Flashcards in Chapter 18 - Fossil Evidence For Evolution Deck (53):
What is a Fossil?
1. A fossil does not have to be a part of an organism.
2. Any preserved trace left by an organism that lived long ago is a fossil.
3. Fossils may therefore include footprints, burrows, faeces or impressions of all or part of an animal or a plant, as well as bones, shells or teeth.
Why are fossil remains extremely important?
-They allow scientists to determine exactly what extinct species were like.
-Other material associated with the bones, such as the rock in which they were found and fossils of other plants and animals, allows the scientist to develop a picture of life in the past - what the organisms ate, what other organisms existed at that time and, some times even what the climate was like.
Which is one of the best known fossil record that has allowed scientists to build up a sequence of evolution for that particular species?
The evolution of the horse.
What is the chance that a plant or animal will be fossilised?
Very small chance; normally, dead organisms are decayed by micro-organisms and no trace of their existence is left.
How do some parts of organisms become fossilised?
-When buried by drifting sand, mud deposited by rivers, volcanic ash or, in the case of some of the more recent human ancestors, other members of the species.
-If buried rapidly, conditions may not be suitable for the activity of decay organisms and decomposition may be slowed or prevented.
Why is the nature of the soil very important for the fossilisation of bone?
1. In wet, acid soils the minerals in the bone are dissolved and fossilisation occurs.
2. However, if such soil contains no oxygen, as in case of peat, complete preservation of the soft tissues and bones of the animal may occur.
3. Bones buried in alkaline soils produce the best fossils since the minerals in the bones are not dissolved.
How does a bone become petrified (turned into rock)?
-When new minerals, often lime or iron oxide, are deposited in the pores of the bone, replacing the organic matter that makes up about 35% by weight of the bone.
-The bone becomes petrified, but details of structure are still preserved.
Why are fossils of human ancestors often found at the edges of ancient lakes and river systems, in caves or in volcanically active areas ?
1. Lakes and rivers build up sediments when flooding occurs or when the water flow slows rapidly.
2. Many caves are in limestone that consists of calcium carbonate.
3. This Chemical may be deposited around dead organisms, or the cave roof or walls may collapse, covering the bodies of animals.
Why is it unusual for animals to be preserved near volcanic eruptions?
Because heat from the volcanic material destroys the organism, but in East Africa ash falls have preserved fossils of many human ancestors.
How does the discovery of fossils take place?
1. Fossils are sometimes found by chance at the surface of the ground where they may have been uncovered by erosion, but more often the discovery of fossils is the result of slow and painstaking excavation of likely sites.
2. Surface discoveries such as fossil fragments, or evidence of human occupation such as that found in many caves, are indications of places where excavations may prove fruitful.
What do Scientists refer an excavation as?
List the steps of Discovering Fossils.
1. The area to be investigated is first surveyed and marked out in sections.
2. Small hand tools are used to remove the soil gently so as not to damage any of the material.
3. The soil removed is usually sieved so that every very small fragments are not overlooked.
4. In the case of fossils of human ancestors, artefacts are often found in association with the fossils.
5. Photographs are taken at every stage of a dig so that detailed studies of positions of uncovered material can be carried out later.
6. Each item is carefully labelled and catalogued for the prolonged study that follows the excavation of the site.
7. In the laboratory, fossil bones are carefully scraped clean, broken parts are pieced together, measurements are made and plaster casts or latex moulds may be made.
-Artefacts are objects that have been deliberately made by humans.
-They include items such as stone tools, beads, carvings, charcoal from cooking fires and cave paintings.
What is Dating?
It is one of the major tasks following the excavation of fossils or artefacts which aims to determine the age of the material.
What's the difference between Actual and Relative dates?
Actual dates is the age of the specimen in years; other methods give relative dates, which tell us whether one sample is older or younger than another.
How are the age (or date) of a fossil or artefact usually presented?
It is usually given in years before the present time.
e.g. A fossil may be said to date from 45,000 years BP, which is another way of saying it is 45,000 years old. BP stands for 'before present'.
What is the potassium-argon technique?
It is based on the decay of radioactive potassium to form calcium and argon.
What is Potassium?
- Potassium (chemical symbol K) is a mixture of three different forms with atomic weights: 39, 40 and 41.
-The isotope potassium -40 is radioactive and decays to form calcium-40 and argon-40.
