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11: Methods

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    • How are human physical characteristics distinguished from other organisms:

      • Centered foramen magnum

      • V-shaped (parabolic) dental arcade; in non-human primates, it is U-shaped

      • Shorter, bowl-shaped pelvis

      • S-shaped vertebral column

      • Longer femur with bicondylar angle, larger and lighter femoral head

      • Arms shorter than legs

      • Non-grasping hallux (big toe)

      • Increased manual dexterity (thumb is longer)

      • Reduced prognathism (flatter face)

      • Reduction in size of dentition

    • Fossilization is a process that preserves remains or traces of a once-living organism

    • Ways fossilization can occur:

      • Organic material (remains) become mineralized

      • Organisms become trapped in tree resin that hardens after the tree is buried (amber-preserved fossils)

      • Freezing preserves remains

      • Tar pits or volcanic ash preserve remains

      • Imprints of animal tracks, shells, leaves, stems, or flowers become preserved

    • The first hominins (group consisting of modern humans, extinct human species, and all our immediate ancestors) originate in the African continent

    • Several fossils have been found in the Rift Valley region, and some have been found in Sterkfontein Valley (southern Africa)

    • The East African Rift Valley is a divergent plate boundary

      • The African and Nubian plates are drifting apart, and ocean is forming between the plates

      • Divergent plate boundaries have volcanic activity, so fossils are often better preserved in this region

      • The presence of volcanic activity means that certain types of radioactive dating methods can be used on specimens here

    • Sterkfontein Valley

      • There is no volcanic activity here, so radioactive dating methods cannot be used

      • Must use relative dating methods

    • Elements

      • Atomic # = # protons (same # of electrons)

      • Atomic weight = protons + neutrons

    • Nuclei with the same atomic number but different mass numbers are isotopes

      • Most elements have more than one stable isotope

    • Isotopes:

      • Stable isotopes - do not decay into another isotope

        • Ex: Both Carbon-12 and Carbon-13 are stable isotopes

      • Radioactive isotopes - decay into another isotope

        • Ex: Carbon-14 is a radioactive isotope

    • Radioactive decay

      • Unstable atomic nucleus loses energy by emitting ionizing particles and radiation -> transforms parent isotope into daughter isotope

    • Radiometric dating

      • Measures the abundance of a naturally occurring radioactive isotope within the  

      • material to the abundance of its decay products (constant rate of decay)

      • Isotopes – differ in the number of neutrons (some are unstable and decay over time: radioactive)

      • Parent isotope -> decays into daughter isotope

    • Carbon-14

      • Can date fossils up to 50,000 ya (years ago)

    • Potassium-argon

      • Can date fossils older than 50,000 ya

      • Use of volcanic rock (Rift Valley)

    • Argon-argon

      • useful for dating smaller samples and volcanic rock

      • greater accuracy than K/Ar for small samples

    • More info: https://humanorigins.si.edu/evidence/dating

    • Relative dating

      • Law of superposition; stratigraphy

      • Comparing unknown fossils with known species (method for chronologically ordering specimens)

    • Paleomagnetism

      • Positions of magnetic north and south poles are interchanged

      • There have been 183 reversals over the last 83 million years

        • Latest occurred 780,000 years ago. 

      • By analyzing magnetic properties, we can determine possible age of the specimen

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    • See Google slides below:

    11: Methods is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

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