6.1 The origins of life on Earth
Fossils are the remains of organisms from many years ago that are found in rocks.
Fossils may be formed in various ways:
- from the hard parts of animals that do not decay easily e.g. bones, teeth, shells and claws
- from parts of organisms that have not decayed because some of the conditions for decay are absent, e.g. preserved in ice
- when parts of the organisms are replaced by other materials, such as minerals as they decay
- as preserved traces of organisms e.g. footprints, burrows and rootlet traces
Soft bodied organisms did not leave fossils, many organisms did not form fossils and some fossils were destroyed over time. This means the fossil record is incomplete.
How fossils are formed
1. The creatures dies and falls to the ground
2. The flesh rots, leaving the skeleton to be covered in sand or soil and clay before in it damaged.
3. Protected, over millions of years, the skeleton become mineralised and turns to rock. The rocks shifted in the earth with the fossil trapped inside.
4. Eventually, the fossil emerges as the rocks move and erosion takes place.
6.2 Exploring the fossil evidence
Extinction is the permanent loss of all the members of a species from the Earth.
Many species have evolved, and then become extinct.
Extinction can be caused by:
- a new disease
- environmental changes
- a new predator
- a new competitor
- a single catastrophic event
6.4 Isolation and the evolution of new species
Natural cycles linked to environmental change result in new species forming and old species dying out.
New species arise when two populations become isolated.
Populations become isolated when they are separated geographically e.g. on islands.
In an isolated population alleles are selected that increase successful breeding in the new environment.
Speciation takes place when an isolated population becomes so different from the original population that successful interbreeding can no longer take place.
Speciation: isolation - genetic variation - alleles selected - interbreeding no longer possible - new species