Evidence for Evolution - Fossils

Fossil

Any preserved trace left by an organism that lived long ago. Can be bones, shells, teeth, footprints, burrows, faeces or impressions of an animal or plant

Fossil usage

Allow scientists to determine exactly what an extinct species was like. Can determine what organism may have eaten, which other organisms may have existed at the time and what the climate may have been like. Can also build up a sequence of evolution of a particular plant or animal

Types of fossils

Index fossil, mould, cast, mineralised fossils

Index fossil

Widely distributed fossils, only existed for a short and specific time period, useful for dating rock layers in which they are found

Features of index fossil

Widespread - allows dating worldwide or at a wide range of locations

Found in large numbers - numerous enough to easily find, increased chance of discovery

Recognisable - easily distinguishable and identifiable

Lived for a short period of time - easily dated (time period of existence), easily identified. Can be used to find accurate relative age of other fossils

Mould

A hardened impression (hollow) that has been made in sediments

Cast

Occurs when a mould fossil is filled with minerals and allowed to harden

Mineralised fossils

Waterborne minerals fill the void spaces within the original material of the specimen

Fossil formation

Most dead organisms are decayed by microorganisms, chance of fossilisation is very small

Cases of fossilisation

Part of organisms may become fossilised when buried by volcanic ash or other members of the species (burial). If burned rapidly, conditions may not be suitable for the activity of decomposers, decomposition may be slowed down/prevented -> better preservation

Effects of soil conditions

Nature of soil, acidic soils, tannins, alkaline soils

Nature of soils

Organisms must be buried rapidly, soil needs to have no oxygen -> slows down/prevents microorganisms decaying the body, completely preserves soft tissue/bones

Acidic soils

Not good for fossil preservation as the minerals in the bone are dissolved. However, if acidic soil has no oxygen, may still preserve the specimen

Tannins

From plants found in peat bogs, act as a preserving agent. Peat is formed in acidic soils with low oxygen as the plant matter in this condition decaus more slowly and accumulates.

Alkaline soils

Minerals in soil (often lime or iron oxide) replace the organic matter in the bone and preserve it as rock (bone is petrified)

Discovery of fossils

Main way - excavation

Process of excavation

1. Site is surveyed and marked into sections

2. Small hand tools are used to remove soil gently

3. Small samples are removed and sieved

4. Photographs are taken at every stage for analysis of photos later

5. Each item is carefully labelled and catalogued.

6. In the laboratory, fossil bones are carefully scraped clean, broken parts are pieced together, measurements are taken and plaster casts or latex mouls may be made

Discovery of human fossils

In the case of fossils with human ancestors, artefacts are often found in association with the fossils

Artefacts

Objects deliberately made by humans, include items such as stone tools, beads, carvings, charcoal from cooking fires and cave paintings

Fossil reconstruction

Fossils are often not whole organisms and must be constructed. Reconstructions are approximately based on the experience of the scientists involved, which can lead to controversy

Location of fossils of human ancestors

Fossils of human ancestors are often found at the edges of ancient lakes and river systems, in caves or in volcanically active areas (organisms can be buried rapidly, preventing decomposition)

Lakes and rivers

Build up sediments when flooding occurs or when the water flow slows rapidly

Caves

Many caves are in limestone, which consists of calcium carbonate - chemical may be deposited around dead organisms, or cave roofs or walls may collapse, covering bodies of animals

Volcanic eruptions

Unusual for animals to be preserved near volcanic eruptions because heat from the volcanic material destroys the organism, but in East Africa ash falls have preserved fossils of many human ancestors

Problems with fossil record

Incomplete, as conditions for fossilisation do not always occur, or occur at irregular periods of time. Fossilisation is a chance occurence so there are gaps in fossil record, and most fossils that do exist have not been discovered (buried too deep, inaccessible, inadvertently destroyed), or what has been found may be impossible to date

Fossilisation conditions

Quick burial of material, presence of hard body parts, absence of decay organisms, long period of staibility - organisms need to be left undisturbed