PLB143 - Lecture 06

Contemporary Methods in the Study of Crop Evolution


© Paul Gepts 2010

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Lecture 06 - Readings

Selected Recent Technologies

  • To recover plant remains:
    • Flotation
  • To date plant remains:
    • Isotopes
  • To identify plant remains or vegetation:
    • Palynology
    • Phytoliths
    • Isotopes

Types of Archaeological Remains

(Zohary and Hopff 1988)

  • Carbonized remains (from B. Smith, The Emergence of Agriculture, 1995; © Scientific American Library)
    • Examples: hearth/oven, drying kiln, storage pit, pottery
    • Baking (charred) under low oxygen ==> charcoal; resistant to bacteria and fungi ==> conserved in most environments
    • Charring slow and mild (<200C): retain morphology; charring more severe (>200C): shortening and swelling, loss of seed coats and glumes. Intensity of deformation = f(water content, spread of heat, temp. reached)
  • Impressions on pottery, bricks
  • Parched (desiccated) plant remains
    • Examples:
      • potatoes (from B. Smith, The Emergence of Agriculture, 1995; © Scientific American Library)
      • beans and maize (© Paul Gepts 1996)
    • Absence of water blocks bacteria and fungi
    • Examples: Peruvian coast, Tehuacán Valley (Mexico), Egypt
  • Water-logged preservation
    • Anaerobic conditions
    • Examples: Swiss lake-shore dwellings, North European bogs (stomach contents)
  • Preservation by metal oxides
    • Bronze (Cu, Sn), Silver (Ag), Iron (Fe): produce metal oxides under humid conditions
    • Impregnate plant tissues and are toxic to bacteria, fungi
  • Digested or partially digested remains
    • Humans cannot digest cellulose: stems, seeds, etc. can retain some of their features
    • "Coprolites": preserved human faeces: parched, waterlogged, charred
  • Carbonized remains
  • Dessicated  potatoes
  • Beans and maize

Recovery of Archaeological Plant Materials through Flotation

(Pearsall 1989)

  • Recovery technique using differences in density of organic  (plant remains) and inorganic (soil) material to achieve separation of plant remains from the soil matrix
  • Advantages:
    • when properly practised, allows recovery of all size classes ==> quantitative analysis
    • simplicity of equipment and ease of processing ==>field processing possible
  • Comments:
    • Watson (1976): "... the widespread and large scale use of  [flotation] has amounted to a revolution in recovery of data relevant to prehistoric subsistence."
    • Hole (1961): "Plant remains were scarce at Ali Kosh."
    • Hole et al. (1969): "Nothing could be further from the truth. The mound is filled with seed from top to bottom."
  • Procedure:
    • Immerse soil in water and agitate in some way so that light  material is floated to the surface and can be skimmed off or washed out
    • Factors: screen mesh size in flotation bucket, mesh size in catch sieve, extent and consistency of agitation, range of densities of charred materials
  • Variations: other liquids than water: CCl4 (density=1.8); ZnCl (density=1.62)
    • Water: separates bones + charred materials from gravel, soil
    • ZnCl2: separates charcoal from bones
  • Manual flotation:
    • Consists of large drum, flotation bucket , and hand sieve (from D.M. Pearsall, Paleoethnobotany, 1989)
    • Circular motions with flotation bucket containing soil sample
    • Scoop of botanical material off the water's surface in the bucket (from D.M. Pearsall, Paleoethnobotany, 1989)
    • Dry remains
  • Machine-assisted flotation:
  • Flotation bucket

  • Scoop of materials

Isotopic Determination of Plant Remain Age

  • All living organisms contain carbon:
    • Most of it is 12C; some of it 14C:
    • Ratio of 1012 : 1 in the atmosphere
  • After death, gradual radioactive decay of 14C:
    • half-life of approx. 5,700 yrs.
  • Measure amount of 14C relative to 12C to get an estimate of the age
  • Is there an independent way of verifying 14C dates?
    • tree rings of bristlecone pines, giant sequoias (dead or alive)

Some related sites of interest

Determination of 14Carbon Isotope Content

  • Conventional (1940s-50s onward) :
    • General procedure:
      • Counting individual decay events: each event corresponds to emission of a particle
      • Periods of several hours
      • Large amount of tissue because of low levels of 14 C: 1-5 g of C
    • Problems:
      • Amount of material needed: seeds are small; bones have less C
    • Solution:
      • Analyze larger remains thought to be of same age, i.e. the same stratum: ) (from B. Smith, The Emergence of Agriculture, 1995; © Scientific American Library)
      • Caveat: Smaller materials may have been displaced upwards or downward
  • Cave of Guilá Naquitz, Oaxaca, Mexico
  • New (1980 on): Accelerator Mass Spectrometer (AMS)
    • General procedure:
      • Major difference with conventional: measures directly number of 14C atoms
      • Smaller amounts of tissues: up to 103 smaller
    • Conventional mass spectrometer: unable to measure 14C:
      • Amount of 14C too small
    • Ions of similar mass: 14N, 13CH
    • Accelerates ionized C atoms
    • Beam-bending magnets
    • Filters: passage of particles with atomic mass 14
    • 14C detector
  • Use of AMS will lead to reevaluation of many published archaeological dates by allowing direct measures on smaller remains (from B. Smith, The Emergence of Agriculture, 1995; © Scientific American Library)
  • Accelerator mass spectrometry (AMS)
  • Size of sample that can be analyzed by AMS

