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1 of © Boardworks Ltd 2011 Fossils and Species Development
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Fossils and Species Development
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Life on Earth
There are many theories of how life first appeared on Earth, but lack of evidence has made it extremely difficult to know which theory is correct.
Life on Earth began approximately 3,500 million years ago.What do you think the earliest life forms were like?
All life on Earth evolved from very simple organisms. Dinosaurs only appeared 3,325 million years later!
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Life on Earth
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Fossils are the preserved remains, impressions or traces of animals, plants and other organisms that lived millions of years ago. The history of life on Earth as shown by fossils is called the fossil record.
How do we know life has changed?
The fossil record can also show how different species evolved from common ancestors.
Although there are gaps in the fossil record, it helps to show how gradual changes in an organism can be linked to changes in its habitat.
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Pentadactyl limb
Fossils have revealed many structural similarities between living and extinct organisms.
These suggest that certain organisms could share a common ancestor.
For example, many vertebrates share a pentadactyl limb: a limb with five digits, a lower section containing two bones, and an upper section containing one bone.
bird human
lizard
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Evidence for change
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Fossilization
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Fossils only form in the absence of microbes, which need food, oxygen, water and warmth.
How do fossils form?
Some fossils contain the mineralized remains of the organism. In other cases, the remains have been completely dissolved and what is left is an impression, which may later fill with minerals to produce a cast of the organism.
Some fossils are only the preserved traces of organisms, such as footprints or root traces.
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A rare event
Because the conditions for fossilization are so specific, fossils are relatively rare.
Other factors that contribute to the rarity of fossils include:
Many early life forms were soft-bodied and soft tissues usually decay.
Many fossils have yet to be found.
Many fossils have likely been destroyed by geological activity.
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The rise of new species
Over time, new species evolve from earlier species. This may happen for a variety of reasons.
Populations of one species may become geographically isolated from one another.
Genetic variation means that individuals of a population have many different alleles that result in different characteristics.
The process of natural selection means that individuals with beneficial characteristics will survive and reproduce.
Eventually, populations can become so different that they can no longer interbreed. This is called speciation.
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Natural selection
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Natural selection leads to evolution
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Why does extinction happen?
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Causes for extinction
Individuals that fail to reproduce, compete effectively for food or survive against new predators will eventually die out.
If the last individual of a species dies, the species is extinct.
A single catastrophic event, such as a volcanic eruption or a comet striking Earth, could cause mass extinctions over a large area.
Speciation can result in the extinction of an earlier, less successful species.
In modern times, habitat destruction, pollution and over-hunting result in roughly one extinction per hour.
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Non-living factors causing change
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Glossary
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Anagrams
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Summary
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Multiple-choice quiz
