Ch. 16 Fossil Evidence for Evolution

Question 1

define fossil

Answer 1

any preserved trace left by an organism that lived a long time ago

Question 2

What can fossils be

Answer 2

o Footprints, burrows, faeces, or impressions of all or part of an animal or a plant
o Bones, teeth, shell

Question 3

Why are rocks important fossils

Answer 3

o Other material associated with bones, i.e. rocks in which they were found and fossils of other plants and animals allows to develop a picture of life in the past
♣ i.e. what organism ate, what other organisms existed at the time, and what the climate was like

Question 4

when can an organism be fossilised (what buries it)

Answer 4

o buried by drifting sand, mud deposited by rivers, volcanic ash, or other members of the species

Question 5

why does burying rapidly increase chance of fossilations

Answer 5

• If buried rapidly, conditions may not be suitable for activity of decay organisms and decomposition may be delayed

Question 6

what condition of soils will not allow fossilisation and why

Answer 6

wet, acidic soils (minerals in bone dissolves)

Question 7

what are the three best soil conditions for fossils

Answer 7

♣ No oxygen (in case of peat, complete preservation of the soft tissues and bones of animal may occur)

♣ Alkaline soils (minerals not dissolved)

♣ New materials (often lime or iron oxide) deposited in pores of bone, replacing the organic matter // turning into rock (petrified) but the details of structure are still preserved

Question 8

Where are human fossils often found

Answer 8

• edges of ancient lakes and river systems, caves, volcanically active areas

Question 9

why are humans fossils found near lakes, caves and volcanic areas

Answer 9

o Lakes and rivers= build up sediments when flooding occurs or when water flow slows rapidly
o Many caves= limestone (CaCO3) // may be deposited covering bodies
o Unusual to be preserved near volcanic areas (heat from the volcanic material destroys the organisms), but ash can fall preserving fossils of many human ancestors

Question 10

what makes surface discoveries possible and what do they indicate

Answer 10

erosion

- Surface discoveries (fossil fragments, evidence of human occupation) indicate places

Question 11

where are fossils usually found

Answer 11

excavation sites

Question 12

define artefacts and examples

Answer 12

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Artefacts: objects deliberately made by humans

o E.g. stone tools, beads, carvings, charcoal from cooking fires and cave paintings

Question 13

define dating

Answer 13

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determining the age of excavated artifacts or fossils

Question 14

what are the two methods of fossil dating

Answer 14

o Absolute dates: actual age of the specimen in years

o Relative dates: whether one sample is older or younger than another

Question 15

what are three types of absolute dating, material used and tie limitations

Answer 15

o Potassium argon dating (volcanic deposits used, 100 000- 200 000)
o Radiocarbon dating (carbon compounds used, 60 000)
o Dendrochronology (wood used, 9 000)

Question 16

what is potassium argon dating based on

Answer 16

• Based on decay of radioactive potassium to form calcium and argon

Question 17

what are the two limitations of K Ar dating

Answer 17

o Not all rocks suitable: Some suitable rock of same as age as fossil must be available This occurs, for example, when rocks produced in volcanic eruptions bury bones
o Only date rocks older than 100 000 to 200 000 years
♣ Earlier than 100 000 = only 0.0053% of potassium would have decayed // limits of detection devices

Question 18

what does K 40 decay to

Answer 18

• Potassium-40= radioactive + decays to form calcium-40 and argon-40

Question 19

how is K Ar dating to calculate age

Answer 19

• Decay takes place at an extremely slow but constant rate

o Determining amounts of potassium-40 and argon-40 in a rock sample enables age of rock to be calculated

Question 20

what is radiocarbon dating based on

Answer 20

Based on decay of radioactive isotope of carbon (carbon-14) to nitrogen

Question 21

explain c14 in atmosphere and how enters animals

Answer 21

• Carbon-14 produced in upper atmosphere by action of cosmic radiation on nitrogen at about the same rate at which it decays
• In atmosphere= ratio of 1: million million (1012) carbon-14 carbon-12
• During photosynthesis when plant use atmospheric carbon dioxide, on atom in every million atoms is carbon 14 (incorporated into tissue)
• Should animal eat plant= the carbon-14 becomes part of animals tissues

Question 22

how is radiocarbon measured to date

Answer 22

• With death C14 intake ceases, but C14 in tissues decays at a fixed rate
• By measuring amount of radiation liberated by a sample, ratio of C14 to C12 can be estimated and // age of sample can be calculated

