Chapter 18 - Fossil Evidence For Evolution Flashcards
What is a Fossil?
- A fossil does not have to be a part of an organism.
- Any preserved trace left by an organism that lived long ago is a fossil.
- 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?
- In wet, acid soils the minerals in the bone are dissolved and fossilisation occurs.
- 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.
- 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 ?
- Lakes and rivers build up sediments when flooding occurs or when the water flow slows rapidly.
- Many caves are in limestone that consists of calcium carbonate.
- 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?
- 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.
- 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?
A ‘dig’.
List the steps of Discovering Fossils.
- The area to be investigated is first surveyed and marked out in sections.
- Small hand tools are used to remove the soil gently so as not to damage any of the material.
- The soil removed is usually sieved so that every very small fragments are not overlooked.
- In the case of fossils of human ancestors, artefacts are often found in association with the fossils.
- Photographs are taken at every stage of a dig so that detailed studies of positions of uncovered material can be carried out later.
- Each item is carefully labelled and catalogued for the prolonged study that follows the excavation of the site.
- 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.
Define Artefacts.
- 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.
