Theory of Evolution

Evolution has taken place over millions of years and is responsible for all the living things on Earth today. There are different views and theories about the origin of life and the evolutionary process.

How did life start?

planet earth, as seen from space


It is not possible to be certain how life on Earth began. We do know that:

  • the Earth is about 4,500 million years old
  • there is evidence that living things existed on Earth at least 3,500 million years ago
  • no one was there to record how life began
  • early Earth was hotter and the atmosphere consisted mostly of carbon dioxide (with other gases such as ammonia and methane)

Scientific theories

Through the study of simple organisms and the chemistry of living things, we can develop scientific theories about how life on Earth began. The main theory is that living things developed from molecules that could replicate, or copy, themselves, rather like DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] does.

There are two possible origins for these replicating molecules:

  • they were produced by the conditions on Earth at the time
  • they came from somewhere else, such as another planet in our Solar System, or further out in space

Over many millions of years these molecules joined with other molecules, becoming gradually more complex and dependent on each other. The process of evolution by natural selection eventually led to all of the different living things that we see on Earth today.

Darwin's theory of evolution

Photograph of Charles Darwin

Charles Darwin (1809 - 1882)

Charles Darwin was an English naturalist who studied variation in plants and animals during a five-year voyage around the world in the 19th century. He explained his ideas on evolution in a book called, 'On the Origin of Species', published in 1859.

Darwin's ideas caused a lot of controversy, and this continues today, because they can be seen as conflicting with religious views about the creation of the world and the creatures in it.

The basic idea behind the theory of evolution is that all the different species have evolved from simple life forms. These simple life forms first developed more than 3 billion years ago (the Earth is about 4.5 billion years old). The timeline below shows some of the key events in the evolution of life on Earth, from the first bacteria [bacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] to the first modern humans.

You can see a more detailed history of life timeline on BBC Nature.

Natural selection

The theory of evolution states that evolution happens by natural selection. The key points are that:

  • individuals in a species show a wide range of variation
  • this variation is because of differences in their genesgenes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production.
  • individuals with characteristics most suited to the environment are more likely to survive and reproduce
  • the genes that allow these individuals to be successful are passed to their offspring

Individuals that are poorly adapted to their environment are less likely to survive and reproduce. This means that their genes are less likely to be passed to the next generation. Given enough time, a species will gradually evolve.

You need to remember that variation can be caused by both genes and the environment. But it is only variation caused by genes that can be passed on to the next generation.

Conditions on Earth

Life on Earth today exists because of the conditions that were present when life was evolving. If, in the past, conditions had been different eg hotter, colder, or the Earth more or less massive, then evolution by natural selection could have produced some very different results. Our bodies would have changed to suit the Earth's conditions.

Selective breeding

Natural selection and selective breeding can both produce changes in animals and plants. The difference between the two is that natural selection occurs in nature, but selective breeding only occurs when humans intervene.

Selective breeding is a process where we choose the characteristics we want in an animal. We then breed together a male and female that show some of those characteristics. From the offspring produced we select those that show the characteristic the most, and breed them together.

This process is repeated over many generations, each time selecting and breeding together those animals that have the characteristics we are looking for. Over a large number of generations, this can produce some surprising results.

Image showing dogs sitting in a line. They are all bred from a common ancestor

All of these dogs were selectively bred from a common ancestor

Farmers have used selective breeding for centuries to increase milk yield in cattle, produce larger eggs from chickens and obtain more grain from wheat.

Mutations - higher only

Causes and effects

Mutations are changes that can occur in genesgenes: The basic units of genetic material inherited from our parents. A gene is a section of DNA which controls part of a cell's chemistry - particularly protein production.. These changes are random and can be caused by background radiation and chemicals that we come into contact with, for example the chemicals in cigarette smoke.

Sometimes these changes can be so severe that the cell [cell: Basic unit of life. Unicellular organisms only have one cell. Multicellular organisms have many cells. ] dies, sometimes the cell can divide uncontrollably and become cancerous, and sometimes the changes are small and the cell survives. Very rarely the changes may even be beneficial to us and produce new and useful characteristics.

Passing on mutations

If these changes occur in normal body cells, the changes are lost when we die. But if the changes occur in our sex cells such as sperm and ova, there is the possibility that the changes in the gene will be passed onto the next generation.

It is when these changes are passed on to the next generation that natural selection can either ensure that they are selected if they are useful, or disappear from the gene pool if they are not.

