S-Cool Revision Summary
S-Cool Revision Summary
Fossils are the remains of organisms that lived millions of years ago. Usually all the original tissue has gone and only the shape is left as a mineral cast.
What are the fossils made from?
Most of the fossils that you find are made from the hard parts of animals. They include such things as teeth, bones and shells.
Fossils are thought to have been buried in silt or mud and broken down only very slowly. Gradually they were replaced by minerals from the surrounding ground as the surrounding ground was turned to rock. So the rock-like substance of the fossil was formed but stays separate and distinct within the rock. It lies there until you dig it up!
Other fossils can be different in that they come from the softer parts of animals or plants. An example of this might be the "petrified forests" that have been found in coal mines.
Petrified means 'turned to stone'. The trees that used to be in a forest where the coal mine's are were slowly turned into mineral fossils. They look like tree stumps and trunks made of coal.
But it is very rare as the conditions were not often right or the rate of decay slow enough.
Charles Darwin is the one who is credited with coming up with the idea of natural selection, although there were others too, notably the American, Alfred Wallace.
Darwin noted four things from his observations:
- All organisms produce an abundance of offspring, many more than is necessary
- There is a fairly constant population size over time for any particular organism
- Within a species there is a wide range of features, due to different alleles
- Some variations are passed on and inherited by the offspring
From these Darwin decided that all organisms struggle for survival and so have to produce many offspring to ensure that some do survive.
He also concluded that for a species to survive, the best (or 'fittest') of them must survive long enough to reproduceand pass on their genes.
As usual, someone else had a different idea.
Jean Baptiste de Lamarck had the idea that plants and animals evolve features according to how much they use them.
Lamarck's idea means that giraffes evolved long necks by reaching up for leaves in trees. At one time they would have had short necks but then they got longer.
Why do some animals 'die out' and become extinct? The dinosaurs, hairy mammoth, dodo and many others are all extinct.
There are 3 ways in which extinction can occur:
- A rapid environmental change to which they can't adapt, this could be a natural change such as flooding or manmade such as the building of a city.
- A new threat may occur, perhaps a new predator or disease kills them
- Failure to compete successfully against a new species.
This theory is basically that all animals and plants on Earth gradually developed, or 'evolved', over millions of years from a common ancestor.
Life first began as simple organisms living in water. From there organisms became more complex and gradually moved onto the land and all over the Earth.
The process took only about 3 billion (3,000,000,000) years. Only!
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