Biology Core- Evolution


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The Evidence

People are always interested in old things, antiques, old books and history. Fossils are another favourite.

There is also a lot to be known about the rocks and soils that give rise to these fascinating things. But we often just overlook them as just being rock and soil.

However they can all give hints and evidence towards understanding one of the big questions:

The origins of life.

  • What are fossils?
  • Where do they come from?
  • What do they tell us?
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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!

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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.

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What conditions are necessary?

Usually a dead plant or animal is broken down very quickly by decay and its molecules used by other organisms.

However if the environmental conditions are suitable the decay process can be slowed so that it takes thousands of years for all the tissue to disappear.

If the dead organism is covered by mud or silt no oxygen will be present. Without oxygen the bacteria and other organisms involved in decay cannot break it down.

Another example is if the tissue were covered in sap or resin from a tree keeping the oxygen out. Over thousands of years the resin turns into the yellow, stone-like mineral called amber.

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Glaciers are another favourite place for animal and plant remains to turn up. There it is too cold for microbes and bacteria that cause decay to work.

The bodies of woolly mammoths have often been found in the frozen, tundra of Siberia, some are 23, 000 years old.

In 1991 in the Oztal Alps between Austria and Italy a man's body was discovered wonderfully preserved. It was named 'Otzi' and was found to be 5300 years old.

Many other human remains have been found in the waterlogged peat bogs of northern and western Europe. The bogs are too acidic for the normal decay.

They've been found in Britain too! 'Lindow man' was found in Lindow Moss in the early 1980's.

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Rocks and Soil

Fossils and the tissue remains give an idea of the organism's shape and structure.

We can work out the remains and fossil's age by looking at the rocks and soil in which they are found.

To do this scientists need to know the age of the rocks and soil themselves. As organisms died and fell into the mud, resin or silt they were slowly buried by more material falling on top.

So if you know the age of the rock surrounding the fossil, you know the age of the fossil itself.

So as time went on younger and younger material covered the fossils. Therefore the deeper you dig down the older the rocks that you are cutting through.

In the Grand Canyon in the USA, the one mile deep hole stretches down to the oldest rocks at the bottom which are about 1 billion (1,000,000,000) years old. So any fossils found there should be that old, or tourists!

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Natural Selection

"Survival of the fittest". That is the bit that everyone remembers about natural selection. But what does it really mean?

The world inhabited by living things is very harsh. They have to compete for everything such as light, food, shelter and mates.

All this and they have to avoid getting eaten!

Only the best suited will survive long enough to reproduce, and so pass on their genes to the next generation.

Organisms try to ensure that their offspring have advantages in survival. Many strategies are used to ensure this; some produce huge numbers of them so that at least a few will survive, others try to protect and shelter them for a long time - and care for them through school!

But where did these ideas about natural selection originate?

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Darwin's ideas

Charles Darwin is the one who is credited with coming up with the idea of natural selection, although there were others too, notably Alfred Wallace.

Darwin noted four things from his observations:

  1. All organisms produce an abundance of offspring, many more than is necessary.
  2. There is a fairly constant population size over time for any particular organism
  3. Within a species there is a wide range of features, due to different alleles
  4. 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 reproduce and pass on their genes.;

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Lamarck's ideas

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 didn't all the other animals get longer necks too?


An experiment that disproved Lamarck's idea was that mice who had their tails cut off - sound's like a nursery rhyme - still gave birth to baby mice with tails!

If Lamarck had been right the parent's wouldn't use their tails and neither would their offspring.

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Mutations are alterations in the DNA sequence of genes.

Most mutations are harmful and do not benefit the organism, they give rise to things like inherited diseases. However occasionally a mutation occurs which gives an organism an advantage over the others within its species. It gives a better chance of survival.

This is quite often seen in bacteria which can develop antibiotic resistance and therefore survive our attempts to kill them.

Other examples could include the development of different shape teeth that enable other types of food to be eaten when the traditional food source is scarce, or stronger legs that could allow you to run faster and avoid getting eaten.

You could think about any characteristic and think about how it might have arisen.

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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:

1. 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.

2. A new threat may occur, perhaps a new predator or disease kills them.

3. Failure to compete successfully against a new species.

While some organisms will die out others will increase in number. Those that can cope do so by gradually developing characteristics that suit the new conditions over many generations.

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We share about 95% of our genes with the Gorilla, and 50% with a banana!

Faced with those numbers most people would answer that it's something to do with evolution. The theory of evolution has had a powerful effect on our views of the world!

It has also caused great debates since the 1840's when Darwin first suggested evolution. Even today scientists still debate this important scientific idea.

The theory of evolution

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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Horsey evidence

Fossils provide a lot of the evidence on which the theory of evolution is based.

They show how organisms that exist today have altered over millions of years from the simpler forms shown in fossils.

One famous series of fossil examples is that of the supposed evolution of the modern horse. The following diagram shows four of the ancestors of the modern horse.

You can see how the animals get larger as time, and presumably evolution, goes on. Initially the animal was about the size of a dog but ends up being much bigger.

Also the bones of the fore leg change from being three separate bones into being one large bone with the hoof of today's horse on the end.

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However fossils are 'flukes'. We have only got fossils when an animal or plant died, fell into a suitable substance and then didn't decay quickly. Most animals or plants at that time would have died where they would decay quickly. In that case we would have no fossil.

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Copyright S-cool (

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The missing links

Although most scientists and people now accept evolution there are some problems with it.

The pattern of evidence we have with fossils is not complete, there are "missing links" in that chain of evidence.

The link between humans and our ancestors is still incomplete.

Archaeologists and Biologists are continually searching for more evidence to support this theory.

So this is still an area where lots of work needs to be done. Fossils and evolution theory is not a dead subject. Perhaps you will add more to them yourself in the future.

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