-Such decay takes place at an extremely slow but constant rate, so that by determining the amounts of potassium-40 and argon-40 in a rock sample the age of the rock can be calculated.
What are isotopes?
Different forms of the same element, with different numbers of neutrons in the nucleus of their atoms.
Potassium-argon dating has limited usefulness:
- Not all rocks are suitable for this method of dating and it can only date rocks older than 100, 000 to 200, 000 years.
- At the earlier date of 100,000 years, only 0.0053% of the potassium-40 in a rock would have decayed to argon-40.
-Such a small amount pushes the limits of detection devices currently in use.
How can the age of a fossil using this method be determined?
- There must be available suitable rock of the same age as the fossil.
eg. This occurs when rocks produced in volcanic eruptions bury bones.
- Despite these limitations, potassium-argon dating is very useful in providing precise dates for those rocks for which it is suitable.
What does the carbon-14 or radiocarbon dating method depend on?
This method is based on the decay of the radioactive isotope of carbon, carbon-14, to nitrogen.
How is Carbon-14 produced?
1. Carbon-14 is produced in the upper atmosphere by the action of cosmic radiation on nitrogen at about the same rate which it decays.
What ratio of carbon-14 atom to atoms of the stable isotope carbon-12 is there?
In the atmosphere there is a ratio of one carbon-14 atom to every million million (10^12) atoms of the stable isotope carbon-12.
What happens when green plants use atmospheric carbon dioxide in photosynthesis?
1. One atom in every million million of the carbon atoms incorporated in the plant tissues is carbon-14.
2. If an animal eat the plant, the carbon-14 then becomes a part of the animal's tissues.
3. With death, an organism's intake of carbon-14 ceases, but the carbon-14 already in the tissues of the organisms continues to decay at a fixed rate.
What is the ratio of radioactive carbon in the tissues of a living organism today?
1. It is one carbon-14 atom to every 10^12 carbon-12 atoms.
2. This ratio declines to 0.5 in 10^12 after 5730 years, and so on.
3. In other words, over a period of 5730 years +/- 40years, half of any given quantity of carbon -14 breaks down.
4. Therefore, this is known as the half-life of radioactive carbon.
What does a normal method of radiocarbon dating requires?
It requires at least three grams of organic material so that the rate of radioactive decay of carbon-14 in the sample can be measured.
What is Accelerator Mass Spectrometry (AMS) radiocarbon dating?
1. It is a more refined technique and it can be used to date a sample as small as 100 micrograms.
2. This technique involves breaking the sample up into its constituent atoms so that the number of atoms of each isotope of carbon can be counted.
What can you find using AMS radiocarbon dating?
1. It has become possible to date cave paintings accurately from tiny samples of pigment.
2. It has been found that such pigments often contain organic material such as charcoal, and honey, milk, blood or oil seed may have been used to bind the pigment particles.
Why can't AMS radiocarbon dating be used?
1. After about 70,000 years, the quantity of carbon-14 left is negligible, this is why this method cannot be used to date back more than about 60,000 years.
2. A further limitation is that the material to be dated must contain organic compound - that is, it must contain carbon.
How can the approximate age of artefacts can be deduced?
By dating the charcoal found in ancient hearths, the approximate age of artefacts can be deduced.
What is another problem with using radiocarbon dating?
- It was once assumed that the ratio of carbon-14 to carbon-12 in the atmosphere was constant, but it is now known that the amount of carbon-14 in the atmosphere varies.
-Thus, radiocarbon dates must be treated with a certain degree of caution.
-Corrections for the fluctuations in the carbon-14 content of the atmosphere are, however, now possible for about the last 9000 years, by reference to tree ring dating.
AKA tree-ring dating is another type of absolute dating that is sometimes of value in the study of human origins.
What does the concentric rings on the surface of a cut tree trunk represent?
Each ring represents one year's growth and the rings differ in width according to how favourable the growing season was.
What can be used as marker rings?
-Certain rings produced in years of exceptional weather conditions.
-Starting with living trees, it is possible to correlate these marker rings with timber taken from ancient human structures and gradually to work back to timber thousands of years old.
How can living trees be dated?
-Living trees can be dated by drilling a small core out of the trunk wood and counting the rings.
-It may be noted, for example, that a particularly wide ring, indicating a year of very favourable growth, occurs near the middle of the trunk.
