Chapter 11 - Other evidence of evolution Flashcards
fossil
any preserved trace left left by an organism that lived long ago. eg, bones, shells, teeth, footprints
what do fossils and the rock in which they are found tell us
what the organisms ate, what other organisms existed at that time and what the climate was like
How are fossils formed
Part of organisms may become fossilised when buried by drifting sand, mud deposited by rivers, volcanic ash or other members of the same species. If buried rapidly, conditions may not be suitable for decomposers and decomposition may be slowed or prevented
which soil doesn’t allow for fossil formation
And wet, acidic soils the minerals in the bone are dissolved and no fossilisation occurs, except if the soil contains no oxygen
which soils produce the best fossils
Bones buried and alkaline soils because the minerals in the bones are not dissolved
Why are fossils often found near lakes and rivers
Because the organism can be buried rapidly, preventing decomposition. Lakes and rivers build up sediments when flooding occurs or when the water flow slows rapidly
what are artefacts
Objects that have been deliberately made by humans. E.g. stone tools, beads, carvings
Absolute dates
The actual age (in years) of a fossil or artefact
relative dates
a comparison of fossils to tell us whether one sample is older or younger than another
Potassium argon dating
A method of calculating the age of a fossil or artefact using the known rate of decay of radioactive potassium.
how does potassium argon dating work
Potassium-40 is a radioactive isotope and decays to form calcium-40 and Argon-40. Such decay takes place at an extremely slow but constant rate and so determining the amounts of potassium-40 and argon-40 in a rock sample enables the age of the rock to be calculated. As the rock ages, the portion of potassium-40 in decreases while that of argon-40 increases.
what are the limitations of potassium argon dating
not all rock types are suitable for this method of dating and it can only date rocks older than 100,000 to 200,000 years as after 100,000 years only 0.0053% of the potassium-40 has decayed and such small amounts are hard to detect
what is half life and what is the half life of potassium-40
The time required for half of any quantity of radioactive material to decay into stable non radioactive material. The half life of potassium-40 is 1250 billion years (1.25 x 10^9 years)
When is potassium argon dating most useful
On samples that are older than 200 000 years
Isotopes
One of two or more atoms of the same element with the same atomic number and number of protons, but different numbers of neutrons
Carbon-14
The radioactive isotope of carbon
Radiocarbon dating
The calculation of the age of a fossil or artefact using the rate of decay of radioactive carbon to nitrogen
Explain carbon-14 dating
Carbon-14 is produced in the upper atmosphere by the action of cosmic radiation on nitrogen at about the same rate at which it decays. When green plants use atmospheric carbon dioxide in photosynthesis, one in every million carbon atoms is carbon -14. If an animal eats the plant, the carbon-14 becomes a part of the animals tissues. With death, intake of carbon-14 ceases, but the carbon-14 already in the tissues continues to decay at a fixed rate. By measuring the amount of radiation liberated by a sample, the ratio of carbon-14 to carbon-12 can be estimated and an age calculated
what is the half life of carbon-14
5730 ± 40 years
what is AMS radiocarbon dating
Accelerator Mass Spectrometry is a technique used to give radiocarbon dates for very small samples of material (100 micrograms). Involves breaking up the sample into its constituent atoms so that the number of atoms of each isotope of carbon can be counted
what are the limitations of radiocarbon dating
- Can only be used up to 60000 years ago
- Material to be dated must contain organic compounds
- Amount of carbon-14 in the atmosphere varies, was thought to be constant
Stratigraphy
The study of the sequence of rock layers as a means of relative dating
what are the two ways in which stratigraphy can be useful in dating fossil material
Principle of superposition and Correlation of rock strata
Principle of superposition
In a vertical sequence, the principle that the sedimentary rock layers on top will be younger than those lower down
Correlation of rock strata
The process of matching rock strata from different places
Index fossils
Fossils or organisms that were on Earth for only a short period of time and therefore useful in the relative dating of rock strata
problems with the principle of superposition
- Distortion of Earth’s crust do occur and a sequence of rock layers may be turned upside down
- Fossils or artefacts may be buried by animals or humans, in this case the specimen may be younger than some of the layers above it
what are the four conditions that are required for fossils to be formed
- Quick burial of the material
- Presence of hard body parts
- Absence of decay organisms
- Long period of stability (organism needs to be left undisturbed)
problems with the fossil record
Fossilisation is a chance occurrence and there are many gaps in the fossil record because organisms have not been preserved and only a very small proportion of the fossils that do exist have actually been discovered, some are buried too deep or are inaccessible. Others may have been destroyed by human activity or not recognised as fossils
Embryology
The study of early development of an organism.
Homologous structures
Structures with a similar structure but not necessarily a similar function. Eg, Feet of amphibians and reptiles, wings of bats and birds, leg of hose, flipper of whale, human hand
Vestigial structures
A structure of reduced size that appears to have no function.
4 examples of vestigial structures
- Nictitating membrane
- Ear muscles
- Wisdom teeth
- Appendix
Phylogenetic tree
a diagram showing evolutionary relationships between related organisms, also called dendrogram