Identification of Plant Materials through Palynology

  • Pollen grain: structures containing male gametes (2 sperm cells/ pollen grain): living cell + intine (cellulase, protein) + exine (sporopollenin, very resistant organic substance)
  • Living cell and intine are short-lived; old pollen is therefore recognized by
exine (from B. Smith, The Emergence of Agriculture, 1995; © Scientific American Library)
  • Features of pollen grains:
    • Size: 5 um to >200 um; constant for a species
    • Shape: usually round: P(olar)/Equatorial axis ratio
    • Openings: pores and furrows (from D.M. Pearsall, Paleoethnobotany, 1989)
      • 0-40
      • dicots: usually 3, monocots: usually 1, at end of polar axis
    • Exine structure and sculpturing:
      • 2 layers: endexine and exexine
      • exexine structure (from D.M. Pearsall, Paleoethnobotany, 1989): roof continuous or not, projections from the roof, columellae simple or digitate
  • Representativity of sample with regard to vegetation:
    • Function of: differential production and dispersal; differential destruction
    • Mechanisms of pollen dispersal ==> quantity and form of pollen grains ==> quantity deposited over the landscape anemophilous >> zoophilous >> hydrophilous, autogamous
    • Current vegetation with known composition
  • Variety of pollen grains from different species and genera

  • Openings and furrows in pollen grains

  • Exexine structures

  • Example 1: Subsistence Reconstruction (Schoenwetter 1974: Salts Cave, KY)
    • Coprolites: sources of pollen:
      • eaten in food, beverage
      • transfer to another stored food
      • pollen rain
      • inhaled
    • Results:
      • High % of insect-pollinated taxa: plants eaten as food: flower buds, foliage, seeds (Chenopods, Compositae)
      • Early spring use of cave: pollen of iris, lily; taxa flowering in late spring and early summer are mostly absent
      • Ingestion of stored foods: hickory, sunflower (harvested in late summer)
  • Example 2: Climate change Schoenwetter and Smith (1986), Valley of Oaxaca, Mexico
    • Analysis of modern vegetation and factors affecting pollen rain:
      • certain vegetation types can be grouped based on common underlying ecological variable: e.g., moisture level
      • by identifying pollen array in a sample, one can assess the value of an ecological variable: moisture, annual temperature, annual rainfall
    • Guilá Naquitz (9,500-8,000 BP)
      • low-moderate moisture, moderate annual temperature, low-moderate rainfall
      • drop in rainfall and moisture from previous periods

Identification of Plant Materials through Phytoliths

  • Phytolith
    • = Silica deposition in stems, leaves, and inflorescences. In some cases, 
    • distinctive size and shape 
    • Often in monocotyledons: e.g., grasses, banana, palms, etc. Also in dicotyledons: e.g., Compositae, Cucurbitaceae, Rosacae, etc. and gymnosperms (Pinaceae)
    • Issue: Taxonomic differentiation?
  • Example of a phytolith

  • Example:  Piperno (1983, 1984): Can we differentiate maize from its wild relatives?
    • Problem: cross-shaped phytoliths are found both within maize and its wild relatives
    • Solution:
      • identified three-dimensional variants of the basic cross-shaped pattern
      • combining size and variant frequencies, was able to distinguish different species
      • maize: high frequency of variants 1 + low frequency of variants 2 and 6
      • teosinte: high frequency of variants 2 and 6
    • Application: Pacific watershed of Panama: When was maize introduced?
      • Sequence: approx. 6,500 BC - AD 500
      • Results: Age 6,610 BC: wild grasses; Period 5,000 - 1,000 BC: maize
  • Strengths:
    • Provides data on plant occurence where other remains are poorly preserved: e.g., lowland areas Inorganic ==> resistant to decay and to mechanical breakage. Only high pH (>9) is detrimental
    • Complement pollen data. Together, more specific botanical identification
    • Several agriculturally significant species have phytoliths
    • Several species important as ecological indicators can be identified
    • Represent local deposition:
      • Not wind-borne
      • No illuviation: downward movement in soil
  • Weaknesses:
    • Many plants do not deposit silica in tissues
    • Similar silica bodies may be produced in widely different taxa
    • Not all phytoliths preserve equally well in soil
    • Differential silica production and conservation ==> over- or underrepresentation