Question 23

what is the half life of carbon 14

Answer 23

5730

Question 24

what are the four limitations of radio carbon dating

Answer 24

• must be organic material
• must be at least 3 grams
• can’t date back longer than 60 000 years
• amount of c14 in atmosphere varies

Question 25

explain the limitation of carbon needing to be at least 3 grams and how is this avoided

Answer 25

o Normally requires at least three grams of organic material so that rate of radioactive decay of C14 in a sample can be measured,
♣ Unless using Accelerator Mass spectrometry (AMS) where sample 100 micrograms can be used
♣ Involves breaking sample up into its constituent atoms so that the number of atoms of each isotope of carbon can be counted
♣ Date cave paintings accurately from tiny samples of pigments

Question 26

explain limitation of variation of c14 in atmosphere

Answer 26

o Amount of C14 in atmosphere varies
♣ Thought to be constant
♣ // radiocarbon dates must be treated with certain degree of caution
♣ Corrections in fluctuations in C14 content of atmosphere are however now possible past 9000 years by reference to tree-ring dating

Question 27

what does each ring in a tree represent

Answer 27

each concentric ring represents one year’s growth and the rings differ in width according to how favorable the growing season was

Question 28

what are marker rings

Answer 28

• Certain rings produced in years of exceptional weather can be used as marker rings
  o Correlate marker rings with living trees and timber taken from ancient structures

Question 29

explain use of dendrochronology to date fossils

Answer 29

• Living trees dated by drilling small core out of trunk and counting rings
  o May be noted for example, particularly wide ring, indicating favorable growth year, occurs in middle of the trunk
  o This wide marker may be found near the outside of a piece of tree used in an ancient structure
  o By such correlation of marker rings, dates for the building of structures can be determined

Question 30

what are the limitations of dendro

Answer 30

o Timber not preserved for more than 9000 years

Question 31

what does relative dating enable

Answer 31

• Enables a sequence of events to be established

Question 32

what is stratigraphy

Answer 32

• Study of layer, or strata

Question 33

what are the three parts to stratigraphy

Answer 33

1. principal of superposition
2. correlation of rock strata
3. index fossils

Question 34

explain principal of superposition

Answer 34

• assumes that in layers of sedimentary rock the layers at the top are younger than material found lower
  o Applied with caution because distortions of Earth’s crust do occur and a sequence of rock layers may be turned upside down
• Fossils can also be buried by animals, or early humans, sometime after deposition of sediment // specimen may be younger than some layers above it

Question 35

explain correlation of rock strata

Answer 35

• Matching layers of rock from different areas
• Done by examining rock itself and also by studying fossils it contains
• Rocks that contain same fossils may be assumed to be the same age

Question 36

explain index fossils

Answer 36

• Certain fossils= great value in correlation as they are widely distributed and were present for only a limited amount of time, making more precise
  o = index fossils

Question 37

how is fossilised pollen grains used

Answer 37

• Example: fossilized pollen grains
  o Can be used as index fossil
  o If not used that way= presence of preserved pollen grains in soil or rock sample can enable construction of type and amount of vegetation existing at the time the deposit is laid
  ♣ > idea of climatic conditions prevailing at time can be worked out
  ♣ this data= confirms or refutes relative dates arrived at by other methods

Question 38

what is the breakdown of the geological time scale

Answer 38

o Consist of eras > subdivided into periods > further divided into epochs

Question 39

what era are humans

Answer 39

-	cainozoic era
o	Primates (group where humans belong) first evolved at beginning of this era

Question 40

what are the 4 problems with the fossil record

Answer 40

1. organisms not preserved
2. small proportion discovered
3. problematic dating
4. controversial reconstruction

Question 41

explain the problem of organisms not preserved

Answer 41

• Conditions for fossilization do not always occur or occur at irregular periods of time // organisms not being preserved
• For fossils to form= four conditions
  > Quick burial
  > Presence of hard body parts
  > Absence of decay organisms
  > Long periods of stability (left undisturbed)
• Fossilization // chance occurrence + many gaps un record often due to organisms not being preserved

Question 42

explain the problem of small proportion discovered

Answer 42

• Some buried too deep or inaccessible, destroyed by human activity (agriculture or industry)

Question 43

explain the problem of problematic dating

Answer 43

conditions and certain materials required for different methods as well as time limitations

Question 44

explain the problem of controversial reconstruction

Answer 44

• Unusual to find fossil of entire organism or whole skeleton > particular true for human ancestors
• Usually few fragments of bone formed // reconstruction of what what organisms may have looked like is formed
• Other scientists may disagree of shape // controversy
• Only resolution find more fossils (but as shown chances are slim)