New species

The combined effect of these mutations, environmental changes, and natural selection, can sometimes produce changes in the organism that are so great that a new species is produced. This does not happen very often and only occurs when the mutated organism can no longer breed with the original species and is capable of producing fertile offspring.

How scientists developed explanations for evolution

Origins of life

Living organisms on Earth today, and the chemicals which work together to keep them alive, are extremely complex, even in single-celled micro-organisms. Some people find it hard to accept that such complexity could have evolved through natural selection. Some religious people believe that all living things on Earth were made by God, or that life was begun by God but then evolved through natural selection. We will probably never be absolutely certain about how life began, as no one was there to observe it. But scientists must base their theories on evidence.


Darwin was not the only person to develop a theory of evolution. Jean-Baptiste Lamarck was a French scientist who developed an alternative theory at the beginning of the 19th century. His theory involved two ideas. These are:

  • the law of use and disuse
  • the law of inheritance of acquired characteristics

His theory stated that a characteristic which is used more and more by an organism becomes bigger and stronger, and one that is not used eventually disappears. Any feature of an organism that is improved through use is passed to its offspring.

This table summarises the two different explanations for giraffes having long necks.

Summary of explanations for giraffes having long necks

A giraffe stretches its neck to reach food high upA giraffe with a longer neck can reach food high up
The giraffe's neck gets longer because it is used a lotThe giraffe is more likely to get enough food to survive and to reproduce
The giraffe's offspring inherit its long neckThe giraffe's offspring inherit its long neck

Lamarck's theory cannot account for all the observations made about life on Earth. For instance, his theory would predict that all organisms gradually become complex, and simple organisms disappear.

Darwin's theory can account for the continued presence of simple organisms.

Developing explanations

Just like Darwin and Lamarck, when presented with information about alternative views about the origin of life and the evolutionary process, you need to be able to:

  • Identify statements that are data and statements that offer an explanation. For example, data is produced from an investigation and often but not always includes numbers. However, an explanation is a hypothesis that can be used to explain the data. If a scientist grows a micro-organism and counts the number of micro-organisms present every thirty minutes, the numbers he or she collects is data. If the scientist notices that the number doubles every thirty minutes, he or she can use the data to produce an explanation, eg that bacteria [bacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] divide every thirty minutes and the numbers increase exponentially.
  • Recognise data or observations that are accounted for, or conflict with, an explanation. For example, fossilsfossils: hardened remains or impressions of an organism that existed in the geological past can be dated by looking at the age of the rock in which they are found. The fossil record shows how organisms have changed over a period of time. The data supports the explanation of the theory of evolution. However, when the fossil data is compared with the theory that all life on Earth started just a few thousand years ago, the data does not support the explanation.
  • Identify imagination and creativity in an explanation. For example, sometimes the data collected from an investigation does not immediately lead to an explanation. When Alexander Fleming noticed that bacteria were not growing next to the penicillium mould it took imagination and creativity for him to think that maybe the mould was producing a chemical that was killing the bacteria. That is how antibiotics were discovered.
  • Identify a scientific question for which there is not an agreed answer. For example, did life evolve on planet Earth or did it arrive on a meteorite from outer space? Because no one was there at the time it is impossible to give an exact answer.
  • Suggest why scientists involved in the same research sometimes disagree with each other. For example, many years ago, when scientists were first trying to date how old a fossil was, they sometimes disagreed. It was only when more evidence became available that they began to agree on the fossil's age.

Contradictory evidence - higher only

You will also need to be able to suggest why some scientists are reluctant to change their minds and explanations, even when new evidence is found that contradicts them. This is often due to the fact that scientists devote their lives to their research. To realise that, after all the years they have spent on it, their theory was wrong, is very difficult to accept.

Evidence for Darwin's theory of evolution


Fossils of ammonites - sea creatures that became extinct about 65 million years ago

Most of the evidence for evolution comes from the fossil record. Fossils show how much, or how little, organisms have changed over time.

One of the problems with the fossil record is that it contains gaps. Not all organisms fossilise well. And many fossils are destroyed by the movements of the Earth, or have simply not been discovered.

Peppered moths

Before the Industrial Revolution in Britain, most peppered moths were of the pale variety. They were camouflaged against the pale birch trees they rested on. Moths with a mutant black colouring were spotted easily by birds and eaten. This gave the white variety an advantage, making them more likely to survive and reproduce.