Use of Isotopes in Archaeology

C and N Metabolism in Plants

  • C metabolism: Photosynthesis
    • C3 plants: Most plants: e.g., wheat, cotton, strawberry, peaches-almonds, potato, etc.
    • C4 plants: monocots: tropical grasses (maize, sorghum, sugar cane), dicots: Chenopodiaceae (Chenopodium, Amaranthus)
    • CAM plants: succulents (Agave, Opuntia, Crassula, Aloe)
  • N metabolism: Nitrogen fixation
    • legumes: symbiotic nitrogen fixation in root nodules : beans, peas, soybean, etc.; Rhizobium, Bradyrhizobium
    • non-legumes: generally no symbiotic nitrogen fixation
  • Distinction between C3, C4, and CAM Plants: 
  • Root nodules
  • Incorporation of CO2

  • Discrimination index

    Delta 13C values
    Plant type Range Average
    C3 plants -21 to -34 o/oo -26 o/oo
    C4, CAM plants -8 to -15 o/oo -13 o/oo
    Delta 15N values
    Nitrogen source Range Average
    Nitrate or ammonium: contemporary +5 to +7 o/oo
    Nitrate or ammonium: prehistoric
    9 o/oo
    Atmospheric nitrogen (N2)
    0 o/oo
  • Example 1: Isotopic Composition of Prehistoric Remains from Peru (Hastorf and DeNiro 1985)
    • Background:
      • Three periods:
        • 200 BC - AD 1000
        • AD 1000 - 1200
        • AD 1200 - 1470
      • Crop Production: extracted plant fragments from soil by flotation techniques; isotope analysis
      • Crop Utilization: i.e. prehistoric cooking: scrape charred remains from ceramic shards


    200 BC - 1000 AD
    1000 AD-1200 AD
    1200 AD-1470 AD
  • Examples of local plants
    • Legumes (C3):
      • lupins: Lupinus mutabilis
      • beans: Phaseolus vulgaris, P. lunatus
    • Non-legume C3:
      • quinoa: Chenopodium quinoa
      • potato: Solanum tuberosum, S. juzepczukii
      • oca: Oxalis tuberosa
    • Non-legume C4:
      • maize: Zea mays
  • Crop Utilization: i.e. prehistoric cuisine
    • Findings:
      • Cooked Non-legume C3 plants alone
      • Cooked Non-legume C3 and C4 plants simultaneously or sequentially
      • Did not cook legumes alone or in combination: toasted or popped
    • Matches modern practices:
      • quinoa (C3) alone
      • maize (C4) with C3 plants
      • nunas or k'opurus: popping beans
  • Example 2: Introduction of maize in Eastern North America
    • Composition of foods will be reflected in the composition of the human body
    • For example, C isotope composition of bones will vary according to the composition of the main staple foods
    • Analyze human bones for 13C isotope composition
  • Evolution in 13C values in bones of  Native Americans

Use of DNA in archaeology

Poinar et al. 2001. A molecular analysis of dietary diversity for three archaic Native Americans. PNAS 98:4317-4322

  • Fossil feces (coprolites):
    • abundant in some caves; contain DNA of both humans and their plant and animal diet
  • In a cave in western TX:
    • dry climate; buried under 2m of soil; age: 2000 BP
  • Amplification of DNA by PCR; cloning; sequencing
    • humans and animals: 12S and 16S ribosomal RNA of mtDNA: 
    • plants: large subunit of rbcL (ribulose bisphosphate carboxylase): 
    • both sequences:
      • essential functions --> conserved sequences
      • can compare organisms across wide taxonomic range
      • cannot separate closely related species, or differences between wild progenitor and domesticated descendant
  • Results:
    • Human mt DNA:
      • Native Americans
    • Plant and animals: large diversity: DNA + microscopic analysis
      • Sample I:
        •   4 animals: pronghorn antelope, cottontail rabbit, packrat, squirrel
            4 plants: hackberry, sunflower family, yucca or agave, opuntia
      • Sample II:
        • 2 animals: packrat and fish
        • 6 plants: hackberry, oak, sunflower family, yucca or agave, nightshade family, legume family
      • Sample III:
        • 3 animals: bighorn sheep, packrat, cotton rat
        • eight plants: buckthorn family, hackberry, oak, sunflower family, yucca or agave, legume family, ocotillo, opuntia
      • Therefore:  wide diversity of foods, reminiscent of the " broad spectrum revolution " of Lecture 4.
  • Coprolite

  • Animal mitochondrial DNA (mtDNA)

  • Chloroplast DNA (cpDNA)

  • Human migrations based on mtDNA data

Contemporary Methods in the Study of Crop Evolution: Archaeology

  • Conclusions:
    • Several methods have been introduced over the last 30 years
    • These methods each have their strengths and weaknesses
    • They have allowed archaeologists to gain an unlikely amount of information about the subsistence of our ancestors, particularly about a crucial phase in our evolution, namely the development of agriculture

Examples of archaeological investigations

  • Çatal Höyük , an important early Neolithic site in Anatolia, Turkey

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