What is a fossil?
Any evidence of living things from the distant past
Outline the different types of fossils
- Original fossil → parts of original organism
- Indirect fossil → an imprint or cast of the organism
- Replacement fossil → mineral chemicals have gradually replaced the organism
- Carbon film fossil → organism has decomposed to leave a black carbon silhouette in rock
When do fossils form?
When dead organisms are not eaten by scavengers and are quickly covered to prevent decomposition by microorganisms or reaction with oxygen
Where are fossils found?
Sedimentary rock
In what environments do fossils tend to form?
- riverbed, seabed, lakebed
- swamps
- special cases - amber, tar pits, deserts, ice
Why do fossils give a limited, biased view of life from the past?
- most fossils are found underneath water → not all creatures were water-dwelling
- hard parts of organisms (e.g. bones, shells) are harder to break down and tend to be preserved → we rarely have fossils of soft animals such as slugs or jellyfish
How does relative dating work?
uses comparison of fossils to find out if one is older/younger than the other
What are two methods of relative dating?
- Steno’s law of superposition is used to compare fossils from one location → states that oldest layers are at the bottom and youngest layers are at the top
- Index fossils are fossils used to compare the age of fossils at different locations → index fossils must be easy to identify, abundant, found in many locations, and present for only a short period of time (e.g. trilobite and ammonite)
What is absolute dating?
A technique used to determine the actual age of the fossil (not 100% accurate)
What is one form of absolute dating?
Radioactive dating
How does radioactive dating work?
- analyses presence of radioisotopes and the stable element it decays to
- percentage that has decayed is calculated
- percentage decayed can be used to determine the age using half-life
What is half-life?
The time is takes for half the radioisotopes to decay
What is a radioisotope?
A radioactive unstable element that loses parts of it’s nucleus to become stable
What does the fossil record show?
- not all organisms that have existed still survive today
- different organisms have been dominant at different periods of time in the Earth’s history, and then disappeared
What is the fossil record?
When all fossils are placed in order from oldest to youngest
What is the geological time scale?
When the history of life on Earth is marked into different time periods according to when species appeared & disappeared
What is an adaptation?
a characteristic of an organism that allows it to survive in its environment
What are the classifications of adaptations?
- structural - a physical structure e.g. webbed feet
- behavioural - a behaviour e.g. huddling to keep warm
- physiological - an internal process/mechanism e.g. heart rate slows when swimming
What is the relationship between a species having a wide variety of genes and it’s ability to survive?
a wide variety of genes will have a wide variety of characteristics, meaning that the species is more likely to survive in a changing environment
Who came up with the theory of evolution by natural selection?
Charles Darwin and Alfred Wallace (separately)
What is the theory of evolution by natural selection?
- When there is a change in an environment, some characteristics allow members of a species to survive better because they suit the particular environment
- Members who survive are more likely to reproduce & pass on their adaptations
- Over successive generations, more members of the species show the same adaptations
What did Darwin notice about finches?
- vegetarian finches have a heavy beak to pull buds from plants
- tree finches have grasping beaks to eat grubs & insects
- ground finches have crushing beaks to eat seeds and cactus
- finches on the Galapagos islands (off the coast of Ecuador) were most similar to finches in South America
What is a selection pressure?
a change in an environment
What is biogeography?
the study of the geographical distribution of living things, both in the present and the past. It shows that a species is similar to species in an adjacent region.