-This wide marker ring could be found near the outside of a piece of tree trunk used in an ancient structure.
-By such correlation of marker rings, dates for the building of the structure can be determined.
Why does tree ring dating have obvious limitations?
-The particular conditions necessary for the use of the method do not occur often and timber is rarely preserved for more than a few thousand years, but given certain circumstances the method can be valuable in calculating absolute ages.
How is each method of absolute dating limited in its application?
-This is because it depends on the occurrence of a particular set of circumstances before it cane be used.
-Together, however, these and other methods do give the anthropologist a number of ways of determining the actual age of ancient material.
Summarise the useful range of common absolute dating methods.
1. Tree Growth Rings - wood - up to 9000
2. Carbon-14 - carbon compounds - up to 60 000
3. Protoactinium - sea sediments - up to 250 000
4. Uranium-thorium - sea sediments - up to 600 000
5. Potassium-argon - Volcanic deposits - 200 000 and earlier
6. Electron spin resonance - calcium carbonate (in shells, coral, teeth) also quartz and flint - up to 100 000 possible 300 000
7. Fission tracks - minerals and glass - 100 years ago to 4550 million
8. Thermoluminescence - sediments, lava, ceramics - 300 years ago to 100 000
What is Relative Dating?
-When it is not possible to determine the actual age of a fossil or artefact, scientists can often determine whether it is older or younger than another sample, or whether it is older or younger than the rock or soil in which it is found.
-Such relative dating enables a sequence of events to be established.
It is the study of layers, or strata.
What are the 2 ways in which stratigraphy can be useful in dating fossil material?
1. Principle of Superposition
2. Correlation of rock strata.
What is Principle of Superposition?
-It assumes that in layers of sedimentary rock the layers at the top are younger than those beneath them.
-Thus, any fossils or other material found in the top layers will be younger than material found lower down.
-This principle must be applied with care because distortions of Earth's curst do occur and a sequence of rock layers may be turned upside down.
-In addition, it is possible for fossils or artefacts to be buried by animals, or perhaps by early humans, some time after the deposition of sediment.
-In this case the specimen may be younger than some of the layers above it.
What is Correlation of Rock Strata?
- It involves matching layers of rock from different areas.
-Matching strata can be done by examining the rock itself and also by studying the fossils it contains.
-Rocks that contain the same fossils may be assumed to be of the same age.
-Certain fossils are of great value in this correlation work because they are widely distributed and were present on Earth only for a limited period of time, making the relative dating of strata more precise.
What are Index fossils?
A fossil that is useful for dating and correlating the strata in which it is found.
How did Fossilised Pollen Grains become an important branch of science?
-Some fossil Pollen Grains are useful as index fossils, but even if they cannot be used in this way, the presence of preserved pollen grains in a soil or rock sample can enable a botanist to construct a picture of the type and amount of vegetation existing at the time the deposit was laid down.
-An idea of the climatic conditions prevailing at the time may then be worked out.
-This data can be used to confirm or refute relative dates arrived at by other methods.
Define Fluorine Dating.
-It is another relative dating technique.
-It is based on the fact that when a bone is left in soil, fluoride ions, which are present in water in the soil,replace some of the ions in the bone itself.
-All the fossil bones in a particular deposit should contain the same amount of fluoride, so that fossils that have been displaced can be detected.
-The older the fossil them more fluoride it contains, so that the relative ages can be established.
-It is not possible to decide absolute ages using this method since the concentration of fluoride in ground water varies from place to place and from time to time.
Why was the geological time scale created?
Because of the tremendous time span involved, Earth's geological history has been divided up into a geological time scale.
What is the Geological Time Scale?
The time scale consists of eras, which are subdivided into periods, and the periods are further subdivided into epochs.
When was the Cainozoic era?
The primates, the group to which humans belong, first evolved at the beginning of this era.
Why is the fossil record very incomplete?
-Because the conditions for fossilisation do not always occur, or they occur at irregular periods of time.
-Fossilisation is therefore a chance occurrence and there are many gaps in the fossil record because organisms have not been preserved.
What are the 4 conditions usually required for a fossil to form?
1. A quick burial of the material.
2. The presence of hard body parts.
3. An absence of decay organisms.
4. A long period of stability - the organism needs to be left undisturbed.