Airborne pollution in industrial areas blackened the birch tree bark with soot. This meant the mutant black moths were now camouflaged, while the white variety became more vulnerable to predators. It gave the black variety an advantage. They were then more likely to survive and reproduce. Over time, the black peppered moths became far more numerous in urban areas than the pale variety.

The pale peppered moths camouflage well against the pale birch tree

The darker mutant peppered moths camouflage well against the blackened birch tree

Antibiotic-resistant bacteria

E. coli bacteria (Photo from Rocky Mountain Laboratories, NIAID, NIH)

Micro-organisms such as bacteria [bacteria: Single-celled microorganisms, some of which are pathogenic in humans, animals and plants. Singular is bacterium. ] and viruses [viruses: ultramicroscopic non-cellular organisms that replicate themselves inside the cells of living hosts ] reproduce rapidly and can evolve in a relatively short time. One example is the bacterium E. coli. Its DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] can be damaged or changed during replication, and most of the time this causes the death of the cell. But occasionally the mutation is beneficial (to the bacteria). For example, it may allow resistance to certain antibiotics [antibiotics: Substances that kill bacteria. ]. When those antibiotics are present, the resistant bacteria have an advantage over the bacteria that are not resistant. Antibiotic-resistant strains of bacteria are an increasing problem in hospitals.


Scientists can now examine the DNA from different species of organism and use the data produced to see how closely related the two species are to each other. By collecting a lot of this data, scientists can compare the results with conventional ideas about how organisms have evolved. What they found was that DNA data supported the conventional theory of evolution.

Ideas about science

Developing explanations - higher only

You need to be able to provide an explanation that links together things that were previously thought to be unrelated. For example, DNA data being used to support the theory of evolution. Also people had noticed that there was a wide variety of different things living on the planet. Darwin noticed that different islands in the Galapagos had different types of finches adapted to eating the type of food found on each island. It was only when Darwin produced his theory of evolution by natural selection that these two unrelated facts were linked together.


Living organisms are dependent on the environment and other species for their survival. When the environment changes, organisms are not as well adapted to it.

Individuals that are poorly adapted to their environment are less likely to survive and reproduce than those that are well adapted. Similarly, it is possible that a species that is poorly adapted to its environment will become extinctextinct: Species of organism that is no longer living.

Here are some factors that can cause a species to become extinct:

  • rapid changes to the environment, such as the climate
  • new diseases
  • new predators
  • new competitors

Because we are all part of a giant food web, changes in one part of the web can have a big impact on other parts of the web. When one food source disappears, a predator will have to find another food source in order to survive.

The fossil record shows that since life on Earth began many species have become extinct. Extinction is still occurring, a lot of it because of human activity. We compete with other living things for space, food and water, and are very successful predators.

The quagga

An image of a quagga, a type of zebra. The quagga has a horse/zebra like face and stripes similar to a zebra.

A Quagga

Some animals such as the quagga are extinct because of the direct activity of humans.

The last quagga alive in the wild was shot in the late 1870s.

The quagga was the first extinct species to have its DNA [DNA: The material inside the nucleus of cells, carrying genetic information. DNA stands for Deoxyribonucleic Acid. ] studied. The DNA showed that the quagga was not a separate species as had been previously thought, but was a type of zebra.

Ideas about science


The DNA result from the quagga is a good example of where new data is in conflict with a previous explanation.

The dodo

The Dodo, a distant relative of the pigeons, is heavily-built, roughly the size of a swan and flightless

Engraving of the extinct dodo

Other animals such as the dodo are extinct because of the indirect activity of humans. The dodo was a large flightless bird that lived in the Mauritius, a group of islands in the Indian Ocean. These islands were uninhabited, and the dodo had no natural predators. Then Mauritius were colonised by the Dutch in 1638. Dodos were hunted for food and easy to catch, because they were not afraid of people. However, it was new competitors brought to the islands, such as pigs, cats and rats, which ate the dodos' eggs and young, that were finally responsible for the extinction of the dodo.


Biodiversity means having as wide a range of different species as possible. Maintaining biodiversity is an important part of using the environment in a sustainable way. Indiscriminate use of the environment, for example cutting down large areas of the rain forest to grow crops such as soya, results in a large number of species becoming extinct and reduces biodiversity.

It is only now that we are beginning to realise the potential medicines and crops that we can obtain from a wide range of different organisms. Only by maintaining biodiversity can we be sure that these benefits will be available for future generations.

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