chapter 16 - fossil evidence for evolution Flashcards
1
Q
what is a fossil / types of fossils and artefacts
A
- any preserved trace left by an organism that lived long ago (footprint, burrows, faeces, impressions of animal, plant, bone, shells, teeth)
- altered: when minerals replace the cellular component of a dead organism and create a remineralised skeleton
- unaltered: retain original composition, preserved in material that does not allow for bacterial decay (amber, tar pits, peat bogs)
- trace: tracks / footprints, true / under track, natural cast (sediment that fills original footprint) preserved near bodies of water
- artefact: object that have been made deliberately by humans, stone, tools, beads, carvings, charcoal, cave paintings)
2
Q
explain the evolution of the horse and fossils of lucy
A
- horse: extensive sequence of evolution, traced through fossil remains from creature not much larger than a dog to the modern day horse (extensive change in leg bones)
- lucy: remains of human ancestor, found in ethiopia, rock in which fossil was found and other material in area provides information about other organisms, life, diet and climate of the time
3
Q
how does fossil formation occur
A
- normally dead organisms are decayed by microorganisms and no trace of existence is left
- formation occurs when burial of organisms are covered by drifting sand, mud deposited rovers, volcanic ash, other species
4
Q
process of fossil formation under water
A
- dead organism is covered by sediment and protected from other animals and natural elements
- minerals replace the cellular components of organisms and a remineralised skeleton is created
- more sediment accumulates, buried deep within bedrock
- rock is distorted / uplifted by geological forces (continental movement), uplifted rock is exposed to weathering and erodes exposing tip of fossil
5
Q
temperature and nature of soil affect fossil formation
A
- cold temperatures inhibit the life of bacteria and other species increasing chances of fossilisation
- wet acid soils (no O2): anoxic conditions, no bacteria, complete preservation of bones and soft tissue, peat
- wet acid soils (O2): minerals in bone are dissolved (no fossil)
- alkaline soils: minerals in bone are not dissolved, fossil
- alkaline soils (lime and oxide): minerals replace organic material of bone, turns to rock (petrified), preservation of fossil
- increased inflow in ocean: increased sediments, fossil
6
Q
where have fossils of human ancestors been found
A
- edges of ancient lakes and river systems (sediment build up)
- caves (limestone preserves body, carbon and carbonate)
- volcanically active areas (unusual for fossilisation however ash falls in east africa have preserved fossils
7
Q
how does the discovery of fossils occur
A
- by chance on surface due to erosion (indication there may be other fossils) or at excavation site
1. area is surveyed and marked out into sections
2. small hand tools are used to gently remove soil and then sieved, fossils / artefacts are found
3. photos are taken at every stage
4. items are labeled / catalogued
5. a study on the dig is conducted
6. fossils are scraped clean in a lab and pieced together, measured and plaster / latex moulds may be made
8
Q
dating of fossils
A
- dating: determine the age of the material excavated, crucial in determining the sequence of changes that have resulted in present day humans
- absolute: actual age of specimen in years (carbon and potassium / argon)
- relative: determines whether one sample is older than another (stratigraphy)
9
Q
describe the technique, best conditions and half life for potassium argon dating
A
- based on decay of radioactive K40 to form Ca40 and Ar40, extremely slow but constant rate
- works best when fossil is surrounded in igneous rock, volcanic rock / ash, feldspar and clay (contain K)
- 1.26 x 10^9 years (1300 million), those older than 100000-200000
10
Q
describe the assumptions and limitations for potassium argon dating
A
- age of rock surrounding fossil = age of fossil, rate of half life is constant, rock loses all Ar prior, no Ar is lost since, no atmospheric Ar has entered, no K has been gained or lost
- at earlier date of 100000 years only 0.0053% Ar is present, pushes limits of detection devices
11
Q
describe the process for potassium argon dating
A
- volcanic material flows over land and atmospheric Ar (inert gas) is driven off due to heat
- lava becomes molten (no Ar left)
- rock cools and hardens (closed system), any Ar produced due to decay of K is trapped inside
12
Q
describe the materials for potassium argon dating
A
- mass spec: of gases released when rock is melted in vacuum (Ar)
- atomic absorption spectroscopy: or flame photometry, quantifies K
13
Q
describe the technique, best conditions and half life for carbon dating
A
- based on decay of radioactive C14 to C12 then N14
- important for dating fossils of organic material (bone, shell, charcoal, linens, teeth, wood, paper)
- half life of 5730 +/- 40 years, more recent origin
14
Q
describe the limitations for carbon dating
A
- found that the amount of C14 in atmosphere varies
- after 70000 years amount of C14 left is negligible (can’t date more than 60000 years old)
- must contain carbon (organic compounds)
15
Q
describe the process for carbon dating
A
- C14 is produced in upper atmosphere through cosmic radiation of N14 (1:1trillion, C14:C12)
- plants absorb atmospheric CO2 during photosynthesis, incorporated in plant tissue
- passed along through food chains of organisms, becomes part of their tissue
- organisms die, intake of C14 ceases, decay continues to nitrogen
