ancient Fossil Examples: Colonial chroococcalean and filamentous Palaeolyngbya found in the Bitter Springs chert of central Australia ($850\,\text{million years old}$).

• The Theory of Endosymbiosis:

  • Describes a relationship where one organism lives within another, eventually forming a single unit.
  • Mitochondria and chloroplasts are believed to have evolved through endosymbiosis involving ancient aerobic and photosynthetic bacteria.
  • Evidence for Endosymbiosis:
    • Mitochondria and chloroplasts have their own DNA.
    • They possess double membranes.
    • They are similar in size to bacteria.
    • They have ribosomes similar to those of bacteria.
    • They divide through a process like bacterial fission.

• Evolution of Complexity:

  • Sexual Reproduction: Increased genetic variation, which is a major evolutionary advantage. It likely led to the evolution of multicellular life.
  • Multicellularity: Developed roughly $100\,\text{million years}$ after the onset of sexual reproduction. Multicellular organisms were more fit and became better competitors, causing evolution to accelerate.
  • Example: Ancient jellyfish from the Precambrian period.