Fossils are the ‘remains’ of plants or animals that lived millions of years ago, and are found in rocks.

The fossil record shows us that simple organisms developed first.

The oldest fossils are found in the deepest, oldest rocks. It is possible to follow how organisms have changed (evolved) over the years.

Fossils also provide evidence of organisms that have become extinct, probably due to changes in the environment.

The fossil record is incomplete because soft-bodied organisms do not form fossils as well as vertebrates and there are fewer fossils of land organisms than those that lived in or near water. Many fossils are buried deep within rocks and are yet to be found.

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Formation of Fossils

Fossils can be formed if the organism is covered in a layer of mud or earth soon after it dies, and the sediment covering it eventually forms sedimentary rock.

There are different types of fossils, for example:

• Hard parts of the organisms, e.g. teeth or shells, which do not decay easily and are preserved unchanged in the rock
• From parts of organisms which have not decayed because one or more of the conditions needed for decay are absent
• When parts of an organism decay and are replaced by other minerals, so it is eventually made of rock
• When the organism decays but leaves an impression in the rock
• As preserved traces of organisms, e.g. footprints, burrows or rootlet traces
  

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The Theory of Evolution

The theory of evolution states that all species of living things that exist today, and many more that are now extinct, evolved from simple life forms that first developed more than three billion years ago.

Evolution takes place very slowly, over millions of years.

In 1856, Charles Darwin first suggested the idea that evolution occurred by a process of natural selection. At the time, his ideas were met with opposition, partly because it went against the beliefs of the Church. His ideas were based on careful observation of fossils and organisms living in different parts of the world.

Natural selection is based on the idea that nature will select the strongest and fittest organisms for survival, whilst weaker organisms will die. It is also called survival of the fittest.

Individual organisms within a species show a wide range of variation because of their genes.

Large numbers of organisms die due to predation, disease and competition.

Individuals with characteristics most suited to the environment, are more likely to survive.

These individuals will breed and pass on the genes to their offspring, many of which will also have the useful characteristic.

Over many years the useful characteristic becomes more common.

The diagram below shows how the horse has evolved from a small, dog-like animal, which is thought to have lived on soft, marshy ground and to have eaten soft vegetation.

Its teeth have become bigger and stronger, so it can chew grass.

The feet have evolved to have hooves, which are better for running fast on dry ground, and the leg bones are longer and stronger.

Other theories exist to explain the diversity of organisms that exists.

Lamarck, a scientist in the 1800s, thought that organs developed if they were used more. He thought giraffes had long necks because they had to stretch to reach the leaves at the tops of trees. He believed that all characteristics of an organism were passed on to the offspring, so the giraffes with long necks would produce offspring with long necks.

Others thought that long species were created by a god, and some thought they appeared spontaneously.

Organisms may become extinct (die out) if they cannot adapt to environmental changes, such as climate change, new predators or diseases, or more competitors for resources.

Evolution is essential, so species become better adapted to survive the changes and prevent extinction.

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Antibiotic-Resistant Bacteria

An antibiotic is a chemical produced by a fungus that kills bacteria. They are very important medicines.

Unfortunately, overuse of antibiotics has led to new strains of bacteria that can resist these drugs, so they no longer work to kill the bacteria.

It is very important to take the full course of an antibiotic, to kill all the bacteria, even if you feel better.

If some bacteria survive, they may mutate. This means a change occurs in the bacterial DNA, which may give the organism resistance to the antibiotic.

Bacteria reproduce very rapidly, so many antibiotic-resistant bacteria are quickly produced, which may infect other organisms.

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