Fossils, extinction and speciation
This topic links nicely with Darwin's work on natural selection and evolution. Fossils show us nice evidence of evolution, extinction and speciation, and are our main insight into the past. They are not, however, a perfect record as you will see.
Fossils
The Earth is about 4.5 billion years old. Much of our evidence for what life was like during the past several million years is based on fossil records. Fossils are the preserved remains of once living organisms and they can be formed in a number of different ways:
The Earth is about 4.5 billion years old. Much of our evidence for what life was like during the past several million years is based on fossil records. Fossils are the preserved remains of once living organisms and they can be formed in a number of different ways:
Hard parts - such as bones, claws or teeth. These don't decay and so can be found thousands or millions of years after the animal diet
Unable to decay - this happens when animals are trapped in a medium which prevents decay. Examples include amber (very Jurassic Park!) and ice (such as the baby mammoth) Mineralisation - this occurs when the skeleton of a preserved animal gets replaced by rock Traces - Examples include footprints and droppings (see left) |
Look at the BBC Nature website to find out more about fossils if you are interested:
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Fossils give us lots of interesting information. They give us an idea about how species alive today have evolved over the years. Because rock forms in layers, and the oldest layers are at the bottom, where a fossil is found can give ideas about when that organism was alive. Interestingly, as we evolved from much simpler organisms, fossils found in older rocks are much simpler.
As interesting as it is, the fossil record is not complete. There are many gaps in it. This is due to a number of reasons:
As interesting as it is, the fossil record is not complete. There are many gaps in it. This is due to a number of reasons:
- Some fossils haven't been found yet and remain buried under layers of rock
- Ancestral species were often soft bodied. These wouldn't not have been fossilised but instead would have decayed leaving no trace of their existence
- Conditions for fossilisation are rare so many dead animals and plants are simply not preserved
- Geological movements over the years (eg. earthquakes, continental drift, volcanoes) have destroyed many fossils
The evolution of the modern horse is a good example of a reasonably complete fossil record. Comparison of the fossils from the leg and foot of the horse has allowed us to piece together how the horse has changed and adapted over time.
The ancestral horse (eohippus) lived about 60 million years ago and was about the size of a small dog. See how it's foot evolved from having 4 spread-out toes, suited for walking in the soft marshy conditions of the time, to one hoof in the modern horse. This is to suit the harder conditions of the ground. In addition, the horse has become significantly bigger (to evade predators maybe). |
Extinction - this is the loss of all organisms of a species. Dinosaurs are classic examples of organisms that have become extinct. Polar bears, tigers, panda bears and the Northern white rhino are all examples of animals that are currently facing extinction, with only a relatively few of each still alive today.
There have been 5 mass extinctions in the past. These occur for a variety of reasons (such as volcanic activity or collision of a giant asteroid) which cause significant changes in the environment or the atmosphere. This results in the loss of huge numbers of species of animals and plants
The last mass extinction was 65 million years ago and result in the loss of approximately 50-70% of the species on Earth, including the dinosaurs. |
These days humans are the cause of much of the extinction that is occurring in the world. Common reasons are:
Hunting and fishing
Climate change - burning of fossil fuels which results in global temperatures rising
Introduction of species into a non-native area (eg, when the grey squirrel was introduced it outcompeted the native red squirrel)
Habitat destruction
Extinction of a species from a food chain
Hunting and fishing
Climate change - burning of fossil fuels which results in global temperatures rising
Introduction of species into a non-native area (eg, when the grey squirrel was introduced it outcompeted the native red squirrel)
Habitat destruction
Extinction of a species from a food chain
Speciation
This is the process by which new species evolve. It brings together a number of important principles: genetic variation, adaptation and natural selection. It is a step-wise process (outlined below), starting off with geographical isolation.
This is the process by which new species evolve. It brings together a number of important principles: genetic variation, adaptation and natural selection. It is a step-wise process (outlined below), starting off with geographical isolation.
1. A population becomes geographically isolated from the original population (eg, by earthquakes, continental drift, a man-made structure such as a road, volcanic eruptions, on an island)
2. There may be different environmental conditions in the new locations 3. Genetic differences within the species leads to variation 4. Natural selection occurs whereby those that are better adapted will survive (survival of the fittest) 5. These individuals are more likely to breed and pass their genes on to their offspring 6. Eventually over time, the population will change so much it is unable to interbreed with the original population to produce fertile offspring - this is speciation Remember:
A species is a group of organisms that are able to breed to produce fertile offspring Lions and tigers can breed to produce ligers but ligers are infertile. This means lions and tigers are different species |